My understanding is that the H2 in an expander-cycle engine is vaporized by the heat from the combustion chamber and nozzle wall. This is used to power a turbine and then get injected, in gas phase, into the combustion chamber.

My question is how startup and the injector design is affected by the engine not being hot enough during startup to vaporize the fuel.

Are the injectors just designed to be able to inject liquid with enough efficiency to get the engine hot?

Is the chamber pre-heated somehow? My understanding is that rocket engines are usually pre-COOLED before ignition.


1 Answer 1


The classic expander cycle engine, the RL-10 starts as follows:

The RL-10 engine starts by using the pressure difference between the fuel tank and the nozzle exit (upper atmospheric pressure), and the ambient heat stored in the metal of the cooling jacket walls. The engine “bootstraps” to full-thrust within two seconds after ignition.

enter image description here

A typical plot of the valve movement during engine start is shown in Figure 2. To initiate start, the FSOV is opened and the fuel-pump discharge cool-down valve (FCV-2) is closed.The interstage cool-down valve (FCV-1) remains partially open in order to avoid stalling of the fuel pump during engine acceleration. The pressure drop between the fuel inlet and the combustion chamber drives fuel through the cooling jacket picking up heat from the warm metal. This pressure difference also drives the heated fluid through the turbine, starting rotation of the pumps, which drive more propellant into the system. At start, the OCV also closes partially, restricting the flow of oxygen into the combustion chamber. This is done to limit chamber pressure and ensure a forward pressure difference across the fuel turbine after ignition of the thrust chamber.

As the turbopumps accelerate, engine pneumatic pressure is used to close the interstage cool-down valve completely and open the OCV at pre-set fuel and LOX pump discharge pressures. The OCV typically opens very quickly and the resultant flood of oxygen into the combustion chamber causes a sharp increase in system pressures. During this period of fast pressure rise, the thrust control valve (TCV) is opened, regulated by a pneumatic lead-lag circuit to control thrust over-shoot. The engine then settles to its normal steady-state operating point.

enter image description here


  • $\begingroup$ Thank you for the explanation. I need some clarification. The body of the engine is at ambient temperature (20°C+ probably) before startup? This temperature is enough to vaporize the hydrogen? I assume the thermal mass of the engine is sufficient to sustain vaporization for the duration before ignition? $\endgroup$ Commented Oct 24, 2019 at 13:15
  • $\begingroup$ That's a lot hotter than liquid hydrogen. $\endgroup$ Commented Oct 24, 2019 at 13:15
  • $\begingroup$ sorry, I hit enter before I was finished. Yeah, I was just wondering because hydrogen has a huge specific heat. I assumed it would chill the metal fast enough that vaporization would be negligible. I guess my hunch was wrong. $\endgroup$ Commented Oct 24, 2019 at 13:20
  • $\begingroup$ Also, at what point in the graph you provided is ignition occurring? I'm assuming around T+1.6 to T+1.7? $\endgroup$ Commented Oct 24, 2019 at 13:24
  • $\begingroup$ It only takes 2 seconds to start up, after that it's generating plenty of heat. $\endgroup$ Commented Oct 24, 2019 at 13:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.