[AR] Re: What makes a useful launch vehicle?
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: arocket <arocket@xxxxxxxxxxxxx>
- Date: Mon, 11 Mar 2019 17:58:05 -0400 (EDT)
On Sat, 9 Mar 2019, David Summers wrote:
Any connectivity at all with the payload increases the complexity and
"things that can go wrong" count. It is required if you are sharing a
ride, because the primary doesn't want you messing something up...
Actually, no, the primary *doesn't* want your spacecraft live for exactly
that reason -- they don't trust your software to keep transmitters off,
thruster valves closed, etc. So it's normal for small cheap secondaries,
e.g. 3U cubesats, to go up not only completely unconnected to the rocket,
but also unconnected even to their own power system until a separation
switch closes.
(Separation switches can be mechanical switches held open by contact with
part of the deployer, or reed switches held open by small permanent
magnets in the deployer. The key thing is that their proper functioning
does not depend on your competence at designing circuits and/or software,
which the primary's owner has no confidence in.)
Cubesat *deployers* typically need an electrical connection for a "go"
signal supplied by the rocket's control system, and some also have a
sensing switch or two (e.g. "door open") whose state they'd like to have
included in the rocket's telemetry. The ones I'm familiar with supply
their own power to operate their mechanisms, because there's too much
diversity in the nature of the "go" signal to rely on it to provide power.
hoping that if you can just leave the sat on for the ride, and the
launch process is really fast, there is no reason to electrically
connect the satellite.
The only electrical favor cubesat owners want is a chance to top up the
charge on the spacecraft batteries, and the deployer batteries if it has
them, not too many days before launch. (Well, they might *want* more than
that, but they're accustomed to not getting it! :-) )
Henry
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