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Space Forum / Space Flight / April 2004SSTO propulsion overview
Catching up on some unread journals, I note that the March/April 2003 issue of JBIS has a very interesting paper: "A comparison of propulsion concepts for SSTO reusable launchers", by Richard Varvill and Alan Bond. While the authors have a mild case of hydrogen religion -- non-hydrogen rockets are never mentioned -- and they obviously have their own axe to grind, they generally give a good overview of the alternatives, including why scramjets are such a lousy idea for space launch.  SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net >Catching up on some unread journals, I note that the March/April 2003 >issue of JBIS has a very interesting paper: "A comparison of propulsion [quoted text clipped - 3 lines] >grind, they generally give a good overview of the alternatives, including >why scramjets are such a lousy idea for space launch. This is all my opinion, of course... After reading this newsgroup for a while, I have come to the conclusion that any type of air-breathing for orbital launch is a waste of time and money. Turbojets, scramjets, whatever. It just makes getting into orbit harder, not easier. The only exception to this is an aircraft carrier 1st stage. There are some advantages to high altitude launch (less altitude compensation needed for your rocket nozzle, for example), and it can give you a lot of flexibility with launch site location. But that's the only exception in my view. I just can't understand why so much time and research dollars are being spent on hypersonic research. If you want to get into orbit, you want to get _out_ of the atmosphere as soon as possible. It is completely counter-intuitive to try and gain lots of velocity while still inside the atmosphere, where you are subject to drag (inefficiency) and heating (exotic materials and/or cooling systems needed). While this is drifting off-topic for this newsgroup, can someone explain to me why so many in the aero/astro field still think hypersonics for orbital launch are a good idea? And are hypersonics a good idea for anything at all? James Graves > While this is drifting off-topic for this newsgroup, can someone explain > to me why so many in the aero/astro field still think hypersonics for > orbital launch are a good idea? 1.) Because the first "A" in NASA stands for "aeronautics." 2.) Because it helps keep several NASA center in business. > And are hypersonics a good idea for anything at all? Not in my opinion. Hypersonic travel combines all the disadvantages of airplanes with all the disadvantages of rocket flight and all the disadvantages of re-entry --- continuously. Above Mach several, you might as well go whole hog and do a sub-orbital lob; it'll be faster, cheaper, and more practical. -- Gordon D. Pusch perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;' g_d_pusch_remove_underscores@xnet.com (Gordon D. Pusch) wrote in message > Hypersonic travel combines all the disadvantages of > airplanes with all the disadvantages of rocket flight and all the > disadvantages of re-entry --- continuously. Very funny --but also very true Greg >...any type of air-breathing for orbital launch is a waste of time >and money. Turbojets, scramjets, whatever. It just makes getting into >orbit harder, not easier. >The only exception to this is an aircraft carrier 1st stage... I'm inclined to say that the jury is still out on LACE and its relatives (such as Alan Bond's concepts) and on things like the original Roton, which airbreathe a little bit at the start of a largely rocket-powered ascent. The idea is plausible; what remains unproven is that it's any better than a pure rocket. I wouldn't build one myself, but wouldn't exclude the possibility of success that way. >While this is drifting off-topic for this newsgroup, can someone explain >to me why so many in the aero/astro field still think hypersonics for >orbital launch are a good idea? To some extent this is a lingering echo of the idea that spaceships are, or *should be*, just especially high-performance aircraft. There has never been any very strong justification for this belief, but it remains an article of faith for many from the "aero" side of aerospace. And to some extent, it's pure public-relations hype, the result of the hypersonics people -- who have been largely rejected by the aero side of the house -- trying to find funding from the space side (which has long been the rich side of the family). >And are hypersonics a good idea for anything at all? For high-speed cruise within the atmosphere -- assuming you have some urgent reason to want to do that -- they look promising. But they have nothing much to do with spaceflight. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net >>And are hypersonics a good idea for anything at all? > >For high-speed cruise within the atmosphere -- assuming you have some >urgent reason to want to do that -- they look promising. But they have >nothing much to do with spaceflight. I think the sonic boom forbids any hypersonics aircraft service. If you want such a possibility, the only practical solution is going out of the atmosphere for most of the trip, so you get back to the rocket solution even if there is no space travel or orbital capabilitiy. So I don't see any use for "hyper" whatever the intended use. Yvan Bozzonetti. >>>And are hypersonics a good idea for anything at all? >>For high-speed cruise within the atmosphere -- assuming you have some >>urgent reason to want to do that -- they look promising... > >I think the sonic boom forbids any hypersonics aircraft service. Not if it's military. And there are some theoretical possibilities for low-boom or no-boom flight, although whether they will work at hypersonic speeds is unclear. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net >>>And are hypersonics a good idea for anything at all? >>For high-speed cruise within the atmosphere -- assuming you have some >>urgent reason to want to do that -- they look promising. But they have >>nothing much to do with spaceflight. >I think the sonic boom forbids any hypersonics aircraft service. Why? The sonic boom has not prohibited supersonic aircraft service. It only ever prohibited supersonic commercial aircraft service over land, and even that may be negotiable at the margins. Lots of supersonic planes have been built and seen service, and hypersonic planes would serve nicely in the same applications. Signature*John Schilling * "Anything worth doing, * *Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" * *Chief Scientist & General Partner * -13th Rule of Acquisition * *White Elephant Research, LLC * "There is no substitute * *schillin@spock.usc.edu * for success" * *661-951-9107 or 661-275-6795 * -58th Rule of Acquisition * > >...any type of air-breathing for orbital launch is a waste of time > >and money. Turbojets, scramjets, whatever. It just makes getting into [quoted text clipped - 27 lines] > urgent reason to want to do that -- they look promising. But they have > nothing much to do with spaceflight. Wouldn't a hypersonic carrier be an ideal lauch platform for a manned shuttle-type vehicle - a type of TSTO or 3STO if the shuttle had some small drop tanks? > Wouldn't a hypersonic carrier be an ideal lauch platform for a manned > shuttle-type vehicle - a type of TSTO or 3STO if the shuttle had some > small drop tanks? No, because even a _supersonic_ separation of two vehicles at any significant atmospheric density is not merely difficult, but outright =DANGEROUS= !!! Attempting a separation at hypersonic velocities, with every leading-edge surface experiencing absurdly high dynamic pressures while glowing red to white hot, is an exercise only for insane lunatics. Look it up near the top of the list of "Stupid Things You Will Not Live Through If You Attempt." -- Gordon D. Pusch perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;' > No, because even a _supersonic_ separation of two vehicles at any > significant atmospheric density is not merely difficult, but [quoted text clipped - 3 lines] > exercise only for insane lunatics. Look it up near the top of the > list of "Stupid Things You Will Not Live Through If You Attempt." What was the atmosphereic density going to be for separation of HyperX (wasn't that the name of the hypersonic test vehicle that was to be boosted by a Pegasus dropped from a B52?) rick jones Signatureoxymoron n, commuter in a gas-guzzling luxury SUV with an American flag these opinions are mine, all mine; HP might not want them