At 12:13 AM 7/1/2002 -0700, Randall Clague wrote:
>On Sat, 15 Jun 2002 12:40:28 -0700, Randall Clague
><[EMAIL PROTECTED]> wrote:
>
> >As you've discovered, SSTO with a small vehicle requires trajectory
> >optimization, which in turn is a balancing act as you trade off drag losses
> >against gravity losses. There is probably a minimum size (for a given
> >shape, mass fraction, propellant type, engine, etc.) below which a single
> >stage vehicle can't make orbit.
>
>So I did a structural model for this really cool pressure fed TSTO.
>Nothing fancy, 6061 T6, about 1000 kg GLOW, 10 kg payload, delta-v
>9000 m/sec. You could take the thing out to the launch site in a
>small U-Haul truck, and put 22 lbs in orbit for about $1000/lb.
>Definitely within the reach of motivated amateurs, and probably
>marketable as well. Way cool.
>
>Next step, trajectory modeling.
>
>Drag *killed* me. It didn't even make a decent sounding rocket.
Of course it did. The typical scratch value for orbital delta-V is
about 10 km/s. Your best bet to salvage the design is to make some changes
to up the delta-V to a worthwhile value. Simplest is probably to switch
materials on the stages to something like 2000 or 7000* series aluminum,
one of the weldable aerospace titanium alloys, or composites. At this size,
composites may even drop the cost, due to the reduced labor and parts count
of filament-wound tanks and structures. This sounds well short of the
world's largest filament winders and autoclaves, so you don't get the
horrible price kick from needing those.
The concept is overall way cool -- here's a launcher that's small
enough to put all of vehicle, support gear, and propellant on a single
trailer small enough to tow behind my truck with room to spare. Of course,
it has ASAT weapon written all over it -- trade some of your fuel for ball
bearings and a dispersal charge...
>I'm currently doing a parametric analysis, varying thrust, propellant
>load, aspect ratio, and payload. What I've seen just from the first
>few runs is that crawling along through the troposphere at M 0.5 or so
>gets me the best final altitude. I throw away a lot of fuel "standing
>on a pillar of fire," as Mitch puts it, but the alternative is burning
>harder to get through the atmosphere quicker. This decreases my
>gravity losses at the price of increasing my drag losses even more.
>(Gravity losses are linear, but drag is proportional to the square of
>the velocity.)
Remember that fooling around with your aspect ratio also affects
your structural fraction. The higher your aspect ratio, the more material
you need to wrap around your fuel.
-p
* 7000 series aluminum is supposedly weldable with friction-stir welding.
Mars or Bust!
www.marssociety.com
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