So many interesting details even in the first post…
• Some GPS modules are crippled to not work very high and very fast so they can't be used on ballistic missiles.
• "Rise time" is an issue with balloons. You want to hurry up to your burst altitude to stay warm and not drift too far. I suppose aerodynamic shapes are out since it has to be able to expand hugely as it rises.
Yes, the rules (http://en.wikipedia.org/wiki/CoCom) dictate that the GPS should cut out if the device exceeds a speed of 1000kts AND an altitude 60,000 feet.
A lot of GPS units wrongly interpret these limits as being 1000kts OR 60,000 ft, and so aren't suitable for high-altitude ballooning.
Of course, it's a silly restriction because if you've got enough money to build a ballistic missile, you've probably got enough to build your own GPS receiver (CU Spaceflight have built a software GPS receiver themselves).
I'm glad that you enjoyed that. I was using the blog post in part to document what I had found. Hopefully, others will found the information useful.
I went to a balloon launch yesterday in Cambridge and the guys there were great. Very helpful and explained many little things that I don't have time to blog about.
So how do you keep it from bursting? Or would reading about ballasthalo explain that?
I wonder what the actual formula is to calculate the correct balloon size? It seems like it might be some complicated differential equation. Here's why:
Say I want to lift 10 lbs. So I figure out I need X volume of helium. So I need a balloon weighing 1 lb to hold the helium. Now I need a balloon big enough to lift 11 lbs instead of 10, repeat, repeat.
If by “complicated differential equation” you mean either “surface area is approximately proportional to radius squared so this could be solved by 8th graders” or “just add 20% margin and it’ll probably work perfectly well”.
We’re talking about a balloon that we want to have rise to the stratosphere in 2-3 hours. If the balloon rises at 4 meters/second, or 7, instead of 5, it’s really not the end of the world. Just pick a size, calculate (rather trivially) how fast it’s going to rise, and then change the amount up or down.
If it really must be exactly 5 meters/second (or whatever), the computation is still at about the level of a slightly-above-average 8th grader.
A 20% margin would reduce space-access costs immensely.
We use exotic alloys, explosive fuels and precision machining because we need to reduce weight to the structural limits. If we hadn't, space access would be cheaper and safer.
Sadly, Earth has just enough gravity to ruin our day.
I have no experience with this stuff, but: instead of a plastic case, could you carve out 2 blocks of polystyrene foam to exactly contain the equipment, with more temperature-sensitive stuff nearer the middle? It's a good thermal insulator and extremely light.
Or will polystyrene burst/disintegrate at such low pressures?
In any case, this sounds like a fantastic project, best of luck!
• Some GPS modules are crippled to not work very high and very fast so they can't be used on ballistic missiles.
• "Rise time" is an issue with balloons. You want to hurry up to your burst altitude to stay warm and not drift too far. I suppose aerodynamic shapes are out since it has to be able to expand hugely as it rises.
… ah, details.