anyway... :) feel free to post, OR email to raj in cup.hp.com but NOT BOTH... >> No, because even a _supersonic_ separation of two vehicles at any >> significant atmospheric density is not merely difficult, but [quoted text clipped - 7 lines] >(wasn't that the name of the hypersonic test vehicle that was to be >boosted by a Pegasus dropped from a B52?) ~100,000ft altitude or a dynamic pressure of 1000psf. Separation was and still is considered one of, if not the highest risk portion of the flight. >> No, because even a _supersonic_ separation of two vehicles at any >> significant atmospheric density is not merely difficult, but [quoted text clipped - 7 lines] > (wasn't that the name of the hypersonic test vehicle that was to be > boosted by a Pegasus dropped from a B52?) At 100,000 ft, atmospheric pressure is about 10 millibars, or 1% sea-level. Temperature is about -20 C (253 K). Since gas density is proportional to pressure divided by temperature, atmospheric density at 100kft is ~1.2%; =NOT= a good vacuum --- even by old-fashioned light-bulb standards! -- Gordon D. Pusch perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;' > > No, because even a _supersonic_ separation of two vehicles at any > > significant atmospheric density is not merely difficult, but [quoted text clipped - 7 lines] > (wasn't that the name of the hypersonic test vehicle that was to be > boosted by a Pegasus dropped from a B52?) I don't know, but by definition in this case, it has to be enough for lift to occur and for airbreathing engines to function.... SignatureYou know what to remove, to reply.... <<Attempting a separation at hypersonic > > velocities, with every leading-edge surface experiencing absurdly > > high dynamic pressures while glowing red to white hot, is an > > exercise only for insane lunatics.>> Another myth-maker emerges. [As if it's so hard to believe that hypersonic craft will be safe and controllable.] ^ //^\\ ~~~ near space elevator ~~~~ ~~~members.aol.com/beanstalkr/~~~ >> Attempting a separation at hypersonic velocities, with every leading- >> edge surface experiencing absurdly high dynamic pressures while glowing >> red to white hot, is an exercise only for insane lunatics. > > Another myth-maker emerges. [As if it's so hard to believe that > hypersonic craft will be safe and controllable.] There is a very distinct difference between attempting to control _one_ hypersonic vehicle controllable under relatively smooth external conditions, versus attempting to control _two_ of them in extremely close proximity, with their shock-waves and plasma-sheaths mutally interacting and impinging on each other, under conditions of very rapidly changing and almost certainly turbulent airflow. The latter case makes attempting to do something as stupid as practicing "touch and goes" on the back of a flying 747 look positively safe and easy by comparison. -- Gordon D. Pusch perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;' <<their shock-waves and plasma-sheaths mutally interacting and impinging on each other,>> Oh by the way, shock waves don't shock. It's just a term that denotes a steeper than usually ocurring pressure gradient. >> >And are hypersonics a good idea for anything at all? >> For high-speed cruise within the atmosphere -- assuming you have some [quoted text clipped - 3 lines] >Wouldn't a hypersonic carrier be an ideal lauch platform for a manned >shuttle-type vehicle... Maybe. As Gordon has already noted, there's a big problem with separating cleanly from such a carrier in a hypersonic environment, and for that matter with surviving that environment long enough to reach separation time. (Launchers normally reach such velocities only in very thin air, but a hypersonic aircraft needs to stay in air that's thick enough for its engines and wings.) That aside, such an aircraft would certainly be interesting for such uses... *if* it already existed. It's very unlikely to be cost-effective to build such a thing solely as part of a launch system. A rocket stage, even a reusable one, is easier and cheaper. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > That aside, such an aircraft would certainly be interesting for such > uses... *if* it already existed. It's very unlikely to be cost-effective > to build such a thing solely as part of a launch system. A rocket stage, > even a reusable one, is easier and cheaper. Burt Rutan did design (and build) an aircraft specifically for his X-Prize entry. It's not hypersonic (AFAIK), but its existence does suggest that designing a custom launch aircraft is a viable approach. SignatureJoe Claffey | "Make no small plans." jrc3@cox.net | -- Daniel Burnham >> That aside, such an aircraft would certainly be interesting for such >> uses... *if* it already existed. It's very unlikely to be cost-effective [quoted text clipped - 3 lines] > Burt Rutan did design (and build) an aircraft specifically for his > X-Prize entry. It's not hypersonic (AFAIK), It's not even _supersonic_ !!! It's basically a big high aspect-ratio sailplane-like design with a couple of jet engines --- essentially a civilian re-invention of the U2 spyplane. It's top speed is at most a few hundred knots! > but its existence does suggest that designing a custom launch aircraft is > a viable approach. Viable for _WHAT_ ?!? Rutan is =NOT= planning to launch payloads into orbit economically --- he is planning to try and win a ONE-SHOT PRIZE, and _maybe_ take up a few tourists with more money than sense !!! -- Gordon D. Pusch perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;' >>>That aside, such an aircraft would certainly be interesting for such >>>uses... *if* it already existed. It's very unlikely to be cost-effective [quoted text clipped - 8 lines] > civilian re-invention of the U2 spyplane. It's top speed is at most > a few hundred knots! Hmm. Are you referring to SS1 or the carrier? They achieved supersonic speeds with SS1. It's not necessary for the carrier to be supersonic, in any case. They have air-launched Pegasus from a B-52 and a Lockheed L1011. (IIRC). Pegasus is comparable in weight to SS1. By any stretch of the imagination, the carrier is cheaper than the aircraft used by Pegasus. >>but its existence does suggest that designing a custom launch aircraft is >>a viable approach. [quoted text clipped - 4 lines] > he is planning to try and win a ONE-SHOT PRIZE, and _maybe_ take up > a few tourists with more money than sense !!! Slight difference here. Rutan is not planning to try to win a one-shot prize. He's planning on building a vehicle he has been contracted to build. He gets paid whether they get the prize, or not. But, this thing is being funded by someone who does not need $10 million and this was in works for years prior to actually being announced and even before the prize was fully funded. >>>> That aside, such an aircraft would certainly be interesting for such >>>> uses... *if* it already existed. It's very unlikely to be [quoted text clipped - 10 lines] > > Hmm. Are you referring to SS1 or the carrier? The carrier, which is very _definitely_ subsonic, not hypersonic! The issue being debated here is orbiter/carrier separation, with an earlier poster suggesting that separation could safely occur at hypersonic velocities. I am pointing out that the separation of Rutan's "SS1" from its subsonic "White Knight" carrier aircraft most definitely does _NOT_ qualify as a sucessful example of a hypersonic separation !!! -- Gordon D. Pusch perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;' The X-15 was "strong justification" for the concept of a plane just going higher and faster until it's in orbit. It's an attempt at myth-creation to claim that supersonic separation is too dangerous to ever use for a space plane launch. Everythingc was dangerous at first but we learn to handle things safely. <<> To some extent this is a lingering echo of the idea that spaceships are, > or *should be*, just especially high-performance aircraft. There has > never been any very strong justification for this belief, but it remains > an article of faith for many from the "aero" side of aerospace.