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Space Forum / Space Flight / December 2003RLV physicaly impossible ?
I am really amazed by all these optimistic people trying to do rlvs and i belive in these peoples cause! however it strikes me that these people all seem a bit to optimistic for ex almoast all the groups i have investigated are into liquid propellant engines, and they seem to think they can make these engines totaly reuseble, let me remind you about the physical laws of thermodynamics (heat) that makes it impossible to turn the engines on and off forever or atleast alot of times! for me it seems almoast impossible to make a liquid rocket engine reuseble. Or am i wrong ? will these people beat the heat laws of rocket engines somehow ? > [...] let me remind you about the physical laws of thermodynamics (heat) > that makes it impossible to turn the engines on and off forever or > at least alot of times! I turn my car engine on and off a lot. Jet engines get turned on and off a lot. Not sure what physical law you are talking about. > Or am i wrong ? I don't want to be the one to break the bad news to you.... > > [...] let me remind you about the physical laws of thermodynamics (heat) > > that makes it impossible to turn the engines on and off forever or [quoted text clipped - 6 lines] > > I don't want to be the one to break the bad news to you.... here is the thing i am really not so sure by my own nolage, its just that i have asked peoeple that i think are credible people that work in the field of physics (not space engineering though) and accoarding to what they have said: the energy and heat stress of going to orbit and back are much more higher than for example what a car experience and therefore it cuts back what is possible to do with space crafts. Either the person i asked is right or wrong because he is no expert in space crafts and is too "theoretical". I still wonder though how long life spans sub/orbital rlv vehicles will have though.. >that i have asked peoeple that i think are credible people that work >in the field of physics (not space engineering though) and accoarding >to what they have said: the energy and heat stress of going to orbit >and back are much more higher than for example what a car experience >and therefore it cuts back what is possible to do with space crafts. The extent of these problems is much exaggerated, especially by people who don't have direct knowledge of space engineering. In some cases, there are real problems but they are artifacts of current design practices, which can and should be changed. For example, rocket engines often experience a great deal of thermal stress during startup, due to very rapid temperature rises. But there is no fundamental reason why their startup sequences need to be so fast. Limiting warmup to rates normally found in jet engines is not a big problem, once designers start caring about reliability and long life rather than absolute maximum performance. In other cases, these beliefs are simply misunderstandings, partly based on authoritative statements from people with vested interests in keeping spaceflight expensive and difficult. (Of *course* NASA will tell you that space is terribly hard; it would be immensely embarrassing for them to admit that they've been wasting your money all these years...) >...I still wonder though how long >life spans sub/orbital rlv vehicles will have though.. It's an open question. The first-generation ones may indeed have somewhat limited lives. There is much speculation about this, most of which boils down to religious arguments about basic assumptions. The only way to know for sure is to try it and see.  SignatureMOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | henry@spsystems.net Virtually all managers and most all engineers that have grown up in the current space hardware design bureaus have been steeped in the "rockets as artillery" school. Design constraints are fire the engine once for testing, put it on the stand for its first flight, then through it away after four or five minutes operation. Any effort or materials to give it a longer life are considered a waste. This does not mean that large rocket engines can't be built for longer service, only that the engineers don't need it for "artillery" use. A good example of a flight weight engine with long life is the RL-10. This engine used on several upper stages was used (in the short bell version) for the DC-X, and has had dozens of starts and hours of total time. Another area that is not considered by most engineers developing RLVs is that propellents used by most designs are about three orders of magnitude lower in cost than flight hardware, and that adding propellent to reduce the quantity of flight hardware will eliminate any failure modes that were possible in the eliminated hardware. An example would be if the