As I sat with my colleagues at one of the outdoor bars at our hotel in Cancun last month, I had an idea for a method of getting into space cheaply. Since this is a problem of enormous general interest and commercial importance, I was sure it must have been thought of before, but when I checked Wikipedia’s non-rocket spacelaunch page (highly recommended, by the way!), I didn’t see my idea listed. So could it possibly be novel after all?
[By happy coincidence, it’s only a few days ago that the world’s first Sushi In Space video was released. Why? I couldn’t tell you. But that’s where this image is taken from.]
As in the sushi example above, balloons have often been used to lift light objects to the edge of space. They have even been used as a launch platform for light rockets — the combination is called a rockoon. The problem with balloons is that as they climb higher, the pressure of the atmosphere decreases, so the gas density difference inside and outside the balloon decreases, and so does its lifting power.
My idea is a progressively evacuated rigid balloon (which we can conveniently abbreviate PERB). I think it can be used to get easily up to — well, I don’t know what height, that would depend on material properties — but high enough to make it a reasonable launch platform for smaller rockets. (Many of the other non-rocket spacelaunch techniques also do not aim to get into orbit as the space elevator does, but just to make a rocket’s task easier by starting from a point some way up through the atmosphere.)
The PERB is simply a large, rigid shell strong enough to withstand a pressure differential of, say, 0.1 atmospheres without imploding. You use a vacuum pump to evacuate air until the pressure inside is down to 0.9 atmospheres. If your PERB is sufficiently voluminous, it will now be floating: what’s needed of course is a volume such that one tenth the mass of contained air at one atmosphere is greater than the mass of the PERB itself: the shell, the pump mechanism and any payload.
The beauty of the PERB is this: if you can make it take off at all, then you can take it higher and higher very easily: you simply keep evacuating the shell to a pressure that is 0.1 atmospheres lower than the surrounding air, and let it float higher and higher. (I am neglecting some details, of course. For example, you’d want to tether it to the ground, and the mass of the tether itself will grow linearly with altitude. I am guessing this needn’t be a killer.)
What is the limit? I suppose the PERB can keep rising while there is enough atmosphere to give a pressure differential capable of lifting it. You’d end up by fully evacuating the shell, at a point where the atmospheric density is something less than a tenth of an atmosphere. From there, you launch your rocket into orbit by conventional means. (Notice that it’s pleasantly simple to bring your PERB back down to ground level: you merely bleed air slowly into the shell.)
If this can be made to work at all, it seems to have many important advantages.
First, the launch cost would seem to be negligible, on a par with running a vacuum cleaner: all the significant cost would be in manufacture of the light, rigid shell. Presumably your tether would double as an electrical supply, so you don’t need to carry any fuel.
Second, simplicity. The PERB has almost no moving parts — only the pump — and (fingers crossed) seems pretty much foolproof.
Third, safety: because everything happens so slowly, if things do go wrong there should be time to do something about it. Provided the shell doesn’t fail catastrophically by buckling — which should be easy to avoid just by evacuating slowly enough to allow the PERB to climb — if a leak develops, it should only cause the PERB to descend slowly.
Fourth, reusability. I don’t see any reason why a PERB should not be used many times.
The big one: do we have a material that the shell can be made from? It needs to be strong, light, and impermeable to air (or nearly so — very slow leakage shouldn’t be a problem, as the vacuum pump will easily overcome it). Here, I am lost. I know almost nothing about material properties. Anyone? Carbon nanotubes?
The PERB would likely be very vulnerable to bad weather. Wind is the obvious hazard — it could conceivably break the tether; it could throw the PERB upwards or downwards too fast for the pump to compensate for pressure changes, causing the shell the implode or explode; it could throw solid objects against the shell causing a local failure. Perhaps even the pressure of rain could be enough to defeat the lifting power, if it acts over a wide enough area (although this only applies at low altitudes).
Would it need to operate at, or near, the equator? I am not clear on why, exactly, but I seem to recall that the space elevator can only be built at the equator for reasons that elude me (and which the Wikipedia article is strangely silent about).
Have I missed any?
Obviously some calculations need to be done here. The key one is this: can we make a shell voluminous and strong enough enough that it can sustain a pressure differential great enough to counteract its mass? And how much additional mass can it carry while maintaining a healthy safety factor? Does anyone out there have the relevant background to work through this with me?
Here is the easy part. Consider a PERB with a shell of mass Ms and volume V, capable of withstanding a density differential of P atmospheres. The mechanism (vacuum pump and associated stuff) has mass Mm and we want to lift a payload of mass Mp. The density of air at one atmosphere is 1.2 kg/m^3, so the difference in mass between the enclosed partially evacuated volume and the equivalent volume of air is 1.2 P V. That buoyancy has to overcome the mass of the shell, mechanism and payload, so we have lift-off when 1.2 P V > Ms + Mm + Mp.
The hard part is calculating the strength of a shell of mass Ms and volume V made from some specific material in some specific shape. Is it capable of sustaining a pressure difference of P atmospheres, where P = (Ms + Mm +Mp)/1.2 V? If so, we are good to go. Does anyone have the background to comment on shell strength for various materials?
The big question: why has no-one done it before? Surely that can’t be the case? Does anyone out there know of any prior art? Has the idea been proposed, shown to be stupid, and discarded? (If so, it should at least be mentioned in the Wikipedia article.) If not — is it stupid anyway, for a reason that I’ve missed?