>> ^ //^\\ ~~~ near space elevator ~~~~ ~~~members.aol.com/beanstalkr/~~~ >> To some extent this is a lingering echo of the idea that spaceships are, >> or *should be*, just especially high-performance aircraft. There has [quoted text clipped - 3 lines] >The X-15 was "strong justification" for the concept of a plane just going >higher and faster until it's in orbit. No more so than the SR-71, the X-1, or the Wright Flyer. The X-15 was a fast aircraft... where "fast" means about 25% of orbital velocity. It had little resemblance to an orbital vehicle, and provided no obvious development path toward one. Indeed, much of its design was strongly driven by the requirement for high-speed flight *within the atmosphere*, which is almost completely irrelevant to spaceflight. This is just more faith-based wishful thinking. The X-15 represents a step along a path to aircraft-like spaceflight only if you believe such a path exists. It's the existence of the path that I am questioning. The performance gap between even the X-15 and an orbital vehicle remains vast, and it's not at all obvious that there is any way to close it while retaining the general characteristics of an aircraft. Aircraft were not built by making cars or locomotives faster and lighter. Spaceships will not be built by making aircraft faster and lighter. Indeed, the single biggest problem of early spaceflight -- reentry -- was solved only when Harvey Allen realized that a reentering spacecraft was *NOT* an aircraft and should not be designed like an aircraft. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net <<Indeed, the single biggest problem of early spaceflight -- reentry -- was solved only when Harvey Allen realized that a reentering spacecraft was *NOT* an aircraft and should not be designed like an aircraft.>> Bless Harvey Allen but some would say that the reentry problem has yet to be solved. We invented better thermal insulation and called it a reentry solution but it's an unsatisfactory way of descending from orbit, as we witnessed with Columbia. It's premature to say airplanes can't reenter. More designs and techniques must be explored. ^ //^\\ ~~~ near space elevator ~~~~ ~~~members.aol.com/beanstalkr/~~~ ><<Indeed, the single biggest problem of early spaceflight -- reentry -- was >solved only when Harvey Allen realized that a reentering spacecraft was [quoted text clipped - 4 lines] >solution but it's an unsatisfactory way of descending from orbit, as we >witnessed with Columbia. Some would say that the reentry problem was solved quite satisfactorily, until the aircraft nuts got their foot in the door and started insisting that spaceships had to look and act like aircraft. The Apollo heatshield had tremendous safety margins, and a little bash from falling debris wouldn't have bothered it in the slightest (not least because the really crucial part of it wasn't exposed during launch). But ablative heatshields don't work very well if you start insisting that the vehicle has to have *wings*. That's what killed Columbia: the long, slow, reentry of a winged vehicle gives it a prolonged roasting rather than a quick blowtorching, requiring thermal protection that radiates heat away rather than soaking it up... and thus needs exotic high-temperature materials, which typically involve compromises in areas like physical durability. > It's premature to say airplanes can't reenter. Nearly half a century ago, we already understood that an airplane shape was not the best choice for reentry. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > [snip] > [quoted text clipped - 5 lines] > materials, which typically involve compromises in areas like physical > durability. Henry, having said that.. what are your thoughts on something along the lines of Rutans' SpaceShipOne "shuttlecock" design? That has wings but would reenter quicker than a conventional airframe shape. Would something like that (but with an Apollo-style ablative shield on the belly) offer any advantages? Thanks, Cameron:-) > > It's premature to say airplanes can't reenter. > [quoted text clipped - 3 lines] > MOST launched 30 June; science observations running | Henry Spencer > since Oct; first surprises seen; papers pending. | henry@spsystems.net >> But ablative heatshields don't work very well if you start insisting that >> the vehicle has to have *wings*... > >Henry, having said that.. what are your thoughts on something along the >lines of Rutans' SpaceShipOne "shuttlecock" design? That has wings but >would reenter quicker than a conventional airframe shape. I don't know quite enough about its reentry profile to confidently compare it. The impression I have is that it's a pure drag device, non-lifting, until it swings its tail back down. That's actually *too* drastic for an orbital reentry, or even a higher suborbital, where you want a little bit of lift to reduce the G-loads. Unless you get quite a bit of lift, though, its reentry is still in the "sharp and short" category where ablators etc. do well. >Would something like that (but with an Apollo-style ablative shield on the >belly) offer any advantages? I would worry about ablators on aerodynamic surfaces, because of concern that uneven ablation would change the shape or leave a rough surface. There are also a lot of surfaces needing protection -- the belly is worst but it's not the only concern -- which is going to run up the ablator mass. That aside, there is a lot of advantage in variable geometry, or more generally, in being able to change vehicle modes to deal with the changing environment. The more you can separate the problems of reentry from those of landing -- for example, by using different vehicle configurations -- the easier the problems are to solve, because you don't have to satisfy several sets of constraints simultaneously. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net >Some would say that the reentry problem was solved quite satisfactorily, >until the aircraft nuts got their foot in the door and started insisting >that spaceships had to look and act like aircraft. The Apollo heatshield >had tremendous safety margins, and a little bash from falling debris >wouldn't have bothered it in the slightest (not least because the really >crucial part of it wasn't exposed during launch). Indeed. It was buried in the middle of the spacecraft during launch and in-flight. Anything that could damage the TPS on Apollo would be doing critically bad things to the rest of the spacecraft. And even when it did take a hit, as it (probably) did with the LOX tank explosion during Apollo 13, it still performed like a champ. Design Margins. Why is that so hard for some people to understand? The Space Shuttle has 5 flight computers, any one of which could fly the spacecraft alone. But it only has _one_ very fragile TPS! There were people saying it was a bad design 30 years ago, and they're still not being listened to. >But ablative heatshields don't work very well if you start insisting that >the vehicle has to have *wings*. That's what killed Columbia: the long, [quoted text clipped - 3 lines] >materials, which typically involve compromises in areas like physical >durability. And if the fragility of the tiles themselves weren't bad enough... Everyone should have already read about the tremendous amount of time and effort needed to maintain the Shuttle's TPS. We're talking tens of thousands of man-hours, for every flight. I'm sorry, but that's just insane. Now compare that to the time and effort needed to attach oak blocks to the bottom of a capsule. Sure, maybe you'd need to hire Norm Abrams to work on your TPS, instead of relying upon your uncle Bob who has a table saw in his garage. But we're talking several orders of magnitude easier to design and impelement. Perhaps this is slight hyperbole, but the TPS inspection could be done with a hammer in 15 minutes. Anything which falls off after being given a good whack needs to be fixed. Simple. Robust. Reliable. I like it. Sure, you can call the Chinese space program primitive. I'd rather be primitive and alive, than sophisticated and dead. - - - - - - - - - - - - - - - - - - BTW, 'reliable' isn't the same thing as 'robust'. Lots of people forget that too. - - - - - - - - - - - - - - - - - - Fuel is Cheap. Simplicty is a Virtue. Design Margins are Good. How many billions have been wasted ignoring these sentiments? - - - - - - - - - - - - - - - - - - What the heck are the wings on the shuttle good for anyway? It isn't as if the SS has any kind of cross-range ability. You can't, at the last minute, abort or even pick a different runway. James Graves >What the heck are the wings on the shuttle good for anyway? It isn't as >if the SS has any kind of cross-range ability. You can't, at the last >minute, abort or even pick a different runway. The shuttle does have considerable cross-range capability earlier in reentry, which is useful because you can reach a landing site that is significantly to one side of your orbit track. This is helpful in normal operations, reducing the need for either in-orbit waits or retrieval from contingency landing sites. Moreover, it's essential if you want to be able to do a once-around mission from Vandenberg, either deliberately or because a problem causes an Abort Once Around, since there's nothing much but water west of Vandenberg. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net Henry, I think all of this applies to horizontal landing winged vehicles only. When you specify wings you are required to have multi kilometer long runways, and that means a lot of maneuvering room for contingencies. For vertical landing vehicles the landings contingencies for maneuvering are much smaller. In an emergency a football stadium could be used, assuming no game was in progress : ). If I recall correctly the Vandenberg facility has been abandoned for Shuttle use, so cross range for this facility is not an issue. Mike Swift > The shuttle does have considerable cross-range capability earlier in > reentry, which is useful because you can reach a landing site that is [quoted text clipped - 4 lines] > because a problem causes an Abort Once Around, since there's nothing much > but water west of Vandenberg. Mike Swift opined >Henry, I think all of this applies to horizontal landing winged vehicles >only. When you specify wings you are required to have multi kilometer [quoted text clipped - 4 lines] >correctly the Vandenberg facility has been abandoned for Shuttle use, so >cross range for this facility is not an issue. A winged vehicle does not necessarily need a long runway. Blackhorse shoould be able to land any where a light twin can get into. >> The shuttle does have considerable cross-range capability earlier in >> reentry, which is useful because you can reach a landing site that is [quoted text clipped - 8 lines] >> since Oct; first surprises seen; papers pending. | >> henry@spsystems.net -ash Cthulhu for President! Why vote for a lesser evil? >><<Indeed, the single biggest problem of early spaceflight -- reentry -- was >>solved only when Harvey Allen realized that a reentering spacecraft was [quoted text clipped - 24 lines] > Nearly half a century ago, we already understood that an airplane shape > was not the best choice for reentry. Faget had an interesting compromise to that by making the re-entry at high attitude which essentially made the winged vehicle a capsule for the re-entry. The wings helped reduce the thermal loading, IIRC. <<That's what killed Columbia: the long, slow, reentry of a winged vehicle gives it a prolonged roasting rather than a quick blowtorching,>> Well, the facts aren't all in. YOu cannot automatically say nasa chose the best reentry technique for all time and for all systems. Maybe a longer and slower glide with less atmospheric compression, producing less heat build-up, would be preferable to the high speed plunge technique?. > Maybe a longer and slower glide with less atmospheric compression, >producing less heat build-up, would be preferable to the high speed plunge >technique?. The former is what the shuttle already does. And as I noted, it makes thermal protection considerably harder, resulting in much less robust systems. If you're suggesting making it still longer and slower, that is easier said than done. Slowing the process down requires staying up in thinner air, which in turn requires better gliding performance -- to be technical, a higher hypersonic L/D (lift/drag) ratio. Alas, the configuration changes needed for that tend to make thermal problems worse, not better. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > > Maybe a longer and slower glide with less atmospheric compression, > >producing less heat build-up, would be preferable to the high speed plunge [quoted text clipped - 9 lines] > a higher hypersonic L/D (lift/drag) ratio. Alas, the configuration > changes needed for that tend to make thermal problems worse, not better. Henry, maybe you can answer a question I have. I have been told that the Shuttle's ET would reenter without burning up if steps were not taken to depressurize it before reentry. Is this true, and if so is it because of its very large surface area compared to its weight? <<I have been told that the Shuttle's ET would reenter without burning up if steps were not taken to depressurize it before reentry. Is this true, and if so is it because of its very large surface area compared to its weight?>> I read that the ET is broken up by explosive charges so that it burns better during reentry. Makes you wonder why nasa didn't go the other way and try to make it burn less and be recoverable. ^ //^\\ ~~~ near space elevator ~~~~ ~~~members.aol.com/beanstalkr/~~~ > I read that the ET is broken up by explosive charges so that it burns better >during reentry. No, it's not. Even the destruct charges that used to be there (fired by command only) are no longer present. > Makes you wonder why nasa didn't go the other way and try to make it burn >less and be recoverable. Because it was designed from the start to be expendable. Recovering it would be difficult, and refurbishing it for re-use would be tricky and expensive. (Even refurbishing the SRBs probably does not actually save NASA any money.) SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net <<Slowing the [reentry] process down requires staying up in thinner > air, which in turn requires better gliding performance -- to be technical, > a higher hypersonic L/D (lift/drag) ratio. Alas, the configuration > changes needed for that tend to make thermal problems worse, not better.>> That was how the old DynaSoar was going to do it. Quick little dips down into thick air to scrub some speed but then skipping back up into thinner air to cool off a bit before gliding back down. I think that's how civilian entrepreneurs will design their reentry ships - more like an airplane than a bowling pin. ^ //^\\ ~~~ near space elevator ~~~~ ~~~members.aol.com/beanstalkr/~~~ > That was how the old DynaSoar was going to do it. Quick little dips down >into thick air to scrub some speed but then skipping back up into thinner air >to cool off a bit before gliding back down. Unfortunately, while this may improve the thermal situation somewhat -- it's not as clear-cut as the Dyna-Soar promoters thought -- it doesn't actually help the aerodynamics. To achieve the same duration of flight, you need the same L/D. Averaged over one cycle, you need the same amount of lift, and hence at the same L/D, you will incur the same amount of drag. > I think that's how civilian entrepreneurs will design their reentry ships - >more like an airplane than a bowling pin. There's no such pattern visible in how civilian entrepreneurs have designed their vehicles to date. A long, slow, high-L/D reentry has major disadvantages as well as advantages. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net So no way to have a massive amount of area, and use the resulting area to act as a parachute on the atmosphere? Asbestos parachute or .. Mike > > That was how the old DynaSoar was going to do it. Quick little dips down > >into thick air to scrub some speed but then skipping back up into thinner air [quoted text clipped - 16 lines] > MOST launched 30 June; science observations running | Henry Spencer > since Oct; first surprises seen; papers pending. | henry@spsystems.net >> ...A long, slow, high-L/D reentry has >> major disadvantages as well as advantages. > >So no way to have a massive amount of area, and use the resulting area >to act as a parachute on the atmosphere? Asbestos parachute or .. You *can* do that, but it's a different concept entirely. With a very low mass per unit area -- via parachute, inverted-umbrella, whatever -- the reentry environment changes in a rather different way. Reentry doesn't get longer or slower; the duration, and the G-loads, are determined almost entirely by L/D, which is a function of shape only. However, *heat* loads do go down dramatically, because the deceleration happens earlier, in thinner air (or alternatively -- you can think of it either way -- because the same amount of heat is spread over a larger surface area). Both the total heat load and the maximum heating rate are greatly reduced... unlike wings, which reduce maximum heating rate at the expense of *increasing* total heat load. It's a promising, but neglected, concept. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net >>> ...A long, slow, high-L/D reentry has >>> major disadvantages as well as advantages. [quoted text clipped - 7 lines] > get longer or slower; the duration, and the G-loads, are determined almost > entirely by L/D, which is a function of shape only. However, *heat* loads The exception is that if you can design the parachute with a variable cross-section (tear-off panels when load exceeds a certain amount, ...) then you can change G-loads a bit, by extending the time of reentry. As Henry said, for a constant mass per unit area, the graphs of accelleration-time look similar. If before the accelleration peaks, you drop the outer parachute, and use a smaller one, you can drop back down the graph, and start again lower down. <<<With a very low > mass per unit area -- via parachute, inverted-umbrella, whatever -- the > reentry environment changes in a rather different way. Reentry doesn't > get longer or slower>>> A first nasa idea for reentry was to have a big disposable heat shield which deployed a Rogallo Wing [hang glider] after the hot part was over and speed was down to 200 mph. The heat shield was parachuted to earth. The capsule hung below the wing which was flown as a glider to a cross range runway. This was too cheap and simple for nasa and it would help the civilians build a CATS vessel so it was, guess what?, cancelled. Hey, reentry's not that hard. ^ //^\\ ~~~ near space elevator ~~~~ ~~~members.aol.com/beanstalkr/~~~ >Henry, maybe you can answer a question I have. I have been told that >the Shuttle's ET would reenter without burning up if steps were not >taken to depressurize it before reentry. Is this true, and if so is it >because of its very large surface area compared to its weight? Just retaining pressure inside would probably not be enough. However, the large area and light weight do help, as does the insulation, which is partly an ablator and makes a fair heatshield. If you could somehow stabilize the tank, so it reentered broadside-on (for maximum frontal area) with a slow spin on its axis (so heating would be spread evenly instead of being concentrated on one side), in theory it should survive reentry. Landing it intact is left as an exercise for the student. :-) In addition to depressurizing it, the tank is deliberately tumbled on separation, to help avoid any chance of this. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > [...] If you could somehow stabilize the tank, so it reentered > broadside-on (for maximum frontal area) with a slow spin on its > axis (so heating would be spread evenly instead of being > concentrated on one side), in theory it should survive reentry. > Landing it intact is left as an exercise for the student. :-) Would it slow to subsonic speeds before hitting the ground? If so, maybe it could deploy a hydrogen gasbag to stay aloft. A 40 m diameter balloon ought to be enough lift for the empty tank, if I've done my sums correctly. >;K > > > Maybe a longer and slower glide with less atmospheric compression, > > >producing less heat build-up, would be preferable to the high speed plunge [quoted text clipped - 14 lines] > taken to depressurize it before reentry. Is this true, and if so is it > because of its very large surface area compared to its weight? I had heard it as "if it could be stabilized for a sideways reentry and spun along the long axis to even out the heating, it could reenter by itself", which is a bit different than just letting it fall. Yes, it is due to having a very low ballistic coefficient. Some plans have been proposed for reentry with extreme area to mass ratios using ballutes and such, which can theoretically be done with very modestly heat resistant materials. John Carmack www.armadilloaerospace.com JC> I had heard it as "if it could be stabilized for a JC> sideways reentry and spun along the long axis to even JC> out the heating, it could reenter by itself", which JC> is a bit different than just letting it fall. Yes, JC> it is due to having a very low ballistic coefficient. JC> Some plans have been proposed for reentry with extreme JC> area to mass ratios using ballutes and such, which can JC> theoretically be done with very modestly heat resistant JC> materials. Heat load during reentry is on the order of 100,000 W/m^2. Spinning the craft will reduce the heat load by a factor of 2. Reentry temperatures are over 10,000 Kelvins (plasma). The external tank of the space shuttle is made of a cryogenic aluminum-lithium alloy. Such alloys have melting point temperature of about 600 degrees Celsius. NASA tested ballutes made from Kapton film reinforced with PBO fiber - they fail when ambient temperature exceeds 600 degrees Celsius. (http://www2.jpl.nasa.gov/adv_tech/ballutes/tech.htm) Kris Cowart made charts depicting heat loads and temperatures during reentry: http://www.ssdl.gatech.edu/main/ssdl_paper_archive/aiaa_99-4806charts.pdf We are not in the ballpark - spinning, empty, aluminum-lithium tanks cannot survive the reentry unless they have walls at least one foot thick. ======================================================================= Allen Meece wrote: AM> That was how the old DynaSoar was going to do it. AM> Quick little dips down into thick air to scrub some AM> speed but then skipping back up into thinner air AM> to cool off a bit before gliding back down. Easier said than done. Thin wings of the spacecraft will melt down in a few seconds, long before the spacecraft returns to the thin air. [...] > Aircraft were not built by making cars or locomotives faster and lighter. Nit: the development of a decent internal combustion engine that made cars lighter and faster was pretty important to powered flight. I'm not sure what the analogue for space flight is; maybe lighter guidance components, but gyro weight probably wasn't much of an issue for a plane. /dps > >Catching up on some unread journals, I note that the March/April 2003 > >issue of JBIS has a very interesting paper: "A comparison of propulsion [quoted text clipped - 27 lines] > to me why so many in the aero/astro field still think hypersonics for > orbital launch are a good idea? "If the only tool you have is a hammer, every problem looks like a nail." And to the less technically inclined, there's the carrot of not carrying your oxidizer along. The price, of course, is taking it in at increasing Mach numbers, and the drag imposed thereby, likely a heavier engine for the same thrust, major thermal issues, etc. Everything you said, and then some. And LOX is cheap. > And are hypersonics a good idea for anything at all? If all you *want* is high-altitude, hypersonic cruise (as opposed to accelerationg to orbital velocity), sure. Recon, *perhaps* commercial flights depending on the economics, etc. And perhaps expendable hypersonic weapons. But airbreathing to orbit, while it might yet be acheived, will be only a niche application. Most access to LEO will be with rockets, until/unless 'beanstalks' can be done, and even those will not completely replace independent spacecraft. > James Graves SignatureYou know what to remove, to reply.... > And are hypersonics a good idea for anything at all? Possibly, yes: Killing Osama bin Laden, et al., when you know exactly where they are *now*, as opposed to where you *hope* they will still be in two hours from now. -- Andrew J. Higgins andrew.higgins@mcgill.ca > And are hypersonics a good idea for anything at all? For long-range flights across the atmosphere. Interesting applications: High-speed reconnaisance platform or Intercontinental bomber, hypercruise missile. Fast things are harder to shoot down. An interesting factoid: There used to be rumours the Russians had some sort of scramjet or ramjet upper stage variant of the Topol-M ICBM when apparently the USA radar network (?) detected an ICBM test with a really weird final trajectory somewhere inside Russia. Turns out the Topol-M has a maneuverable warhead to elude missile defense systems, but it still uses Solid rockets. quasarstrider@yahoo.com.br (quasarstrider): >An interesting factoid: >There used to be rumours the Russians had some sort of scramjet or [quoted text clipped - 3 lines] >maneuverable warhead to elude missile defense systems, but it still >uses Solid rockets. There is the "Gosht" missile with some air-intake capability, may be the only practical system of its kind ( see Astronautix) Yvan Bozzonetti. I agree that hypersonic SCRAMJET propulsion research is completely pointless. But there is a region between standstill and about mach 6 where air breathing propulsion is very attractive. Simple calculations show that you can achieve a factor of 10 savings in fuel consumption if you use ramjet like engines. If your design is correct you get a 15 to 1 air fuel mixture ratio which results in an Isp of about 4000 at standstill. As you start to move the