Airship to Orbit, A cheap way to LEO? Options

[Guest_DonPMitchell_*]

What do people think about this concept being explored by JP Aerospace? Is it feasible?

[abalone]

Is it feasible?

I have done a few back of the envelope calculations and I get the feeling it might be "pie in the sky" if you can excuse the pun.
It is important to remember two relevant facts.
1. The bouyancy of the atmosphere at 65,000 m (200,00ft) is very small.
2. Getting a payload to that altitude only represents about 8% of the energy needed to achieve a stable minimum orbit. Each kilogram of mass to orbit from there still needs an additional 70 megajoules, or, for the MER buffs, about 20,000 W.h. If we assume an average of say 400W.h per day for the MERs that is how much each will have collected in total in 50 sols. (Slight adjustment after checking calcs)

I have assumed that
(1) solar cell efficiency is 20% (10% if you assume they are in the Earths shadow for 1/2 the time)
(2) ion drive efficiency is 50%
(3) that cargo represents 50% of total mass, this is probably optimistic
(4) that 50% of energy is lost through air resistance.

With these assumptions you would need 40 sq meters /kg to supply enough energy to get each kg to orbit in (say)3 days. A 5,000kg payload would need a solar cell area of 200,000 sq meters. This of course would be limited to an equal mass as the payload and include to balloon and fuel as well.
I have not bothered to calculate the volume of a balloon needed at 65,000 m to support such a mass, but I will add that any publication that still persists in using feet and pounds as units in a scientific field is doomed to crash anyway.

[Guest_DonPMitchell_*]

It sounds like they are proposing a combination of boyancy (primarily to get out of the thickest atmosphere), airfoil lift, and at the highest altitude where air resistance begins to disappear propulsion would accelerate to orbital velocity.

Energy to reach orbit is not really about altitude, but velocity. I think the intersting physics question is, what is the terminal velocity at that altitude, given the thrust of their engine. Can they keep accelerating to orbital velocity and make the transition from aerodynamic lift to centrifugal lift?

The use of pure solar power seems problematic.

The idea is the first plausible alternative to rocket power that I've heard, so I hope it is studied.

[helvick]

what is the terminal velocity at that altitude, given the thrust of their engine. Can they keep accelerating to orbital velocity and make the transition from aerodynamic lift to centrifugal lift?

Highly unlikely. Terminal velocity isn't really the right term but it is a reasonable analog for the drag limited practical maximum velocity. Excluding shock effects it is proportional to the square root of the atmospheric density. AT 60km the atmospheric denisty is somewhere in the region of 0.1g/m^3 and Vt for a fairly dense (iron) sphere would be around 40,000km/h. Which is the right order of magnitude.

However these things are not dense iron spheres they are huge objects that by definition have to have a lower density than 0.1g/m^3. The terminal velocity for such a thing is no faster than 4km/h and that's assuming you have an engine that can impart 1g of acceleration to it.

Not going to fly to orbit I fear.

[Guest_DonPMitchell_*]

Aren't you ignoring lift? The object is an airfoil.

[helvick]

Aren't you ignoring lift? The object is an airfoil.

No - Lift and drag are inherently linked you have to be able to overcome drag at least initially. You will get lift only if you have sufficient thrust to overcome drag and can get moving. If you go lighter than air to get up which is energy efficient (presumably) then when you want lift you have to move something relatively fast which is damn hard when your density is in below 0.1g/m^3.

Now if you could progressively shrink the effective cross section of the craft, and speed it up at the same time so that you traded off buoyancy with lift then you could morph your lifter into a dart shape that might be able to reach an escape velocity before hitting the terminal velocity\ drag wall. That is a neat idea and would be one hell of an impressive machine to watch in action but I can't even begin to imagine the materials technology that would be involved (starting off with a density <0.1g/m^3 and ending up with something capable of handling 40000km/h travel through the upper atmosphere).

[Guest_DonPMitchell_*]

Well who knows. Presumably they have real engineers who have looked at all of this. But it could be a boondoggle. I've seen dumber ideas getting funded by NIAC, and sometimes you do wonder if there is just a shortage of scientists to check ideas before money is spent.

[tty]

Now if you could progressively shrink the effective cross section of the craft, and speed it up at the same time so that you traded off buoyancy with lift then you could morph your lifter into a dart shape that might be able to reach an escape velocity before hitting the terminal velocity\ drag wall. That is a neat idea and would be one hell of an impressive machine to watch in action but I can't even begin to imagine the materials technology that would be involved (starting off with a density <0.1g/m^3 and ending up with something capable of handling 40000km/h travel through the upper atmosphere).

They will have to change shape for another reason. The illustrated moderately swept thick profile flying-wing shape is very effective at subsonic speed but drag will go up dramatically at transsonic, not to mention supersonic speed. Also I wonder if it is possible to get enough stiffness in a 0.1 gm-3 structure to withstand aerodynamic loads at high speeds, however mild they may be at 200,000 feet. Not to mention the heat loads, which are not mild at hypersonic speeds even at that altitude.

In any case it is very difficult to get a good L/D at supersonic/hypersonic speed which is the basic reason supersonic flight is not much used. The best shape known at present is probably a wave-riding bulbous "lifting-body" shape with downturned wingtips.

I agree, it would take an impressive machine to morph from a flying wing into a lifting body.

tty

[ups]

It would be quite a rush to take a ride up and jump out.