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Space Forum / Space Flight / December 2005Large rocket engines cannot be reusable
Large rocket engines cannot be reusable because they are damaged by large Reynolds number. The Reynolds number = Re = V*D*S/N where: V = gas velocity D = diameter of the chamber S = gas density N = gas viscosity Gas viscosity is primarily a function of temperature. The impact of pressure is minor and the viscosity correction for pressure is less than 10% for up to 3.5 MPa. This means that the Reynolds number is proportional to the chamber's diameter and to the gas density, which is proportional to its pressure. The large Reynolds number is the primary cause of turbulence, combustion instability, and cavitation. Turbulence disturbs the protective layer of cool gas adjacent to the chamber's wall. Cavitation and vibration damages turbopumps. All of these fatal diseases are associated with large combustion chambers and large turbopumps. They do not exist in small combustion chambers and well designed, small turbopumps because their Reynolds number is small. This means that large rocket engines cannot be reusable and they are suitable for nuclear missiles only. Small rocket engines have another important advantage: they have superior thrust-to-weight ratio due to the Cube-Square Law which states that as scale is reduced, properties which are a function of volume (mass) will decrease faster than those which are a function of area (thrust and strength). Small, simple engines can be made by robots, so they can be very cheap. Large engines must be made by rocket plumbers. A rocket launcher made of a few hundred small engines is very reliable because the failure of a few engines out of a few hundred is not a catastrophe. The small engines have a small flaw: their small combustion chambers do not mix fuel and oxidizer well. This means that only volatile propellants (e.g., oxygen and methane) can be used in those engines, and the injector holes must be small. Small holes are more difficult to make than large holes and they may plug up with dirt. The best example of small, robust engines that can be made by industrial robots are engine clusters. They are robust enough to survive reentry, splashdown, and handling on a bobbing ship: http://www.islandone.org/LEOBiblio/SPBI101.HTM#engine_cluster >...This means that large rocket engines >cannot be reusable and they are suitable for nuclear missiles only. This would surprise the people who built the F-1, whose specs demanded that it be reliably capable of 20 starts and a total run time exceeding half an hour, even though its operational use required one start and a run time of 2.5 minutes. Demonstrating the rated life required six test engines to achieve more than double it without incident, which they did. Your "fatal diseases" of large chambers and turbopumps are indeed problems, but they can be avoided, and repeatedly have been. >Small rocket engines have another important advantage: they >have superior thrust-to-weight ratio due to the Cube-Square >Law which states that as scale is reduced, properties which >are a function of volume (mass) will decrease faster than >those which are a function of area (thrust and strength). It is not that simple, alas. You don't make a small engine by just scaling down a big one. In particular, achieving adequate chamber residence time for efficient combustion, other things being equal, tends to require scaling only two of the three dimensions, which makes a hash of your proposed application of the square-cube law. >Small, simple engines can be made by robots, so they can >be very cheap. So can large, simple engines. Ask the Russians, who invested much more heavily in production automation (and in design for easy production) than the US ever did.  Signaturespsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | henry@spsystems.net henry@spsystems.net (Henry Spencer) : > In article <433F6EE2.E1192B83@nospam.com>, > Andrew Nowicki <andrew@nospam.com> wrote: [quoted text clipped - 6 lines] > time of 2.5 minutes. Demonstrating the rated life required six test > engines to achieve more than double it without incident, which they did. Question, are not the engines of the Space Shuttle considered large engines? It is my understanding that they are presently up to 4 firings before being refurbished. And the inspections after each flight are just that, inspections. They are not needed, but NASA being a government organizations knows that if there is a failure and they had not done an inspection that heads will roll so they spend the extra money to cover thier butts not because they are needed. 