Re: Space Access Update #112 9/19/05

Pat,

> Yup, if everything is working right, nothing will go wrong.

Did you even read what I said? The igniter either works or it doesn't.
The ignition detection system is set in such a way that while it might
give a false negative (ie the igniter lights but the ignition detection
fails to trigger), it can't give a false positive. The solenoid valve
to the actuator for the main fire valves are linked to the ignition
detection system in such
a way that there is no physical way it can be energized without the
ignition detection system triggering. Even if something goes wrong,
the worst that will happen is a no-light. Since they have an HTHL
vehicle, that either means they don't take off, or if it fails to
relight in flight, they dump LOX and come in for a glide landing.

There is a very good AIAA paper that Doug Jones wrote on the matter.
If you actually want to learn a bit about how to make reliable rocket
systems, this is a pretty good start. It's like $25 if you're not an
AIAA member, and $15 if you are. It doesn't give away all their
secrets, but it goes a long way towards explaining their system.

> I stood next to my 100 mm homemade cannon when it was fired on several
> occasions; I was dumb, and later got blown up while putting powder in my
> homemade siege mortar.

I'm sorry to hear you were that dumb. It annoys me though that you're
slandering a company that does a darned fine job with designing safe
rocket systems, especially since your slander doesn't appear to have
any basis in fact or knowledge about their system.

> They've done over 2000 firings total of all their engines without a hard
> start yet, but the question was if the Xcor engines in the Long-EZ makes
> it as safe as one powered by a Lycoming piston engine. I do not think
> that has been proven yet.

Proven as in statistically measured and demonstrated in fact? No. And
it never will be. They'll never fly the EZ Rocket the millions of
times it would take to prove that. However, the safety and reliability
they've shown with their systems shows that there's no inherent reason
why rockets have to do stupid things like blow-up. Jet engines are far
more complicated than many rocket engines, operate close to their
structural limits in many cases, and yet don't blow up on a regular
basis. The whole thought that there is something mystically magical
about rockets that makes them somehow impossible to make safe or
reliable smacks of ignorance, not engineering. Sure, it'll take a lot
of practice, many iterations, and thousands of flights until we have
rocket vehicles that are as safe as even general aviation planes, but
there is nothing physically impossible, or economically impossible
about getting up to that safety level.

> I also think that the blast containment shield is a little worrying in
> regards to their engine if it's so completely safe.

As I said, the XCOR guys are paranoid. As their safety office Randall
Clague
put it: when you want to keep your pants up, you put on suspenders.
The belt backs up the suspenders in case they come off, then you add
duct tape just to be sure. And when working with rocket engines, you
add armor. They're pretty darned sure that they have their system to
the point where it can't explode/detonate, but just in case they're
being smart and providing one more layer of safety.

> If I were going to choose an aircraft for use as an exotic engine
> testbed, it would be something more robust than a Long-EZ, and it would
> have a zero-zero ejection seat fitted to it. :-)

How exotic really is a 400lbf, 300psi pc LOX/IPA engine? I mean
really? Crud we've been doing rockets for decades now.

~Jon

.

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