Shuttle had no wings there would have been no wing leading edge failure. RLVs require companies and engineers willing to try new paradigm, not just incremental improvements. The Space Shuttle was a try, however many of its design requirements were made for political reasons not economic or technical. Mike > >that i have asked peoeple that i think are credible people that work > >in the field of physics (not space engineering though) and accoarding > >to what they have said: the energy and heat stress of going to orbit > >and back are much more higher than for example what a car experience > >and therefore it cuts back what is possible to do with space crafts. > >...I still wonder though how long > >life spans sub/orbital rlv vehicles will have though.. Another area that is not considered by most engineers developing RLVs is that propellents used by most designs are about three orders of magnitude lower in cost than flight hardware, and that adding propellent to reduce the quantity of flight hardware will eliminate any failure modes that were possible in the eliminated hardware. An example would be if the Shuttle had no wings there would have been no wing leading edge failure. I suppose you mean an approach that is simple is prefered, to make rlvs possible ? anyway i tend to like "simple" and "clean" approaches... like thouse of scaled composites and armadillo earospace. It seems the space shuttle as you meantioned is really "complex" > I suppose you mean an approach that is simple is prefered, to make > rlvs possible ? anyway i tend to like "simple" and "clean" > approaches... like thouse of scaled composites and armadillo > earospace. It seems the space shuttle as you meantioned is really > "complex" The shuttle also had to do a lot of things that the Scaled Composites and Armadillo Aerospace X-Prize vehicles did not. For example, the shuttle had to reach orbital velocities and return from them. No X-Prize vehicle that I know of is approaching 1/4 of orbital velocity (17500mph); I think the Scaled Composites vehicle is topping out at ~2500mph. The shuttle had to meet a lot of military needs, like a 1500-mile cross-range so it could land at its launch site after a single polar orbit. Before the USAF joined the shuttle project, some shuttle designs featured small, stub wings optimized for low-speed performance. On designs like Faget's "stub wing orbiter," the shuttle would aerobrake with its belly (~60-degree angle of attack) with leading edges...well, they weren't really leading edges. But Faget was designing for a civilian vehicle that could accept 200-300 miles of cross-range and wait in orbit until it was again lined up with its landing site. Metallic heat shields were also considered before the USAF jumped/was shoved aboard. After the USAF signed up, only ceramic heat shields would get the job done. A lot was asked of the shuttle, more than most current RLV designs are expected to do. Mike Miller, Materials Engineer > > > I suppose you mean an approach that is simple is prefered, to make [quoted text clipped - 5 lines] > The shuttle also had to do a lot of things that the Scaled Composites > and Armadillo Aerospace X-Prize vehicles did not. Well ofcourse thats why burt rutan and amradillo are doing suborbital vehecles. But one still have to wonder "if it is so damn easy to make suborbital rlvs" then why havent NASA allready made one of them? well ofcourse they made x15 but what good is that vehicle to me ? > For example, the shuttle had to reach orbital velocities and return > from them. No X-Prize vehicle that I know of is approaching 1/4 of > orbital velocity (17500mph); I think the Scaled Composites vehicle is > topping out at ~2500mph. Let me correct you, non of the xprize teams have made a suborbital trip yet, Burt Rutan has been at altitudeds of 50000 feet but i have not herd about him fireing off the hybrit rocket yet. So i wont give xprize teams any credit until the prove me wrong. However it is only a matter of time til some of thouse teams do. > The shuttle had to meet a lot of military needs, like a 1500-mile > cross-range so it could land at its launch site after a single polar [quoted text clipped - 11 lines] > A lot was asked of the shuttle, more than most current RLV designs are > expected to do. I dont blame NASA for anything, im not from America so i dont pay my taxes there. But if i would pay my taxes there i would not think the shuttle project was worth anything of it. I am not to happy with the thought that people can build stuff in their garage and NASA needs a army to do things, even though they make slightly diffrent things. Anyway i think you catch my drift. Paul. > The extent of these problems is much exaggerated, especially by people who > don't have direct knowledge of space engineering. I tend to belive this is true. I get the feeling they know more about mathematical