On 2016-06-21 06:09, Jeff Findley wrote:
> It should. There is nothing fundamental about a liquid fueled rocket
> stage from an orbital launch vehicle that prevents reuse. It's just
> that no one has *tried* all that hard to reuse one, until now.
And there is nothing fundamental about jet engines to prevent reuse. And
this is done every day by jetliners.
HOWEVER, the airlines and FAA learned via experience that jet engines
can only run so many hours before needing certain level of inspection,
and after another set of hours, require a heavy maintenance check.
Oh, and after a "hard" landing, the aircraft needs a maintenance check.
So SpaceX will learn via experience how many times a stage can be
reflown with just a flashlight inpection, how hard a landing the stage
can take without needing more serious inspection and after how many
flights is a major inspection required (and whether such major
maintenance is economic or not).
When you look at the Orbital ATK fireworks at Wallops, didn't they run
the engines to test them before ? All seemed fine and yet, engines failed.
Doing a flight test (including landing) of engines 10 times is cool. But
you really need to test until they fail. There is a big difference if
they fail on the 11th simulation vs on the 25th simulation. (on the
25th, it means that when you fly the 10th flight, the engines are still
far from failure).
> fired on the test stand to test and qualify them. There is no
> fundamental problem here that needs solving.
It is a question of validating the predictions. They've weeded out the
problems they know about. They haven't weeded out problems they don't
know about YET.
It could very well be that they can do 25 flights instead of 10. Or it
may be just 5. It could very well be that a major inspection can extend
the life of stage by X more flights, but still remains to be seen if
that major inspection is economically sound or if building a new one
ends up cheaper.
Building new ones may present advantages if there are continuing
improvements to the rocket being made, at which point, the older rockets
without improvements are less attractive. There are a lot of variables
in this.
> What "new stuff"? Visual inspections? Validation firings on a test
> stand? What do you expect them to do between flights?
That is the question. How do you validate a rocket for reflight to
ensure nothing untoward happened during last flight. This is what SpaceX
needs to develop. This is more about procedures than engineering.
> Possibly, but I doubt it since that's what the crushable inner core is
> supposed to do. It wasn't unexpected at all.
If a leg "collapsed" because of hard landing, it still means it was a
hard landing. If the collapsible core was fully compressed, it means
additional un-absorbed G forces would have been transmitted to the
stage. Just just stating that because the gear absorbed some of the
excess G force doesn't mean that it absorbed all of it.
> Certainly, but if there had been a significant problem during that main
> burn, there would not have been enough fuel left for a landing in the
> first place.
Not talking about significant problem. Talking about some minor problem
that might have been significant. Consider Challenger. Imagine a
scenario where video showed SRB exhaust coming out of a o-ring but
directed away from ET. The flight might have gone on normally, but video
would have shown a problem that needs correcting.
You can have a jetliner taking off normally, but plume of smoke when
engine starts indicates that the engine should get a check.
> Again, possibly. But, pressurized tanks, which make up most of the
> structure of the stage, are really freaking strong.
How pressurized are they at landing ?
> As long as they
> don't rupture, the stage as a whole is very likely o.k. I'm sure SpaceX
> has done analysis and possibly even ground testing for this case
Yes. And they may be confident that the rocket can do X flights. The
issue here is problems that you have not foreseen.
> Somehow, the launch vehicle guys in both the US and U.S.S.R. got a free
> pass to ignore all of that experience with rocket engines and flight
> vehicles and clung to the mistaken belief that launch vehicles must be
> expendable simply because they started their engineering work with
> missiles instead of aircraft. Madness. Simply madness.
In a race to space, you focus to get there first, no matter how much it
costs. You don't have time to develop a landing system. And did
computers and sensors of the 1960s have the power to do a controlled
landing of a rocket ?
Also, in the USA, I somehow suspect that rocket manufacturers lobbied
government hard on the fact that re-using rockets wasn't safe.
As long as US manufacturers had no competition, there was little
incentive to add re-usability to a rocket.
When Soyuz became available commercially and started to compete in the
launch market with lower prices, the US rocket makers didn't really
react, and this left a big spot for SpaceX to come in. And one way to
compete against Soyuz is to look are re-using first stage.
Out of curiosity:
does Falcon9 have simular ratio of delta-V given by first vs second
stage compared to other non-reusable rockets ?
Say I have a rockets whose first stage goes much much further and
provides much greater delta-V, its re-entry would be more problematic
than the Falcon 9.