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 > Question, are not the engines of the Space Shuttle considered large engines? > It is my understanding that they are presently up to 4 firings before being > refurbished. So why is it that the SSMEs need refurbishing after a launch is aborted after they have fired and run for just a few seconds? SignatureAndy Clews University of Sussex IT Services (Remove DENTURES if replying by email) > > Question, are not the engines of the Space Shuttle considered large > > engines? [quoted text clipped - 3 lines] > So why is it that the SSMEs need refurbishing after a launch is aborted > after they have fired and run for just a few seconds? Operational policy. > So why is it that the SSMEs need refurbishing after a launch is aborted > after they have fired and run for just a few seconds? Got a link to support that need for refurbishment after a few seconds of operation? Mike Miller >> Question, are not the engines of the Space Shuttle considered large >> engines? It is my understanding that they are presently up to 4 >> firings before being refurbished. > > So why is it that the SSMEs need refurbishing after a launch is > aborted after they have fired and run for just a few seconds? They don't need it. They need inspection. Refurbishment is needed once every ten flights. That NASA does it more often than needed is based on conservatism and a desire to gather trend data on the engines (as implied by the recommendations of both accident review boards to treat the shuttle as an experimental vehicle). SignatureJRF Reply-to address spam-proofed - to reply by E-mail, check "Organization" (I am not assimilated) and think one step ahead of IBM. Am Wed, 30 Nov 2005 11:21:51 +0000 (UTC) schrieb "Andy Clews": >> Question, are not the engines of the Space Shuttle considered large engines? >> It is my understanding that they are presently up to 4 firings before being >> refurbished. > >So why is it that the SSMEs need refurbishing after a launch is aborted >after they have fired and run for just a few seconds? There must be a reloading of igniters and checking, WHY the abort occured - but normally no real overhaul/refurbishment at that point... cu, ZiLi aka HKZL (Heinrich Zinndorf-Linker) Signature"Abusus non tollit usum" - Latin: Abuse is no argument against proper use. mailto: heinrich@zili.de http://zili.de > This would surprise the people who built the F-1, whose > specs demanded that it be reliably capable of 20 starts > and a total run time exceeding half an hour... That is how long you can run a car engine after you have drained its oil... Winged rockets like the Shuttle and the Baikal are too heavy and too expensive. An economical rocket launcher is a wingless launcher that can survive splashdown and rough handling, and can be reused many times without expensive maintenance. If all the launcher engines are small, you do not have to test and repair them as thoroughly as the Space Shuttle Main Engine. If they are simple, pressure-fed engines, all they need is ultrasonic cleanup and new propellant filters after every flight. If one percent of your small engines fell apart, and your technicians are too drunk to replace them with new engines before the next launch -- you go ahead with the launch and replace the engines later. If your technicians are half sober while replacing the engines, they can do no harm because the engine replacement is a no brainer. If all your small, simple engines have the same design, any machine shop can make the engines in a week -- you just post their design on the web, call a few machine shops on the phone, and you will have the new engines delivered in a week. Andrew Nowicki wrote: > Small, simple engines can be made by robots, so > they can be very cheap. > So can large, simple engines. Ask the Russians, who > invested much more heavily in production automation > (and in design for easy production) than the US ever did. Yes, but it takes a small, cheap robot to make a small engine, but a big, expensive robot to make the big engine. Furthermore, it takes a big piece (stock) of aluminum to make the big engine, so you have to custom order the aluminum piece and wait until the foundry makes it. Big things are dinosaurs. > > This would surprise the people who built the F-1, whose > > specs demanded that it be reliably capable of 20 starts > > and a total run time exceeding half an hour... > > That is how long you can run a car engine after > you have drained its oil... Go on then-- try it. 30min not a chance and thats a plain car. High perfomance engines like Nascar or F1 would not even last 10's of seconds. > Big things are dinosaurs. this really small engine thing lacks substance. I have made small rocket engines for amature rockets, and guess what- its not at all easy and high perfomance is still next to imposable untill you size them up a bit. The truth is there are scaling laws, and relabity issues as well as cost issues that means there is some kind of "optimal size". Its not really really big but then its not really small either. Greg > Large rocket engines cannot be reusable because they are > damaged by large Reynolds number. Of course, considering how large some reusable engines have gotten (2 million pounds thrust), I suppose that large engines only hit that "non-reusable" level of size at some point beyond the largest engines that have seen service. Mike Miller |
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