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The Ars Technica articles

both include this "file photo" Tweeted by Elon Musk 2:27 PM - 18 Jun 2012:

Just fired our Superdraco escape rocket engine at full thrust! Needed to carry astronauts on Dragon

Why does the exhaust look so under-expanded? It looks like it doubles in diameter within a distance of only one diameter, and soon thereafter to a factor of 3 of it's exit diameter.

Doesn't that result in a substantial loss of thrust available to the spacecraft?

below: found in NASA Spaceflight Forum

Superdraco first firing twetted

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    $\begingroup$ It is not uncommon to test rocket engines without the full nozzle installed. Don't know for certain if that's the case here though. $\endgroup$
    – Hobbes
    Commented May 3, 2019 at 9:00
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    $\begingroup$ @JanDoggen slide 17 ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140002716.pdf I'm pretty sure the usage is correct. Also see aerospaceweb.org/question/propulsion/q0220.shtml $\endgroup$
    – uhoh
    Commented May 3, 2019 at 12:05
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    $\begingroup$ Underexpanded is correct. Since the nozzle is too short, the exit plane pressure has not dropped enough (the flow has not expanded enough) to match ambient pressure, so the plume has to expand to do so after exiting the nozzle. $\endgroup$
    – Organic Marble
    Commented May 3, 2019 at 12:30
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    $\begingroup$ "Twetted" is a great typo $\endgroup$
    – ANone
    Commented May 3, 2019 at 14:48
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    $\begingroup$ Related: TR-106 underexpansion in sea level test $\endgroup$
    – Russell Borogove
    Commented May 3, 2019 at 17:24

2 Answers 2

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In this photo, the exhaust doesn't expand so much. The nozzle looks to be a bit longer. So they've played with various nozzles.

Superdraco engine test with a nozzle (cropped and adjusted for visibility)

Superdraco engine test with a nozzle

And the Ars Technica article has this mosaic, which shows variations in the exhaust (maybe due to throttling?). It also shows how little room there is. The exhaust impinges on the shielding. This seems to be from 2015. The second image in the mosaic shows an exhaust flow that matches the image above pretty well.

Superdraco engine tests

So it looks like they switched to a slightly longer nozzle later on in the development.

There were more changes during development: in 2013, they switched to a 3D printed combustion chamber.

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    $\begingroup$ @uhoh the heat shield looks pretty flat but it's certainly expending in one axis. Could It be that they use the heatshield as something near a linear aerospike? $\endgroup$
    – Christoph
    Commented May 3, 2019 at 11:33
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    $\begingroup$ @Christoph that's an interesting idea, you can consider asking it as a new question if you like. $\endgroup$
    – uhoh
    Commented May 3, 2019 at 11:34
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    $\begingroup$ I'm pretty sure this is a test-only thing rather than a change in the nozzle design during development. $\endgroup$
    – Russell Borogove
    Commented May 3, 2019 at 17:25
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I think its clear the lack of a nozzle is the cause. As to the "why" I can think of a few reasons that might be partly responsible (other than it's just testing):

  • They are not designed to be used for long durations. Mass and TWR are more important relative to ISP than for 'main' engines.

  • They need to tuck in for minimum impact on the aero-dynamic profile of the capsule.

  • Combustion stability is likely very important and needs to be achieved in a short time. Under expanded nozzles are generally more stable.

  • There may (as pointed out) be some nozzling effect from the heat shield and the engine acting in pairs.

All that said, the photo posted does look very under-expanded. More than I would have predicted (but take that for the very little it's worth).

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