forumlas than what they are really talking about. > In some cases, there are real problems but they are artifacts of current > design practices, which can and should be changed. For example, rocket [quoted text clipped - 4 lines] > caring about reliability and long life rather than absolute maximum > performance. I belive that this is the solution. Alot of small changes in our way of thinking that could make rlvs possible. There should be a website out there that count up all these small things thats could make rlvs possible. I think its sad that there are people out there that knows approaches on how to make rlvs but there really is no good information source for it??? Where is the websites? Where is the books? I read armadillo aerospace website witch i think is good, but is there more? Paul. >...I think its sad that there are people out there that knows >approaches on how to make rlvs but there really is no good information >source for it??? Where is the websites? Where is the books? I read >armadillo aerospace website witch i think is good, but is there more? There is, but it's thinly spread and not easy to find. Much of it is not on the Internet at all, in fact. SignatureMOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | henry@spsystems.net fherhkmfbfb@hotmail.com (Paul Spielmann) : > > In article <7927e4ca.0311230605.2bb1c016@posting.google.com>, > > Paul Spielmann <fherhkmfbfb@hotmail.com> wrote: [quoted text clipped - 21 lines] > source for it??? Where is the websites? Where is the books? I read > armadillo aerospace website witch i think is good, but is there more? First, you say that you read armadillo aerospace website. Notice that he has already restarted, and even flew his rocket designs more than once. That automaticly tells you that rockets can be fired more than once. Second, most RLV designers are only looking for 10-100 flights before scrapping a craft. Logical if the the total cost of maintenance and the cost of the craft total is less then the cost of the same number of flights of one-shot craft then you are saving money. Notice you car does not last forever either, but imagine what it would cost to drive your car if after every trip you needed to buy a new one? Earl Colby Pottinger SignatureI make public email sent to me! Hydrogen Peroxide Rockets, OpenBeos, SerialTransfer 3.0, RAMDISK, BoatBuilding, DIY TabletPC. What happened to the time? http://webhome.idirect.com/~earlcp >...liquid propellant engines, and they seem to >think they can make these engines totaly reuseble, let me remind you >about the physical laws of thermodynamics (heat) that makes it >impossible to turn the engines on and off forever or atleast alot of >times! How, exactly, do the laws of thermodynamics make it impossible to turn a rocket engine on and off a lot? Be specific. The small engines used as attitude-control thrusters in spacecraft are sometimes rated (by testing, not theoretical calculation) for 300,000 or more firings. SignatureMOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | henry@spsystems.net >>...liquid propellant engines, and they seem to >>think they can make these engines totaly reuseble, let me remind you [quoted text clipped - 6 lines] > attitude-control thrusters in spacecraft are sometimes rated (by testing, > not theoretical calculation) for 300,000 or more firings. I have an engine sitting on my desk at work that went for 632000 pulses - and still performed the last pulse within 5% of what it did on the first! Laws of thermodynamics (??) be damned! Brett > I am really amazed by all these optimistic people trying to do rlvs > and i belive in these peoples cause! however it strikes me that these [quoted text clipped - 6 lines] > engine reuseble. Or am i wrong ? will these people beat the heat laws > of rocket engines somehow ? Have you heard of jet engines? The Space Shuttle Main Engine? Are you aware that most reusable rocket engine designs (apart from thrusters) employ active cooling? You design them to not operate close to the physical margins (not even as close as the SSME does). This is why the engine in your car needs less repair and maintenance than engines used in various forms of racing, where they're typically pushed to the limit and kept there, sometimes for hours. And in all the above examples, materials science doesn't stand still. What's not to understand? SignatureYou know what to remove, to reply.... Please perform a web search on something called the "X-15". You may be surprised to learn that this RLV flew 40 years ago. > I am really amazed by all these optimistic people trying to do rlvs > and i belive in these peoples cause! however it strikes me that these [quoted text clipped - 6 lines] > engine reuseble. Or am i wrong ? will these people beat the heat laws > of rocket engines somehow ? |
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