[AR] LOX/kero layout (was Re: Above 65000 ft for free)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 3 Sep 2018 23:07:27 -0400 (EDT)
On Tue, 4 Sep 2018, Peter Fairbrother wrote:
So: 1st rocket. LOX/kero, 400N turbopumped...
Which tank goes on top, LOX or kero? Does the duct from the top tank go
through the bottom tank, or outside it? Design decisions which have been
made and remade before now.
Answer to both is "it depends". :-)
Putting the LOX forward is good for aerodynamic stability -- there's more
of it and it's denser, so this moves the CM forward. But moving mass
forward means greater structural loads and hence structure mass, although
that may not matter too much at your scale. Greater LOX hydrostatic head
reduces tank pressurization needed to suppress pump cavitation, although
again probably not much at small scale. Putting the cold tank on the
bottom might require insulating its base to reduce ice formation in the
engine compartment (Delta, with LOX on bottom, had to do this; Atlas, with
LOX on top, had no insulation anywhere). Here the choice seems to turn on
details and I don't see a clear preference.
Running the duct through the bottom tank complicates design (shape of
bottom tank is more complex for pressure loads etc.), insulation against
temperature differences, and fabrication (especially if you want to make
sure it's leakproof without fussy process control). It does put the duct
in a more protected location and avoids aerodynamic asymmetry, and it may
shorten the vehicle a bit (having the duct swing off to the side and then
back to the center can require more spacing between tanks and above
engine). Here the usual winner seems to be duct outside.
For a first vehicle, there will be enough problems making it work even if
it's kept as simple as possible. That strongly favors duct outside, and
probably at least weakly favors LOX on top.
Is it ok/sensible to run detcord through/along the
duct for range safety purposes?
If you're going to have a bomb on board :-(, you want it easily
accessible, if only for disarming it when something goes wrong -- i.e.,
putting it inside plumbing is a really bad idea. Alongside, not so bad,
although if it's LOX in the duct (or in the tank right beside it), how
well does detcord handle cryo temperatures? (The answer might be "nobody
knows", in which case it's one more complication that's best avoided.)
There are major practical advantages in *not* having explosives on board.
Especially in Britain, which I gather has a long history of being somewhat
paranoid on the subject. Non-explosive range safety is highly desirable.
Not least, range-safety systems need established reliability. This is one
place where "we think it'll probably work" almost certainly will not be
acceptable; if the vehicle needs range-safety systems at all, very high
confidence in their reliability is mandatory. Which means that amateur
designs for such systems almost certainly will need full-scale live tests.
This will be noisy and expensive if explosive destruction of hardware is
involved.
Henry
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