As we know that for the upper stages of the rocket, we need high propulsion efficiency thereby high specific impulse. For that we employ a nozzle with higher expansion ratios.

Now, if we conduct the static tests of such nozzles at sea-level, it will result into an over-expanded plume.

So, how so we get the details of upper-stage engines accurately?


1 Answer 1


You need a massive facility that can maintain a near vacuum while dealing with the engine exhaust. There are (were) a couple in the US.

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(picture from this informative paper)

These facilities typically use steam ejector systems to keep the test cell pumped down in the presence of huge amounts of exhaust entering it.

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Note: these facilities typically don't claim to produce pure vacuum conditions. Commonly they advertise 100,000 ft (~30 km) altitude conditions (~ 0.2 psi or ~ 0.0014 MPa)

  • 3
    $\begingroup$ Wow, this is extreme. Are there any such sites in the former Soviet Union or in Europe? I guess there should be one in Europe, for the development of the Vinci engine. $\endgroup$ Commented Jul 29, 2019 at 10:53
  • $\begingroup$ Good question, I don't know. I would imagine the Soviets had one. They are expensive to maintain and not used a lot; reading between the lines it seems Plum Brook is falling into disrepair. $\endgroup$ Commented Jul 29, 2019 at 10:56
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    $\begingroup$ @EverydayAstronaut I would conjecture that nowadays these facilities aren't as important anymore, since there are much better fluid-dynamics simulations available. They give pretty reliable predictions of how the flow in the engine bell will behave. Real tests are still indispendable for the nitty-gritty narrow pipes and certainly for turbopumps, but those operate at such high pressure that I'd expect sea-level vs. vacuum won't make much difference. So, I wouldn't be surprised if most of the development of the Vinci is done without vacuum nozzle, and only a few such tests outsourced to the US. $\endgroup$ Commented Jul 29, 2019 at 12:26
  • $\begingroup$ The Plum Brook station is limited to engines of 10,000 lbf thrust for a few seconds. The J-2 engine of Saturn V second and third stage had a vacuum thrust of 232,250 lbf. $\endgroup$
    – Uwe
    Commented Jul 29, 2019 at 15:31
  • $\begingroup$ @Uwe when Plum Brook was fully working " Rocket engines producing up to 100,000 lbf thrust may be run for durations up to 270 seconds with the LOX/LH2 propellant combination. " As I stated in an earlier comment, I don't think the facility has been well maintained. $\endgroup$ Commented Jul 29, 2019 at 15:54

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