The Space Show - John Powell
I made a brief call in to The Space Show with Dr. David Livingston on Tuesday night. The show's guest was John Powell of JP Aerospace. I almost missed the show, but managed to catch the last half hour. I plan on going back and listening to the show in its entirety when I have a couple of hours to spare.
I have been following the progress of JP Aerospace for about ten years now, and I have to say they have come up with some very imaginative concepts that will be amazing technological feats if they ever come to pass. For example, their stratostation concept involves lofting a huge lighter-than-air platform to about 140 thousand feet. This station will serve as a near-Earth observation platform, as well as a possible communications link, an astronomical observatory, a scientific laboratory, and probably dozens of other uses which haven't even been thought of yet.
One proposed use for this station is a point of departure - a harbor if you will - for another one of their concepts, the airship-to-orbit. This is an enormous V-shaped lighter-than-aircraft which might be able to reach orbit by gradually accelerating to orbital speeds over several days. Using primarily buoyancy and aerodynamic lift to overcome gravity losses, this mode of reaching low Earth orbit could have a profound impact on how we think about getting off this planet - at least as much as the space elevator, and potentially much sooner.
My question had to do with the propulsion systems on the proposed airship-to-orbit concept. I've been thinking about this problem for a while. The concept is simple, but the physics are hard. Let me try to explain.
In order to accelerate from approximately rest with respect to the Earth's surface (and about 100 thousand feet up), the airship must accelerate to orbital velocities, which are on the order of 7-8 km/s, and orbital altitudes which are way above 330 thousand feet (~100 km) that marks the semi-official boundary of space.
There are several physical constraints which must be overcome when trying to reach orbit with an airship. First, it must have sufficient thrust to overcome atmospheric drag during the early part of its flight. If you can't overcome the drag, then you can't accelerate. The drag increases with velocity, linearly at first, but later exponentially. Once the thrust and drag are equal, you've reached a sort of terminal velocity. The only option is to fly higher where the density of the air is less, and therefore the atmospheric drag is less.
Second, until the airship reaches orbital velocity, there must be some way of providing lift to overcome the gravity losses. In other words, to keep the ship from losing altitude. During the early part of the flight, this is mostly accomplished with buoyancy and also any aerodynamic lift which might be generated by the airship (which looks like two huge wings stuck together in a V-shape). However, as the airship climbs higher - to get out of the denser air dragging on the ship - the effects of buoyancy and lift are diminished. Mr. Powell confirmed for me that above 300 thousand feet, buoyancy and lift effects are practically negligible.
Unfortunately, by the time the ship reaches 300 thousand feet, orbital velocity has not yet been obtained. Therefore, there must be some other means of supplying the lift required to overcome gravity until orbital velocity and altitude is reached. Thus my question to Mr. Powell was something like this: What kind of propulsion techniques are you considering which will have sufficient thrust to both overcome aerodynamic drag during the early part of the flight, and also provide lift during the later part of the flight, while still being efficient enough to accelerate for days on end without requiring enormous amounts of propellant? His response was that they are working on form of hybrid ion/chemical based engine with a theoretical ISP of around 900 seconds.
I'm still not completely convinced that this approach is viable with existing propulsion technologies. However, I think I now have enough information to do a slightly more detailed analysis of the airship-to-orbit concept. If I can find some time to actually crunch the number, I'll post my findings here.