Airship To Orbit

[mikegarrison]

2 hours ago, CBase said:

Are there any serious numbers out there how much mass you actually save from any high altitude launch (20km+) ?

It's not a lot. Most of the energy you need to go to orbit is horizontal speed, not vertical height.

The main thing is that you can avoid having to use rocket engines optimized for sea level ambient pressure, but that turns out not to save you a whole lot.

The other thing you can do with a mobile launch platform (air or sea) is to move it to the most optimal launch position for any individual mission.

But mostly those are offset by the cost benefits of having a fixed ground base rather than putting everything onto a ship or having a flying launch platform.

[kerbiloid]

10 km/s you need to get in LEO, 1 km/s is taken by the air drag.

So, the balloonery doesn't give much to the rocketry.

[Piscator]

19 hours ago, sevenperforce said:

22 hours ago, farmerben said:

I've wondered about de-orbiting an airship.  The whole problem is with overheating.  I don't think we know conclusively if it would work.  

If it’s light enough, large enough, and stiff enough, it will deorbit with just fine with minimal heat shielding required.

Actually, we might be close to the point where we can try this. Depending on the final figures for dry mass, internal volume and how you pressurize said internal volume, Starship (usual disclaimers apply) might be just buoyant enough to float in the lower parts of Venus' atmosphere. It wouldn't excatly qualify as an airship and would likely have not practical value whatsoever, but it might not touch the ground for a while either.

[sevenperforce]

1 hour ago, Piscator said:

Actually, we might be close to the point where we can try this. Depending on the final figures for dry mass, internal volume and how you pressurize said internal volume, Starship (usual disclaimers apply) might be just buoyant enough to float in the lower parts of Venus' atmosphere. It wouldn't excatly qualify as an airship and would likely have not practical value whatsoever, but it might not touch the ground for a while either.

An expendable stripped-down Starship with three engines is 40 tonnes dry. Dump the engines and you lose about 6 tonnes so that's 34 tonnes. Bulk density of methalox at Raptor mixture is 989.2 kg/m³ and it has a prop load of 1200 tonnes so that means it has an internal volume of roughly 1213 cubic meters.

We know the pressure and temperature relative to elevation on Venus, and we know that CO₂ has a density of 1.87 kg/m³ at 15°C and 1 atm, so by the ideal gas law we can easily calculate the density of the Venusian atmosphere at any altitude:

You can therefore see that Starship would need to be below 15 km before it would be able to displace even its own dry mass in atmosphere. But of course Starship will need to be pressurized in order to keep the atmosphere from crushing it. If you press it with residual methane, which has a density of  0.671 kg/m³ at 15°C and 1 atm, then you can plot buoyancy vs altitude:

A little interpolation gives us neutral buoyancy for Starship at ~5.8 km.

Deorbiting Starship from interplanetary velocities, however, will be left as an exercise for the reader.

[Piscator]

Thanks for confirming my back-of-the-napkin calculations. I was using a dry mass closer to 100 tons but I also took the additional ~1000 cubic meters of payload volume into account. Since Wikipedia gives slightly higher surface densities for Venus' atmosphere at ground level (probably due to not being an ideal gas and/or being supercritical) and since it can also be assumed that the tank's contents will warm somewhat during decent, things might in fact even look a bit more favourably.

Since Starship wouldn't have to be stripped down too much, re-entering  form interplanetary velocities might not actually be much of a problem. That's what it's designed to do anyway, if I'm not mistaken. If push comes to shove, there should be a sufficient delta-V budget to slow down propulsively.

[benzman]

23 hours ago, sevenperforce said:

Balloons have been inflated in orbit before; you just have to get the pressure right.

A balloon pressurized to 3 atm at sea level is under the same stress as a balloon pressurized to 2 atm in a vacuum. 

I was being a bit flippant. I am aware of things like the ECKO project.