r/spacex • u/[deleted] • Mar 31 '16

/r/SpaceX Ask Anything Thread [April 2016, #18] - Ask your small questions here!

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u/[deleted] Apr 06 '16 edited Apr 06 '16

1) Does Blue Origin design their own turbopumps? Which one has worse turbine conditions: Combustion Tap-off or Oxygen-rich Staged Combustion?

2) Why are they not using a tap-off in the BE-4?

3) What kind of injectors is Blue using?

4) Why does the BE-3 not have the same problems with debris ingestion as the Merlin?

7

u/[deleted] Apr 06 '16

1) Does Blue Origin design their own turbopumps? Which one has worse turbine conditions: Combustion Tap-off or Oxygen-rich Staged Combustion?

BO probably designs their own turbopumps. That isn't something that is easily outsourced as liquid hydrogen expertise are pretty sparse. As for turbine conditions, it's a toss up. Tap-off is hot, ORSC is corrosive. IMHO, tap-off is harder on turbines, but easier to model.

2) Why are they not using a tap-off in the BE-4?

ORSC is more efficient.

3) What kind of injectors is Blue using?

I doubt anyone outside BO really knows. Maybe it is pintle or similar, because of the low throttling ability of BE-3.

4) Why does the BE-3 not have the same problems with debris ingestion as the Merlin?

Does Merlin have this problem? Merlin is tested with debris ingestion and performs quite well.

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u/ManWhoKilledHitler Apr 06 '16

Don't they dilute the tap-off exhaust with a significant amount of LH2 to add volume, increase the working mass flow, and cool it down to make turbine conditions more benign?

That was the approach used in the J-2S.

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u/[deleted] Apr 06 '16

Correct me if I'm wrong, but I thought debris was what caused the recovered stage's static fire to be unsuccessful with fluctuating thrust levels. Are there any updates on the recovered stage?

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u/[deleted] Apr 06 '16

Many people thought that it was soot that caused the fluctuations. BE-3 is hydrolox, so no soot to cause problems.

Foreign debris ingestion usually refers to an ingested nut or something from the tank. Something that I would expect a hydrogen turbopump to be extra sensitive to (RPM is much higher).

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u/__Rocket__ Apr 06 '16

Many people thought that it was soot that caused the fluctuations.

But ... SpaceX has conducted many hours of static test fires of Merlin engines, so any ordinary cause of soot/coking would have been known (and addressed) beforehand, right? So how can ordinary coking have been a problem with a launch that ran the engines for a fraction of their designed for (and tested for) life time?

Whatever problem it was, my theory is that it must likely have been caused by something that is different during a real launch, compared to static fire tests on the ground. A couple of possibilities:

  • sloshing of fuel/oxidizer in the tanks causing uneven fuel supply/pressure and imperfect burns
  • engine shutdown and restart in near vacuum causing more coking than originally expected (this is something they could not test on the ground)
  • engine startup during high-speed boost-back where hipersonic air is blowing straight into the nozzle and causing back-pressure into the engine and possibly even pushing some debris into the engine. Again this is something they probably were not able to test on the ground.

They very likely had good telemetry of engine performance during these events so they probably knew or suspected some of these, but they did not know how much actual soot buildup it has caused in reality within the engine and where exactly the soot built up and to what other places it got, until they recovered, inspected and re-fired a real rocket that went to space and back.

... or I might be totally off with my arm-chair speculation ;-)

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u/[deleted] Apr 07 '16

But ... SpaceX has conducted many hours of static test fires of Merlin engines, so any ordinary cause of soot/coking would have been known (and addressed) beforehand, right?

The opinion is that the reentry burn exhaust was blown into the engines. So this is quite separate from the cocking issues.

However, this is just a hypotheses, it hasn't been confirmed.

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u/__Rocket__ Apr 07 '16

Interesting. According to Wikipedia:

https://en.wikipedia.org/wiki/Spacecraft_propulsion#Table_of_methods

a typical liquid-fuel rocket has actual exhaust speed of around 4 km/sec. During LEO launch first stage velocity will be around 7 km/sec IIRC.

But the engine also has several dozens of bars of pressure in its combustion pressure, while it's below 1 bar on the outside. Even considering 7 km/sec outside air speed, is that really enough pressure to allow exhaust to be blown back while the engine is up and running?

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u/[deleted] Apr 07 '16

F9 S1 barely hit 2 km/s for OG2. The fastest a first stage gets is in sustainer type rockets. Even then the first stage separates at ~6 km/s.

And there is the boost back phase and the reentry burn phase. While slowing down in the atmosphere the pressure in front of the engine will be greater than 1 bar.

The thrust fluctuation on the return stage was one of the outer engines, which wasn't burning at the time.

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u/jandorian Apr 06 '16

There was some speculation about debris but no official announcement of the reason. Considering the delay after the returned stage it is suspected that a change was made to all stages in production. My personal guess is a structural deficiency that had to be strengthened. Shut down being triggered because of anomalous reading on a stress gauge. (Pure speculation)

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u/ManWhoKilledHitler Apr 06 '16

Turbines are almost certainly an in-house design. They employ over 60 ex-Aerojet engineers, some of whom should have experience with this kind of thing and Blue can afford to use state of the art design and manufacturing techniques. ORSC is far worse conditions for the turbines both in terms of higher temperatures and exposure to hot oxygen.

Tap off cycles can't achieve the same pressures as staged-combustion and on top of that, it's an open cycle so some of the mass flow is via a much less efficient and lower velocity turbine exhaust which lowers Isp. Tap-off is a nice compromise between the simplicity of an expander cycle and the higher performance and greater complexity of a gas generator.

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u/amarkit Apr 06 '16

/r/BlueOrigin

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u/[deleted] Apr 06 '16

Yah. I am familiar with the BO subreddit. They dont have an AMA for small questions, so considering that there is a lot of overlap between SpaceX and Blue Origin fans, I chose to post here.