Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Reviving the F-1 (not!)
Date: Fri, 23 Oct 1998 17:20:25 GMT

In article <70omop$7ki$1@nnrp1.dejanews.com>,
 <jakemcguire@my-dejanews.com> wrote:
>> Sure there is:  materials problems and manufacturing defects, especially
>> in the ablator.  Yes, ablative nozzles can and do fail...
>
>  Given that testing something before using it is a good idea, is there
>any particular reason that you couldn't test-fire the LRB before using it?  Do
>heat cycles wear out the ablator more than a long continuous burn?

In general, cycling *is* harder on an ablator; they work best in a steady
state, with a flow of pyrolysis gases from the lower layers cooling and
protecting the upper layers, and startup/shutdown transients are hard on
them.

However, that's not exactly what I was thinking of.  Yes, you can
test-fire an ablative engine, if you're brief about it and avoid eating
into the ablator's operational life too much.  However, this doesn't tell
you about ablator flaws in lower layers -- defects which would make
themselves felt only during prolonged firing.  In fact, those are the
real troublemakers, because defects on the surface are easy to spot.

An ablator is a miniature case of the whole expendable-hardware issue:  by
its nature, it can only be used once, so the best you can do is test some
of it (the top layer) and hope that careful manufacturing makes the test
case typical of the rest.  This is quite different from a reusable system,
where you test the actual hardware and materials that will be used in
flight.
--
Being the last man on the Moon is a |  Henry Spencer   henry@spsystems.net
very dubious honor. -- Gene Cernan  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Reviving the F-1 (not!)
Date: Fri, 23 Oct 1998 17:24:31 GMT

In article <70q539$7uh$1@inf6serv.rug.ac.be>,
Filip De Vos <fidevos@eduserv1.rug.ac.be> wrote:
>: couple of reasons.  Most obviously, it means high refurbishment costs.
>                                               ^^^^^^^^^^^^^^^^^^
>Hmmm. I recall from col. London's book (which I don't have with me at teh
>moment) that a biggish pressure-fed engine was built for less than hundred
>thousand dollars, ablative liner included. Those refurbu\ishment costs
>should not be _that high. Surely the SSMEs cost more to refurbish?

Yeah, but we're not talking about using SSMEs here. :-)  More typical
liquid engines cost very little to refurbish; DC-X's RL10s had nearly
zero maintenance during their entire career, dozens of burns.

Groups that have looked at refurbishing ablators have generally concluded
that it's a costly business.  This is one of the things that terminated
the souped-up X-15A's life early:  the ablative protective coating did not
work nearly as well as expected and was going to need replacing more often
than planned.
--
Being the last man on the Moon is a |  Henry Spencer   henry@spsystems.net
very dubious honor. -- Gene Cernan  |      (aka henry@zoo.toronto.edu)