I understand that a shock wave cannot happen in reverse (since you don't get shock waves in nozzles), but what trips me up is this; stagnation (fluid-brought-to-a-stop) pressure decreases in the shocks of the intake. At the SAME TIME, stagnation temperature and the enthalpy (energy contained in fluid) is constant. So where is the lost pressure energy? And why does reversibility have any relation?
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asked Feb 12, 2020 at 13:08
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1 Answer
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The missing energy shows up in heat. The stagnated air is hotter than what could be expected in an inviscid, adiabatic compression. Friction in the shocks of the inlet transforms some of the kinetic energy into heat. Of course this is an irreversible process.
answered Feb 14, 2020 at 3:26
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$\begingroup$ So the energy recovered by the precooler in SABRE is this lost pressure, is it? $\endgroup$ Commented Feb 14, 2020 at 16:49
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$\begingroup$ @ABJX That precooler is meant to remove the compression heat of the air. Friction heat is only a very minor contribution to the overall heating. This cooling allows to burn that air with hydrogen at temperatures that can be tolerated by modern materials and with the combustion energy expanding the gas. Burning it without precooling would happen at a temperature where a lot of the combustion energy is wasted by ionising the combustion products, besides melting the engine. Also, to talk of recovered energy is misleading – getting rid of that heat is a major problem. $\endgroup$ Commented Feb 15, 2020 at 7:48
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$\begingroup$ Oh. Since you said that the lost pressure in the inlet shows up as heat, I thought REL had used a lightweight poor recovery inlet and used the precooler at high Mach to get that heat. After all, the precooler drives the compressor. $\endgroup$ Commented Feb 16, 2020 at 9:44
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$\begingroup$ @ABJX I don't know the details of the design, but liquid hydrogen is a very popular heat sink in cryogenic rocket engines. By heating and evaporating the hydrogen, it can drive a turbine which in turn will power the compressor. That sounds like the most likely explanation to me. $\endgroup$ Commented Feb 16, 2020 at 20:49
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$\begingroup$ Well, they use only 2 shocks in the intake, all the way to Mach 5. Preburners are used at low speed. $\endgroup$ Commented Feb 17, 2020 at 11:05