Scaling up rocket engines seeems to be difficult/costly. I'm not talking about changing the cycle or increasing chamber pressure, only about increasing thrust. I wonder why this is the case and to what extent higher thrust engines are more expensive in development as well as in production.
I've been thinking, about using different propellant for the TP of the GG (like the RD-107/8), but on the expander cycle case. If you were to do a bleed expander rocket, wouldn't using He to expand and move the TP give better performance? The Heat of vaporization of H2 is 0.904kJ·mol−1, while the He's is 0.0829 kJ·mol−1. If I understand that right, it would allow for a lot extra thrust for a given heat output.
Quote from: baldusi on 08/02/2013 04:09 pmI've been thinking, about using different propellant for the TP of the GG (like the RD-107/8), but on the expander cycle case. If you were to do a bleed expander rocket, wouldn't using He to expand and move the TP give better performance? The Heat of vaporization of H2 is 0.904kJ·mol−1, while the He's is 0.0829 kJ·mol−1. If I understand that right, it would allow for a lot extra thrust for a given heat output.Strangequark has some nice relevant posts on expander/cooling issueshttp://forum.nasaspaceflight.com/index.php?topic=16279.msg986192#msg986192http://forum.nasaspaceflight.com/index.php?topic=30910.msg1006206#msg1006206http://forum.nasaspaceflight.com/index.php?topic=31040.msg1009889#msg1009889In short, specific heat capacity (Cp) matters more and hydrogen is in it's own league. Also remember the He is getting more and more expensive, also difficult to store efficiently (AIUI rockets don't store it liquefied but compressed and chilled in supercritical form).
Hello I am wondering the following:Does the difference in Isp for an engine under vacuum conditions and under sea level conditions solely arise from the pressure related term (Pe-Pa)*Ae?If so, does that mean that the mass flow of an engine is constant throughout flight? Just to refresh my memory.Regards
What rocket engines using hydrogen peroxide/kerosene are there in current service? How good would they perform as a restartable upper stage engine?(I'm asking because China is apparently making a new multi-restartable upper stage based on a 35 kN thrust H2O2/RP-1 closed cycle engine and plans to put it into service next year)
Am I right in my understanding that in the expander cycle rockets is purely liquid? In other words, the expander part works by upping the pressure of the liquid but avoiding the phase change? Thus, the injector is liquid-liquid (as the Russians say)?
Quote from: baldusi on 09/05/2013 01:30 pmAm I right in my understanding that in the expander cycle rockets is purely liquid? In other words, the expander part works by upping the pressure of the liquid but avoiding the phase change? Thus, the injector is liquid-liquid (as the Russians say)?I'm stretching a bit out of my expertise, but I believe expanders typically run supercritical. In short, once it gets to the injector, the fuel will be a "gas", but the process to get there does not involve a sharp phase change (like boiling). As for the pressure, it will actually drop through the cooling jacket. The energy increase is expressed in the temperature of the fluid, not the pressure.
I forgot, you have pressure drop and temperature goes up. Silly me. So, if critical point of H2 is (33K,1.3MPa), as long as you keep it above 1.3MPa, you don't have nasty things like bubbles/cavitation, right? My question is, then, by the time it arrives at the turbine, how's energy transferred? I can understand that a gas expands and since the other side is incompressible you trade movement (which in a gas is temperature or pressure?) for potential energy (higher pressure) on the pump side. In the case of super critical H2, does it behave like a gas by the time it reaches the turbine?
Does anybody know how much the ISP of a rocket engine changes throughout its atmospheric flight? Is there any quick way to calculate this? For example, if a rocket engine is sea-level optimized, what would how much would its ISP change at 10 km, 25 km, etc.? Is there a table for this?
If the other two engine families where not also fist stage engines, I would say it had something to do with a sealevel optimized engine, but that is not the case here.
Vulcain 1/2 and LE-7/A are, basically, sustainer engines. When you use a booster augmented h2/lox core (like Ariane 5, H-II or STS did), you want to optimize for vacuum performance. When you need an engine to propel the whole stack alone (like RS-68 might do) you want to focus more on sea level performance.