Discussion Between Henry Vanderbilt
and Henry Spencer
11/12/93
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: source of problem in recent DC-X flight
Date: Tue, 12 Oct 1993 19:18:16 GMT
In article <1993Oct12.122833.16758@iti.org> aws@iti.org (Allen W. Sherzer) writes:
>>And what causes a helium bubble (presumeably from the
>>tank pressurization) to get into the engines?
>
>I'm speculating but perhaps it was helium from the last fuel system purge?
>Maybe some helium from the pressurization system disolved in the fuel and
>came out while the fuel was being pre-heated?
The story, courtesy of Henry Vanderbilt and the SAS updates over in
sci.space.news, is that a helium bubbling system didn't shut off as
it was supposed to.
HV wasn't sure what it was there for, but I think I know. It's normal to
take measures to ensure that cryogenic fuels circulate through rocket
plumbing, to avoid boiling and gas bubbles in warm areas (in particular,
the pipes are typically warmer than the tanks because of greater surface
area) and resulting annoyances like violent burping back into the tanks.
One simple way of doing this is just to bubble a small flow of helium
through the system from the lowest point: the stream of small helium
bubbles carries liquid with it, stirring things up and providing some
circulation to even out temperatures.
--
One flight test is worth | Henry Spencer @ U of Toronto Zoology
a thousand simulations. | henry@zoo.toronto.edu utzoo!henry
From: hvanderbilt@BIX.com (hvanderbilt on BIX)
Subject: Re: Space Access Update #24 10/11/93
Date: 12 Oct 93 19:23:37 GMT
A bit of followup here. Henry Spencer informs me that helium pressurant
to liquid hydrogen tanks is typically injected at the lowest point in the
system that contains liquid hydrogen when the engines aren't running. This
keeps the liquid hydrogen stirred up as the helium bubbles rise through it,
preventing "warm" spots from developing near the engine feed pipes, with
consequent risk of hydrogen bubbles in the fuel lines when the engines start.
DC-X's helium feed entering the LH2 tank at the bottom would thus seem to
be pretty much standard practice, a point I'd wondered about.
Laern something new every day, I say.
Henry Vanderbilt hvanderbilt@bix.com
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Space Access Update #24 10/11/93
Date: Tue, 12 Oct 1993 21:01:51 GMT
In article hvanderbilt@BIX.com (hvanderbilt on
BIX) writes:
>A bit of followup here. Henry Spencer informs me that helium pressurant
>to liquid hydrogen tanks is typically injected at the lowest point in the
>system that contains liquid hydrogen when the engines aren't running...
Well, not quite what I said (in private mail), although my wording may
have lent itself to that interpretation. The bulk of the pressurizing
gas is probably going to be injected at the obvious place: the top
of the tank. However, a bit of helium bubbling from the bottom is one
way of improving circulation and avoiding warm-spot problems while you're
waiting for engine start. There are other ways; for example, you can
bleed off a bit of fuel from the lowest point, and either dump it
overboard or put it back into the tank (the latter being what the
shuttle does, as I recall). The problem is usually warm spots in the
plumbing below the tank, rather than in the tank itself.
--
One flight test is worth | Henry Spencer @ U of Toronto Zoology
a thousand simulations. | henry@zoo.toronto.edu utzoo!henry