mass flow increases and the engine leans out, just getting better and better with speed, up to about 2km/s at which point it is just easier to close the air intakes and use on-board oxidizer. There have been numerous real hardware experiments and proofs of concept that show the performance of ramjets, ejector ramjets, strut jets etc. An example of an air augmented rocket is the russian GNOM missile see: http://www.astronautix.com/lvs/gnom.htm This missile has less than half the mass of a rocket. Basically my calculations show that you can save the entire first stage if you build an air breathing booster. I have actually invented and built an engine, see at vtol.net that gets an Isp of 4000 at standstill. An air breathing rocket can be built that uses air first and then switches to on board oxidizer and gets into orbit with an overall mass ratio of 6. See also vtol.net/air.htm I keep repeating myself, nobody ever reads the archives. It is very easy to search for stuff and read it. Zoltan > I agree that hypersonic SCRAMJET propulsion research is completely > pointless. Agree. > But there is a region between standstill and about mach 6 where air > breathing propulsion is very attractive. Simple calculations show that > you can achieve a factor of 10 savings in fuel consumption if you use > ramjet like engines. Your simple calculations are a bit off. A dense fuel SSTO would have a mass ratio of 16. Even from mach 6, the upper stage would have a mass ratio of about 6. Since you mention this below, you must know better. This would be a factor of 2 &2/3 if the ramjet used no fuel at all. It also does not include getting the ramjet up to supersonic speed where it becomes operational. Also, fuel is much cheaper than the hardware it burns in. > If your design is correct you get a 15 to 1 air fuel mixture ratio > which results in an Isp of about 4000 at standstill. As you start to > move the mass flow increases and the engine leans out, just getting > better and better with speed, up to about 2km/s at which point it is > just easier to close the air intakes and use on-board oxidizer. Your exhaust velocity will be on the order of 1,000 m/s, which gives a real Isp in the 1,500 range. This is at standstill if you compress the air sufficiently. As you move faster, inlet drag becomes a factor which heavily penalizes lean operation, which reduces effective Isp from your engines. > There have been numerous real hardware experiments and proofs of > concept that show the performance of ramjets, ejector ramjets, strut [quoted text clipped - 4 lines] > > This missile has less than half the mass of a rocket. All of your examples seem to be about fuel consumption, ignoring the increased mass, cost, and complexity of the hardware and flight profile. A Saturn V class rocket might have $1M in fuel. This is not a consideration in the current market. > Basically my calculations show that you can save the entire first > stage if you build an air breathing booster. I have actually invented > and built an engine, see at vtol.net that gets an Isp of 4000 at > standstill. An air breathing rocket can be built that uses air first > and then switches to on board oxidizer and gets into orbit with an > overall mass ratio of 6. See also vtol.net/air.htm What is the T/W of this 4,000 second engine? Have you calculated inlet mass at all? Are you aware of the mass and complexity of effective supersonic inlets? How are you compressing the air? > I keep repeating myself, nobody ever reads the archives. It is very > easy to search for stuff and read it. Quit repeating yourself and say something new and convincing. Some of us have read on the subject a bit. > Zoltan >But there is a region between standstill and about mach 6 where air >breathing propulsion is very attractive. Simple calculations show that >you can achieve a factor of 10 savings in fuel consumption if you use >ramjet like engines. Unfortunately, that's much more attractive for cruising missions than for accelerating ones, because the price is much heavier engines. Even at modest altitudes, the oxygen content of air is *four orders of magnitude* less, per unit volume, than that of LOX. So you inevitably need big heavy machinery to handle air. (If you thought liquid hydrogen was a "fluffy" propellant, awkward to handle because of its bulk, atmospheric air is enormously worse.) >There have been numerous real hardware experiments and proofs of >concept that show the performance of ramjets, ejector ramjets, strut >jets etc. An example of an air augmented rocket is the russian GNOM >missile... Yeah, they're fairly interesting for *cruise* missions. But that's a very different class of problem. >This missile has less than half the mass of a rocket. Which is of almost no importance, for launchers. The added mass of the rocket is almost all LOX. Liquid oxygen is one of the cheapest substances on Earth. In particular, it's much cheaper than airbreathing engines. >Basically my calculations show that you can save the entire first >stage if you build an air breathing booster. That's plausible. But so what? You've turned a rocket first stage into a jet first stage. In the process, you've made it harder to build and more difficult to develop. For what? To save *LOX*? WHY??? SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > >But there is a region between standstill and about mach 6 where air > >breathing propulsion is very attractive. Simple calculations show that [quoted text clipped - 6 lines] > less, per unit volume, than that of LOX. So you inevitably need big heavy > machinery to handle air. What is the maximum possible T/W you see from a turbine based air breathing engine. How inevitable is the question. Do you see a fundamental T/W limit at 100, 40, 15, or some other number? At what T/W do air breathing engines become performance competative with the lower stage rocket thrust they replace? Competative does not necessarily mean desirable in this case, just not a penalty. During a previous discussion I accepted that 120/M seemed to be a reasonable break even for an air breather that supplies all the acceleration from the ground. I suggested a few weeks ago that for a VTVL SSTO, 28 to 43 might be a reasonable requirement for units designed for the landing mass only, not operating supersonic at all during launch phase. Would you agree with these requirements for break even performance? How much better would they have to get to be desirable as opposed to break even? What is the performance requirement for an ICH tee shirt? :-) >> ...the oxygen content of air is *four orders of magnitude* >> less, per unit volume, than that of LOX. So you inevitably need big heavy [quoted text clipped - 3 lines] >air breathing engine. How inevitable is the question. Do you see >a fundamental T/W limit at 100, 40, 15, or some other number? I'm not a turbine-engine guy, so it's a little hard for me to call. My understanding is that the fighter-engine guys are now in the 10-11 range, and it's taken them thirty years to get there from the 7-8 range. The air temperature at the turbine inlet is now well above the melting point of the turbine blades (!). (The blades are single crystals of very stubborn alloys, with cooling vents blowing [relatively] cool air out onto their surfaces to keep the hot stuff at a distance.) That technology isn't too far from its limits. 