The comments section of this post, while interesting, has drifted way off topic, so I thought I’d start a new, more focused one. Alfred Differ of JP Aerospace stopped by, and a few people have been asking him questions about their concept for getting to orbit with a lighter-than-air vehicle.

Paul Dietz asks:

First: what is the desired thrust/weight ratio?

Second: how fast will it be traveling in the atmosphere, and what is the L/D ratio at those speeds?

I’m not sure that either of those parameters are useful in evaluating the performance of such a beast (assuming, of course, that it will work at all). T/W is meaningless because it’s lighter than air, and similarly, L/D isn’t relevant because it’s getting its lift from buoyancy, rather than aerodynamics. The only really important characteristic, it seems to me, is thrust/drag ratio. In other words, as it goes faster and faster, can it continue to accelerate against the prevailing atmosphere at whatever altitude it is?

Intuitively, what I thought was being proposed is maintaining altitude via buoyancy, and staying high enough that the drag is small enough to allow acceleration, eventually to orbital speeds. Having given it some thought over the past week, my intuition also says that this won’t work, because I don’t see how you can displace enough air to support you while at the same time having to accelerate through it. If the idea is to just use the upper atmosphere as a starting point and getting into orbit with conventional thrusting, then the T/W does have to exceed one, at least at the beginning, but if that’s the case, I don’t understand how/why it takes five days to get to orbit.

For now, just color me confused. I hope Alfred will stop by again to elaborate, if he can.