[AR] Re: fatigue life (was Re: Re: SpaceX F9 Launch/Update...)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Sat, 9 Jan 2016 02:25:36 -0500 (EST)
(catching up on some old messages...)
On Fri, 1 Jan 2016, Richard Garcia wrote:
...If you're a launch vehicle startup, unless you're backed by a
dedicated billionaire, you'll probably have very modest funding for the
task at hand. The longer it takes to develop a vehicle and the longer it
takes to reach reusability, the more likely you'll run out of funding
before you reach profitability...
I agree in general, but want to nitpick :-) about one bit of wording here:
speaking of "reaching reusability" as if it's something that happens late
in development. There is a strong argument to be made that that is doing
things exactly backwards, the SpaceX example notwithstanding. A large
part of the benefit of reusability is during development, because you
(usually) get your test rocket back! There are major benefits to
"reaching reusability" right at the start, and then proceeding toward
meeting your other goals, rather than vice-versa. Not least, it lets you
do a proper test program -- dozens or even hundreds of tests *before* you
fly paying payloads -- like the aircraft people do, which is impossible
when you're throwing away costly hardware every time.
I suspect that even SpaceX wouldn't do reusability last if they were
starting over. They ended up doing things this way because their original
re-use concept was a complete failure and they had to go to Plan B.
So that's where I'm coming from when thinking about transpiration
cooling. A launch company of today needs laser like focus, and only do
R&D on what will probably be an enabling technology.
Agreed in general, with the caveat that what's an enabling technology
depends on what you are trying to enable. For example, your long-term
plans might call for truly long-lived engines that don't need a detailed
inspection after every few flights. (The fact that 20 flights looks like
a laudable goal for engine/vehicle life reflects how badly rocketry's
world view has been warped by the artillery-rocket mindset.) If so, it's
not obvious that short-lived limited-reuse engines are a worthwhile first
step toward that. Maybe they are, but maybe it's too big a detour and you
want to get serious about engine life from the start. In which case, you
could easily decide that avoiding chamber-wall low-cycle-fatigue problems
entirely is an enabling technology.
when I look at something like transpiration cooling, I see a technology
that was developed by experience people who made a genuine attempt with
significant money and manpower, but couldn't overcome the difficulties.
Here I think Richard has erred, just slightly but I think it's important.
The problem is not that the experienced people with significant resources
couldn't make transpiration cooling work, because *some of them did*, like
the P&W high-pressure-engine group. The problem is that the results from
the experienced-people-with-resources have been *inconsistent*, which says
not that it's intractable, but that it's complicated and that fiddling
with the details doesn't help if you picked the wrong basic approach.
If indefinite chamber life doesn't look important to you, then yes, this
inconsistency means it's a scary obstacle that is best avoided. If, on
the other hand, you think truly long life could be quite valuable, then
figuring out how to do it might confer a real competitive advantage, which
you throw away if you decide it's too scary.
Also there are off the shelf technology that work well enough.
For sufficiently small values of "well enough". I agree that if you want
an incremental improvement on today's launchers, you can do that with
existing technology. But if you think the real payoff comes from changing
the world, and you want to be the guy who does it, rather than one of the
guys whose business model he destroys, that might not be enough.
Henry
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