15, maybe? Radical design changes might perhaps take it farther. But that's harder to predict. I'd be surprised to see 25. (I do get surprised sometimes.) Systems which don't use turbomachinery can do better on mass, but they have a hard time doing as well on air handling, and they generally don't work at low speeds. (Mind you, the turbomachinery tends not to work very well beyond about Mach 3.) Hybrid systems, rocket/airbreather combinations, can do still better. The question there is whether there's enough Isp gain to be worth it. >At what T/W do air breathing engines become performance >competative with the lower stage rocket thrust they replace? >Competative does not necessarily mean desirable in this case, >just not a penalty. Given the other constraints they impose -- for example, they tend to need reasonably clean airflow, which is not easy to come by on the surface of a lower stage -- I think I'd call for at least 40, and that's not going to be easy, especially as speed builds up. (Good LOX/kerosene rocket engines with sea-level nozzles are up around 125.) >During a previous discussion I accepted that 120/M seemed to >be a reasonable break even for an air breather that supplies all [quoted text clipped - 3 lines] >all during launch phase. Would you agree with these requirements >for break even performance? I wouldn't strongly *disagree*, but that reflects limited feel for the problem rather than deep conviction that those are good numbers. :-) SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > >> ...the oxygen content of air is *four orders of magnitude* > >> less, per unit volume, than that of LOX. So you inevitably need big heavy [quoted text clipped - 15 lines] > Radical design changes might perhaps take it farther. But that's harder > to predict. I'd be surprised to see 25. (I do get surprised sometimes.) I am actively working on surprising you. Not building a full up unit in my garage. Just demonstration hardware of a different approach that should reach that goal after real engineers do some analysis and a little CFD work. > Systems which don't use turbomachinery can do better on mass, but they > have a hard time doing as well on air handling, and they generally don't > work at low speeds. (Mind you, the turbomachinery tends not to work very > well beyond about Mach 3.) Mach 3 is past time to get out of the air anyway. IMO, that is where thermal issues on the rest of the airframe begin to clearly out weigh any theoretical gain from an airbreather. Unless the airbreather can provide much more acceleration than commonly assumed. Which implies a better T/W including intakes than I believe likely in the near term. > Hybrid systems, rocket/airbreather combinations, can do still better. > The question there is whether there's enough Isp gain to be worth it. That is the question. There is obviously a curve in there. Less clear is exactly where the curve is. > >At what T/W do air breathing engines become performance > >competative with the lower stage rocket thrust they replace? [quoted text clipped - 6 lines] > be easy, especially as speed builds up. (Good LOX/kerosene rocket engines > with sea-level nozzles are up around 125.) Sounds like a reasonable requirement if the airbreather has other uses. At one point I figured that an airbreather would have to exceed the T/W of a rocket if only used in the launch phase. > >During a previous discussion I accepted that 120/M seemed to > >be a reasonable break even for an air breather that supplies all [quoted text clipped - 6 lines] > I wouldn't strongly *disagree*, but that reflects limited feel for the > problem rather than deep conviction that those are good numbers. :-) I have been sort of digging for a well researched paper that would give a range of honest curves. It seems quite strange that the vast quantity of material on the subject would not have a clear requirements breakdown somewhere handy. A generic set of go-no go curves would be nice. All I have been able to find so far is single project justifications, usually GLOW based. > MOST launched 30 June; science observations running | Henry Spencer > since Oct; first surprises seen; papers pending. | henry@spsystems.net > >Basically my calculations show that you can save the entire first > >stage if you build an air breathing booster. > > That's plausible. But so what? You've turned a rocket first stage into > a jet first stage. In the process, you've made it harder to build and > more difficult to develop. For what? To save *LOX*? WHY??? A reusable flyback booster? > > >Basically my calculations show that you can save the entire first > > >stage if you build an air breathing booster. [quoted text clipped - 4 lines] > > A reusable flyback booster? Which still need not be hypersonic (or supersonic at all) to be effective. Super/hypersonic seperation of stages is not a trivial issue, and a subsonic air-breathing carrier that gets you into less dense air, and a rocket powered second stage whose exhaust expansion is less of the inevitable compromise between sea-level and vacuum is a desirable thing.... SignatureYou know what to remove, to reply.... > > >Basically my calculations show that you can save the entire first > > >stage if you build an air breathing booster. [quoted text clipped - 4 lines] > > A reusable flyback booster? You can have that without using jets on the ascent, only during the return flight. That is a solved problem. Heck, they can even use the same fuel as the ascent rocket engines, for sane choices of fuel :_> > > > >Basically my calculations show that you can save the entire first > > > >stage if you build an air breathing booster. [quoted text clipped - 8 lines] > flight. That is a solved problem. Heck, they can even use the same fuel as > the ascent rocket engines, for sane choices of fuel :_> So, you're saying that a very tiny jet engine used during the return flight makes more sense than a very large jet engine used during ascent. I guess I agree with this idea, since it moves us in the right direction - namely, zero jet engines! :) Let's look at things in terms of delta-vee. To get to orbit you've got to be capable of carrying a vehicle through a delta vee of about 9 km/sec. Orbital velocity near Earth's surface is about 7 km/sec - and you've got about 2 km/sec of gravity and air drag losses if you do things efficiently - to get you 9 km/sec. If you want some legs on your vehicle you, to reach more than minimum altitudes, you might want to add a few tenths of a km/sec more. Of course, to reach higher orbits requires additional delta vee. But those a generally reserved for kick stages operated on orbit, so lets not try to do everything at once. Okay, so you've got 9+ a fraction km/sec delta vee to achieve. On return atmospheric drag does most of the work for you, taking speed away. So, all you've got to do to return is slow the vehicle down enough to lower its perigee to about 50 km to 70 km or so. If you only added say 2 tenths of a km/sec to your minimum orbital speed, you only need the same amount to be able to deorbit. So, now your total delta vee is about 9.4 km/sec, say. The atmosphere will slow you down to subsonic speeds since your surface area is large and your mass is low. You're basically piloting a big propellant tank system - no matter if its one stage or two. So, you start out at near orbital velocity and end up subsonic. The speed of sound is around 0.3 km/sec (depending on local temps) and with proper vehicle shaping you could easily get your terminal speed down to about 0.2 km/sec. With wings, even less (as the Space Shuttle so ably demonstrates) Now, does it make sense to carry wings that mass more than three times your payload to orbit and back to cancel this final 2 tenths of a km/sec? No. Smaller wings, or lifting body shapes, or parachutes, sure. A small quantity of rocket propellant burnt at the last possible second to slow a ballistic descent to zero at the surface, sure. Another question given the range of speeds we're talking about; Does it make sense --using today's propellant combinations and materials-- to spend inordinate effort to build SSTO, rather than TSTO? No. What does make sense; (a) Build vehicles around existing proven technologies that quickly give us the cost and performance we need to carry space travel to the next stage and do so for far less than the President's proposed $12 billion study of the subject. (b) Continue research in truly ground breaking technologies, which include; (i) High performance chemical fuels - such as monatomic fuels (ii) High peformance chemical rockets - such as ARPA's propulsive skin concept (iii) Improved materials and propellants for improved structural fractions (iv) High performance nuclear thermal rockets (v) High performance nuclear electric rockets (vi) High performance nuclear pulse rockets (vii) High performance laser detonation rockets (viii) High performance laser light sails (ix) Tethers (x) Large space structures A near term vehicle that would meet the President's goal of sending people to mars might consist of seven flight elements all similar, built around a fully resuable Space Shuttle External Tank. This improved External Tank would have the minimum thermal protection needed and have small deployable wings like that of a cruise missle. It would be equipped with five to seven SSME derived engines, but these improved engines would have a far lower recurring cost than today's SSME. These tanks would be equipped with cross-feeding. And they would incorporate the latest materials improvements that could easily and cheaply be used to maintain a very reasonable structural fraction (the same one a plain vanilla ET has today) despite the added hardware just described. Now, these seven improved ETs would operate together at launch. From above they would have the following configuration; (1)(2) (3)(4)(5) (6)(7) Now, propellant would flow from 1 to 3 and from 6 to 3 and then from 3 to 4, and propellant would flow from 2 to 5 and from 7 to 5 and from 5 to 4 in such a way as to drain 1,2,6,7 during launch. This would be the first stage. These four elements would be drained and dropped, and they would slow, and deploy their winglets, and be snagged with a towline downrange, where jets modified from airline jets, would tow them back to the launch center for release and landing. Meanwhile, elements 3 and 5 are feeding themselves along with 4. Until 3 and 5 are drained. 