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Comment by termain | Hacker News Reader

"The most stupid thing they've done is stay with an experimental rubber/nitrous hybrid engine this whole time, instead of switching to liquid/liquid engine, a well proven technology."

1. They didn't stay with rubber/nitrous.

2. Like the sort used on Antares?

"Seeing only the first paragraph, I already know that some rubber in the hybrid engine detached blocking the nozzle and causing the whole engine to explode."

But what you know is wrong, they weren't using rubber.

I think a good case can be made that they didn't do enough ground testing, but your post mixing disdain and falsity doesn't advance that case.

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Point 1 is a good one, point 2 is not. Yes, liquid engines like the sort used on Antares, Soyuz (most reliable rocket ever built), Falcon, Proton, Atlas, Shuttle, Delta, Ariane, Saturn, and nearly every other orbital launch platform ever. Liquid/liquid engines, by any measure, are orders of magnitude more stable and well proven than hybrid rocket engines.

My point is that they still fail. Both liquid and solid boosters are a lot more mature than hybrids, no doubt. But I'm not so sure that either is fundamentally more reliable. And for hybrids to mature, somebody has to use them.

"Liquid/liquid engines, by any measure, are orders of magnitude more stable."

What do you mean by stable?

A reason to go with hybrids, at least on paper, is that they are mechanically simple but throttlable, hopefully combining the simplicity of a solid booster with the ability to throttle and shutdown of a liquid.

Of course, much of that also applies to blowdown liquid systems, and I personally find a blowdown methane/lox system particularly compelling. I figure Virgin wanted to avoid cryogens, though.

"2. Like the sort used on Antares?"

I think Paul McCartney was about 23 years old when the engines powering that Antares launch vehicle were fabricated in Russia in their race against the Americans to the moon. An antique liquid/liquid vs an inadequately qualified hybrid: no winners.

I think they're a little newer than that, but that's a good point. Still, I'm not sure either engine was inadequately qualified.

I'm not sure what constitutes adequate qualification, though. Both engines were ground tested at least some. And in Virgin's case, this flight was one of tests.

> I think they're a little newer than that, but that's a good point.

http://en.wikipedia.org/wiki/NK-33 says the AJ-26 engines are from the 1960s and 1970s.

"1. They didn't stay with rubber/nitrous."

This you believe so strongly because a news story stated it?

Wait for the accident report. It will say "rubber".

Aviation Week reported it back in May. So, yes. I'll keep an eye out for the accident report.

http://m.aviationweek.com/space/new-fuel-boost-spaceshiptwo

Well you could be right, but it is very hard to imagine that being true since the plastic hybrid has only been tested once on the ground. Doing a flight test so soon would be totally insane. In any case, the disaster was undoubtably a result of inadequate ground testing.

Problems like these are almost always the result of management pushing things faster than the engineering progress warrants.

I think it's also possible that a bad fuel grain slipped through whatever QA process they had. You might count that as insufficient ground testing, though.

"Problems like these are almost always the result of management pushing things faster than the engineering progress warrants."

Coincidentally, we were talking about such an instance today at work, the Conestoga 1620. As relayed to me, the "low frequency noise" mentioned in the wikipedia post was a low frequency structural mode that was missed because management didn't want to pay for a high enough fidelity finite element analysis.

https://en.wikipedia.org/wiki/Conestoga_%28rocket%29

Virgin has to get Brandon to space this year or their funding is subject to considerable clawback.

Do they still get to keep the funding if he returns?

You don't know what the problem was.

Officials decided this spring to modify the space plane’s rocket motor to use a different type of fuel. Instead of burning a rubber-based fuel mix known as HTPB used on SpaceShipOne and SpaceShipTwo’s initial test flights, Virgin Galactic selected a new propellant grain known as polyamide which officials said offered improved performance.

“This was a new fuel formulation that had been proven and tested on the ground many times,” Mickey said in a press conference.

http://spaceflightnow.com/2014/10/31/virgin-galactics-spaces...

Let us hope for Virgin's sake that "many times" doesn't mean six times for ten seconds each.

I have heard of one long duration test on the plastic fuel reported by the "Virgin watchers" in Mojave, so unless they are doing long duration tests somewhere else it doesn't seem like much. Rocket engine testing is extremely noisy so long duration tests are quite noticeable.

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