John Carmack wrote:
I thought they weighed in at around $20 k; didn't Johns cost that? I think that's fairly expensive. Are there cheaper ones? I was wondering about how to do guidance for an orbital capable vehicle I'm thinking about. Probably need under a degree accuracy over 5 minutes.I don't really see the problem with FOGs. They're not particularly expensive when you get anywhere above the scale we're working at now,
The cheap KVH FOG gyros were $1.5k per axis. The fully-integrated-and-compensated, digital output, 6DOF Crossbow FOG unit was $9,500, but their new-and-improved model just bumped the price to $11,500.
Not quite as bad then. That's the way I would go if I had the money.
The non-fog crossbow 6DOF IMUs were $3995 for the 400 series, and $2995 for the 300 series.My limited understanding of them suggests that the accuracy is largely just clever filtering, and that a bit better than 1 degree per second is all they can really achieve currently. The crossbow units appear to be correlating multiple sensors (gravity and magnetic, and the attitude sensors) to zero the drift out, which would work great for aviation, most of the time, although even then, they seem to lack FAA approval, which is suspicious. If I'm right, you'd want to steer clear for your application- since there's no direct way to measure gravity once a rocket has left the ground.
"solid state" gyros should perform much better than the 1 deg/sec that tuning-fork style gyros like the gyration parts get. I am certainly interested in hearing results from work with them.
Probably, possibly, maybe. I'm currently looking at how small a vehicle can be made, with a payload and still make orbit and hopefully come back. $10k is actually significant, and I may lose a few... The guidance looks to be the most expensive subsystem at the moment, but that probably just means I haven't understood my problems yet.I believe they have a drift around 1 degree/second, great if you have GPS to compare against, but GPS capable of orbital velocity cost $10k or so?and for our current scale, decent solid state gyros should work fine ($50-$150 range).
When you have a vehicle capable of orbital velocity, $10k won't be a problem.
In any case, GPS by itself doesn't let you re-sync your gyros, you need a multi-antenna GPS attitude sensing system, not just a position output GPS.During a burn the GPS track acceleration is going to be the same as the attitude, pretty much; atleast when outside the atmosphere. Within the atmosphere it's not quite as simple, but it's earlier in the flight, so the error is smaller, and my vehicle may be nose heavy which helps keep the track and the attitude aligned.
This trick doesn't get you roll, but a sun sensor can reset the roll gyros.
John Carmack
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