[AR] Re: Cubesats orbital grenades ?
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 13 Dec 2017 22:17:12 -0500 (EST)
On Wed, 13 Dec 2017, Ben Brockert wrote:
In non-dead man switch land, as discussed previously on the list, fluid
dynamics in space don't work identically to fluid dynamics at higher
pressures. So it isn't perfectly true to say that you sweep out the same
area no matter what...
Actually, it's (almost) perfectly true *because* molecular flow is so
different. To a pretty good first approximation, a molecule hitting your
drag brake comes to a stop, transferring almost all of its momentum to the
brake and dissipating most of its kinetic energy as heat, sticks for a
moment, and then wanders off at relatively low speed. Reducing the
satellite's orbital energy enough to bring it down into the fringes of the
normal atmosphere is then purely a matter of how much mass you sweep up,
and apart from air-density variation with solar activity, that's pretty
much purely a matter of how much volume you sweep out. There are all
sorts of complications on top of that, but they're pretty much all
second-order (or higher) effects.
...It also seems clear to me that hitting some other debris with
a balloon skin is much better than hitting it with a satellite even if
you are sweeping equivalent volumes, as all the secondary debris from
a balloon with come down much faster than pieces of a metal from
hitting the satellite.
Although if there's enough mass there to shatter the *other* debris, this
won't help as much as you might hope. The NASA debris-prevention
guidelines have long said "drag brakes aren't considered sufficient
*unless* you can show that they will not fragment objects they hit", but
the latter is much easier said than done. (It got looked at, a bit, for
the CanX-7 drag sails, but saying anything definitive is way too hard.)
To keep it 'inflated' even when full of little holes, cover the balloon
with UV-setting resin before packing it into its lightproof container.
And pray that it won't set up in advance due to temperature extremes etc.
during the satellite's operational life!
The solar-sail community, which absolutely needs very long, halfway stiff
and strong, very lightweight booms that can scrunch down very small for
launch, has looked at *many* ways of rigidizing inflatable structures.
All sorts of ideas, some of which I can't talk about because of NDAs. I
wouldn't say that any of them are 100% satisfactory yet.
Henry
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