Author Topic: It really is rocket science  (Read 22339 times)

Offline Bob B.

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Re: It really is rocket science
« Reply #60 on: March 30, 2012, 07:36:02 PM »
It's in Fortran 77.

I might be able to decipher how it works if I could view the source code; unfortunately all I have is an executable.  Does anybody have a suggestion on how I might be able to decompile it?

Offline Tanalia

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Re: It really is rocket science
« Reply #61 on: March 30, 2012, 09:33:58 PM »
Fortran is easy enough to work with or decipher, but getting the source may be difficult -- the source originally had to be purchased (a Windows executable was free), but it appears that it is no longer supported.

A newer, open-source package called Cantera may be worrh investigating.

Offline ipearse

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Re: It really is rocket science
« Reply #62 on: March 31, 2012, 05:28:54 AM »
Wow, that brings back memories. I haven't touched FORTRAN for ages... and then it was the old FORTRAN IV, we never moved on to modern stuff like 77!  ;D 

Could I ask a favour - could one of you chaps recommend a beginners guide to rocket engines, if there is such a thing? I'd love to read a bit more on this if someone can point me in the right direction.
"The Earth is the cradle of the mind, but we cannot live in the cradle forever" - Konstantin Tsiolkovski

Offline ka9q

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Re: It really is rocket science
« Reply #63 on: March 31, 2012, 06:32:26 AM »
It also (in the case of LOX/RP1 rockets, anyway) uses the stuff you already have large, mass-efficient tanks full of and which you already need to pump around
Yes, but...

Turbopumps in large rocket engines are powerful. As in far-more-powerful-than-a-speeding-locomotive powerful. A few messages back I computed a net mechanical power of 17.5 megawatts for the pumps in each F-1 engine. A typical Diesel locomotive generates only 2.2 - 2.5 megawatts. The turbopumps in the five F-1 engines on a S-IC stage required more power (87.5 MW) than all six Diesel engines on the late Costa Concordia combined (75.6 MW, total weight 1,000 tons).

Even the 100% efficient generation of that kind of power for several minutes requires a seriously large amount of fuel (and oxidizer). The less efficient the generation, the more fuel and oxidizer you'll need. Providing a separate fuel and/or oxidizer supply for the turbines might well be worthwhile from a mass standpoint if the turbine can operate more efficiently than it could by burning the main propellants.

Bob B. commented that staged cycle engines can achieve higher Isps by operating their main combustion chambers at higher pressures than gas-generator cycle engines. The F-1 engine (which uses the gas-generator cycle) would seem to bear this out; it achieves a sea level Isp of only 263 sec as compared to the RD-180, which uses a staged combustion cycle and achieves 311 sec at sea level. That's a big improvement.

Edited to add: Checking the F-1 documentation, I see that the actual turbine output power was 41 MW. I got my figure of only 17.5 MW from the actual work done by the pumps on the propellants; I knew the actual turbine power had to be considerably greater to overcome the usual losses. This further strengthens the case for more efficient turbines even at the expense of a dedicated fuel/oxidizer supply.

Edited again to add: Hey Bob, wanna figure the performance of a Saturn V in which the F-1 engines have been replaced with ten RD-180s?




« Last Edit: March 31, 2012, 06:45:23 AM by ka9q »

Offline Bob B.

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Re: It really is rocket science
« Reply #64 on: March 31, 2012, 03:10:40 PM »
Bob B. commented that staged cycle engines can achieve higher Isps by operating their main combustion chambers at higher pressures than gas-generator cycle engines. The F-1 engine (which uses the gas-generator cycle) would seem to bear this out; it achieves a sea level Isp of only 263 sec as compared to the RD-180, which uses a staged combustion cycle and achieves 311 sec at sea level. That's a big improvement.

Staged combustion engines definitely provide better performance than gas generator engines, but they're also more complex and expensive to manufacture.  When the engine gets thrown away after a few minutes of work, the cost has to be factored in.

The United States has favored cheaper gas generators, making up for the lower Isp by using high-performing LOX/LH2 in the core/upper stages.  The Soviets/Russians never mastered LOX/LH2 propulsion.  Instead they perfected staged combustion, thus getting better performance out of the engines rather than the propellant.

As far as I know, the only American staged combustion engine to reach full operational status was the SSME.  Of course the SSME was reuseable, so the extra complexity and cost was deemed worthwhile.  In expendable applications, the trend is still toward gas generators.  In fact, the philosophy behind the Rocketdyne RS-68 engine was to build an engine as cheaply as possible at the sacrifice of some performance (gas generator, ablative cooling, etc.).

Cheaper still are solid rocket motors.  I can't think of any Russian rockets that use SRMs, but they've been used widely in the USA for the last four decades.  SRMs have lower Isp than liquid propellant engines, but they can deliever the same impulse for less money.  They can also deliver very high thrusts, making them ideal for thrust augmentation at liftoff.  Strapping a couple SRMs onto a liquid core can produce a high liftoff thrust-to-weight ratio, meaning a smaller percentage of the thrust is used in counteracting gravity.  This makes the effective performance of solids a bit better than the Isp alone might imply.

Offline ka9q

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Re: It really is rocket science
« Reply #65 on: April 01, 2012, 02:49:56 PM »
Staged combustion engines definitely provide better performance than gas generator engines, but they're also more complex and expensive to manufacture.  When the engine gets thrown away after a few minutes of work, the cost has to be factored in.
I've been thinking -- instead of trying to make a vehicle like the shuttle that is completely reusable, why not make one where you retrieve and return only the expensive components like the high-performance upper stage engines, assuming you use cheap solids in the lower stages?

Somehow they would have to detach from the upper stage once in orbit, deorbit, protect themselves with a heat shield on the way down, and then land on parachutes.

I have no idea if this idea is remotely feasible, but it would be a compromise between fully expendable and fully reusable launchers, neither if which are getting any cheaper.