On Tue, 07 Jan 2003 19:02:02 +0000, Ian Woollard
<[EMAIL PROTECTED]> wrote:
>Anyway I changed the aspect ratio to give a more realistic blunt reentry
>shape (say, 5:1 height:width) and added some reentry shielding, but now
>my wet vehicle mass has jumped by almost an order of magnitude to 28
>tonnes or so. The problem seems to be mainly the aerodynamics.
It'll get you, no question.
>The optimised trajectory is interesting though- it goes subsonic, and
>very nearly vertical up to about 30km and then separates and the second
>stage burns briefly for a lob up to about 200km and then it restarts its
>engines and does its orbital insertion burn.
That sounds darn weird. Are you getting any advantage at all from
Pythagoras? If most of your burns are either mostly horizontal or
mostly vertical, and not a combination, gravity losses are going to
eat your lunch.
>But the first stage really bugs me. I mean it works, mostly, but I'm
>taking off vertically at about 1/2 the speed of sound and I stay there
>as my fuel drops from 27 tonnes down to 4 tonnes, and I haven't even
>gone supersonic ;-(
>
>Darned air gets in the way; if I try to go faster I burn all my fuel too
>soon. If I go slower, then gravity losses do me. Argh.
Sounds to me like you've got the drag numbers wrong. They shouldn't
be so bad that you're subsonic all the way to 30 km. Caveat here: for
a sounding rocket, going straight up, that is indeed the way to go,
according to (either Dan Delong or Doug Jones, whoever worked on
Hummingbird - I really need a memory upgrade). For going to orbit,
though, you're not looking for altitude; you're looking for horizontal
speed. The only reason to go up first is to get out of the soup. So
you should be burning for some horizontal component as you're
climbing.
>It would be nice to airbreath, but that sounds rather too complex and
>lox and tanks are dirt cheap, and you need them above 10km or so anyway.
>Now John Carmack and Gary Hudson had tried a rocket powered rotor; but
>both had abandoned them. And I wasn't entirely clear why that would
>help, what's the theory?
As Gary put it a few years ago, "Rocket engineers have been fighting
the atmosphere since the days of Goddard. We're going to make it work
for us." It was interesting to watch the Roton design develop over
the years. Recently Gary confessed that the rotors were an
opportunistic design he and Bevin came up with. "We wanted to get the
engine out on booms, and spin the booms - instant pump. We had to
fair the booms. We thought, 'Why not use the booms to generate
lift?'" It increases the effective Isp by an order of magnitude, by
using the air as reaction mass.
>Anyway, woke up this morning, and suddenly spotted the theory of why the
>HMX had the rotating rocket tipped wings at takeoff . It reduces the
>effective ISP of the engine.
No, it increases the Isp, by increasing the chamber pressure. Roton
was going to use some obscenely high chamber pressure. I don't
remember how obscene, but something around 5000 psi comes to mind.
>I think HMX abandoned this idea because of weight growth didn't they?
It doesn't scale well. Given current rotor blade technology, there's
a maximum size you can make a Roton. It turns out that's too small to
confidently say you can use the technology to make a commercial SSTO.
Now, if you're flying tourists at a couple hundred pounds per, instead
of cargo at a few thousand pounds per, you might be back in the realm
of feasibility. But I suspect the failure of Rotary Rocket is going
to sour financiers on Rotons for some time to come.
-R
--
"You haven't been lost until you've been lost at Mach 3."
-- Paul Crickmore
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