> We need to get payloads into space, and cost per kilogram is what matters. Why do this?
They're fuzzy on how they get from ground to 140,000 feet. But the hard part of orbit is speed, not altitude.
Electric propulsion can be orders of magnitude more efficient than chemical rockets [1]. The problem is their thrust is too low for anything beyond cruising. Cruising into orbit doesn't typically work because you'll fall into the ground before you get up to speed. But if you're using the atmosphere to keep off the ground, maybe it could work? (I'm sceptical.)
Using the atmosphere to keep you off the ground feels like you have a problem where for every atmospheric molecule bouncing off your bottom pushing you upwards you're incurring a drag cost. My gut says that that means it's never going to work, but guts aren't always very good at engineering.
It *may* work if you had some impressive wireless energy transfer, but that means you probably would not actually save any cost in the grand scheme of things.
Blimps sound like they should be good for space, but as a concept they're oversized and full of hot air unfortunately.
I might not understand your concern but to me this seemed clear enough:
> The first stage is an airship that travels from the ground to 140,000 feet. There it will dock with a waystation floating at the top of the atmosphere. Cargo and crew then transfer to a large 'Orbital Airship' for the nine day journey to orbit.
> The first stage is an airship that travels from the ground to 140,000 feet
Right, this is the entirety of their description of that airship. An airship which is supposed to go higher than any plane has ever flown [1], within the realm of high-altitude balloons [2].
EDIT: It looks like it just balloons up to 140k feet.
How much heating would accelerating to orbital velocity at the “edge” of the atmosphere create? I’d expect that if there’s enough atmosphere to create lift there’s probably enough to cause the craft to burn up.
> if there’s enough atmosphere to create lift there’s probably enough to cause the craft to burn up
There is generally a window between these speeds. Lifting bodies are good at staying in that window. (As your speed increases, lift increases, which pushes your aircraft up and into thinner air.)
This element of the problem--transitioning from aerodynamic lift to orbital speeds--is shared with SSTO. There are a lot of tough things about SSTO. But not burning up as you accelerate isn't one of them.
SSTO doesn't rely on aerodynamic lift, it can fly higher than a lift-based vehicle can to avoid air resistance/heating. I suspect you can fly high enough that burning up isn't a problem, but I don't see how you reach that as a conclusion by analogy to single stage to orbit rockets.
> SSTO doesn't rely on aerodynamic lift, it can fly higher than a lift-based vehicle can to avoid air resistance/heating
The only reason to do SSTO is so you can use an air-breathing engine. That's why practically every recent SSTO draft looks like an airplane.
This team appears to be using a reaction engine for thrust, so I'm not sure how they're planning on dividing the work between buoyancy, lift and reaction. But the whole point of using an airship and making it massive is so it can generate lift in air too thin to keep a metal tube flying.
> don't see how you reach that as a conclusion by analogy to single stage to orbit rockets
Because SSTO has been extensively studied, this is a problem SSTO would have to deal with, and in no case was it a dealbreaker.
Single Stage To Orbit doesn't have to be an atmosphere breathing multi-engine lifting body craft, it can be a conventional rocket that just doesn't care about being efficient.
Burning up as you accelerate is also an engineering problem that is not 100% solved for hypersonics. Almost solved, but not risk free.
As an example, Starship can theoretically SSTO on Earth, just with no payload and no means of returning. Totally inefficient and actually useless, but technically SSTO.
Only when that 2-stage stack is properly validated.
TBH, Superheavy+Starship, right now, is probably cheaper than the fully expendable SLS.
Also, if all the materials will be cannibalized in orbit (or never land on Earth again, like the Lunar Starship), it makes no sense to send the mass in a reusable vehicle.
L/D ratio at hypersonic speeds is bad. So, the thrust has to still be a significant fraction of the weight of the vehicle. There's no "slowly" accelerating to orbit.
They're fuzzy on how they get from ground to 140,000 feet. But the hard part of orbit is speed, not altitude.
Electric propulsion can be orders of magnitude more efficient than chemical rockets [1]. The problem is their thrust is too low for anything beyond cruising. Cruising into orbit doesn't typically work because you'll fall into the ground before you get up to speed. But if you're using the atmosphere to keep off the ground, maybe it could work? (I'm sceptical.)
[1] https://en.wikipedia.org/wiki/Specific_impulse#Examples