3 and 5 are jettisoned, they re-enter the atmosphere, slow, and descend on winglets they deploy, to a point where additional tow jets are waiting to snag them and tow them back to the launch center, using techniques developed way back in the 1950s for recovering film from orbit. Element 4 continues on to nearly orbital speed and orbital altitude. 4 doesn't quite attain orbit as its perigee will bring it to ground 44 minutes after reaching apogee. However, while at apogee 4 releases its payload, and begins its descent while the payload's internal engine circularizes its orbit. 4 continues around the Earth, descending to the point where the landing center was 88 minutes earlier. But at this point too, after slowing and deploying winglets, there waits a tow jet to take 4 back to the launch center for release and landing at the airstrip there. The payload lofted to orbit this way is 550 tons. More than four times the capacity of the Saturn moonship. The cost to build it, less than $12 billion - about what President Bush proposed spending on studying our return to mars. The time to get it flight ready, less than three years. Adapting what we've learned durig Apollo and building the space station to quickly build manned flight elements capable of returning to the moon and going on to mars - would yeild similar savings. Using today's computing and material processing technologies we could send a small crew on a Mars flyby using a 2 year orbit connecting Earth and Mars. This brings the crew back to Earth in 2 years - without any further propulsive inputs. But, it gives a two week window where they approach mars closely - and can deploy hardware that falls to Mars' surface via airbraking - hardware that is controlled realtime via telerobotics, without the delay of distance. Telerobots that could later be controlled by Earth based researchers - and by later crews that return to orbit the Red Planet. Mission 2, could orbit Mars and 'land' on Phobos and Diemos, again deploying remotely controlled robots from orbit. Mission 3, could land directly on Mars, using Zubrin's excellent approach - and having tested most of the elements in the previous two missions. This would get us to mars quickly, cheaply, and sustainably. All without SSTO I'm afraid. Even so, I do believe SSTO deserves research and development funding. I just don't think we should tie our success in space to this technology. Its fine for small payloads where the massive quantities of propellant per useful payload aren't too troubling. And it has a great spinoff in that quick response weapons and spy hardware with global reach are possible with SSTO technology. >> ...You've turned a rocket first stage into >> a jet first stage. In the process, you've made it harder to build and >> more difficult to develop. For what? To save *LOX*? WHY??? > >A reusable flyback booster? Almost certainly, the flyback propulsion system is going to end up being separate from the boost propulsion system. For flyback, you want maximum economy and modest thrust at medium-subsonic cruise speed; for boost, reasonable economy and high thrust over a wide speed range. Flyback propulsion almost certainly wants to be jets (although Kistler proposed to do rocket lob-back), but that doesn't mean boost propulsion has to be jets. The mother of all flyback boosters was the "Flyback F-1" proposed for the shuttle: a Saturn V first stage with a big delta wing and a row of ten jet engines under it. The jets didn't get involved until after reentry. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > >> ...You've turned a rocket first stage into > >> a jet first stage. In the process, you've made it harder to build and [quoted text clipped - 13 lines] > and a row of ten jet engines under it. The jets didn't get involved until > after reentry. Doesnt high-altitude balloon or airship, like the one JP Aerospace and some X-Prize people are trying to do, accomplish much of the goal of subsonic flybacks ? I.e getting the rocket out of most of the amtosphere. -kert >Doesnt high-altitude balloon or airship, like the one JP Aerospace and >some X-Prize people are trying to do, accomplish much of the goal of >subsonic flybacks ? I.e getting the rocket out of most of the >amtosphere. It's not as good as a full-blown first stage (be that a flyback design or not), which contributes quite a bit of vertical and horizontal velocity as well. It *is* about as good as air launch from a subsonic aircraft. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > > >Basically my calculations show that you can save the entire first > > >stage if you build an air breathing booster. [quoted text clipped - 4 lines] > > A reusable flyback booster? http://www.grc.nasa.gov/WWW/RT1998/5000/5830palaszewski.html Atomic rocket fuels - aka mon-atomic rocket fuels - contain vast amounts of energy and yeild very low molecular weights, thus, produce (theoretically) very high performances of 600 to 1,600 sec Isp. High enough for SSTO performance with quite modest rocket designs. Basically, H2 is converted into a stable form of 2 H atoms and mixed with normal liquid hydrogen and kept at liquid helium temps to keep it all from exploding. You end up with a monopropellant that's very powerful, and a very simple rocket - either VTVL, HTHL, or some combo - take your pick - all with structural fractions we've already achieved. http://sbir.grc.nasa.gov/launch/foctopsb.htm Here's another roadmap to high performance. > Atomic rocket fuels - aka mon-atomic rocket fuels - contain vast > amounts of energy and yeild very low molecular weights, thus, produce > (theoretically) very high performances of 600 to 1,600 sec Isp. High > enough for SSTO performance with quite modest rocket designs. > > Basically, H2 is converted into a stable form of 2 H atoms Which doesn't exist. > and mixed > with normal liquid hydrogen and kept at ..well below... > liquid helium temps to keep it > all from exploding. If you're lucky. > You end up with a monopropellant that's very > powerful, and a very simple rocket - either VTVL, HTHL, or some combo > - take your pick - all with structural fractions we've already > achieved. It's ease of use quite handily explains why every one is using it today to get to orbit. > > Atomic rocket fuels - aka mon-atomic rocket fuels - contain vast > > amounts of energy and yeild very low molecular weights, thus, produce [quoted text clipped - 4 lines] > > Which doesn't exist. Rot check out; http://www.space.com/news/hydrogen_helium.html http://www.islandone.org/APC/Chemical/05.html > > and mixed > > with normal liquid hydrogen and kept at liquid helium temps > ..well below... Self-contradictory on two counts; (1) If monatomic hydrogen doesn't exist as you assert above, it cannot exist well below the temperature of liquid helium (which you now claim it does exist in stable form)and (2) the lambda point of liquid helium occurs at around 2.2 K, plainly its not possible to get 'well below' this temperature since 0K is an unattainable lower limit. > > to keep it all from exploding. > > If you're lucky. Agreed. The instability of this every energetic compound is the central difficulty surrounding its use, which is clearly reported in the literature and the subject of ongoing research into its practicality. > > You end up with a monopropellant that's very > > powerful, and a very simple rocket - either VTVL, HTHL, or some combo [quoted text clipped - 3 lines] > It's ease of use quite handily explains why every > one is using it today to get to orbit. If you would take the time to learn a thing or two before making monumentally ignorant statements you'd see that experts in the field say they're a few decades away from taming this source of energy, if ever. As with all things, before the problems are resolved, things seem impossible. However, after they're solved, they become commonplace. Attitudes such as yours do not help.< |
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