2

In this question I posed the problem of getting an inhabitant of the planet Earthtoo into low orbit (LEtooO) using conventional rocket. Judging by the great answers there, it is - to my surprise - quite possible. Earthtoo has twice the diameter of the earth and the same average density, so the surface gravity is also twice that of earth. The density of the atmosphere at the surface is also twice that of earth (they need more oxygen to climb stairs), and the scale height is half that of earth.

Now the tootronaut is ready to come home. Total spacecraft mass is 1000kg, orbital velocity around Earthtoo is about twice that of Earth, about 15.8 km/s. Yikes! The kinetic energy to be dissipated is about four times from LEtooO than it would be from LEO, and as soon as you start loosing energy, you start falling into an atmosphere which is increasing in density far more rapidly than the Earth's atmosphere would, because of the much smaller scale height.

Could this be done with similar materials and techniques used for reentry on earth? Or would the higher velocity and shorter scale height be extremely difficult to handle using Earth's current heat shield technology? Would it need to rely heavily on propulsive deceleration? Would it have to be airplane-shaped? The Tooians jokingly call the reentry vehicle the "Flying Tile" for a reason.

I'm not looking for opinions or maybes here. Please use an appropriate amount of math and physics to back up your answer. Thanks!

Community
  • 1
uhoh
  • 148,791
  • 53
  • 476
  • 1,473
  • 1
    Apollo capsules re-entered at about 11km/s. I actually think deceleration g-force might be a bigger problem than heat - the lower scale height means that the atmospheric deceleration has to occur over a shorter period of time. How tough is a Toonian? – Russell Borogove Jul 25 '16 at 03:17
  • @RussellBorogove ha you almost got me to answer. But having just voted to close my own question last night because it wasn't about space exploration, I'm not going to bite :-) Let's just focus on the physics of the heat here. Assume structural integrity is managed with advanced materials, and Tootronaut integrity is managed by encapsulating them in Tooian Jello or something. Let's say they have roughy the same mass as Earthians, but they selected a very healthy but lighter-than-average individual for the mission. – uhoh Jul 25 '16 at 03:22
  • Okay, then put an Apollo-grade heat shield on, and a retrorocket capable of delivering 4.8km/s delta-v. The shorter deceleration period mean less total heat load to deal with, but your 1 ton capsule needs a 4-ton retro package, so your LEO ascent system grows to about 60,000 tons. – Russell Borogove Jul 25 '16 at 03:28
  • Look at the trajectory now - especially the radial velocity. It is precipitously sub-orbital How fast is it falling down into the atmosphere now under twice Earth's gravity? How fast is the atmosphere getting denser, how hot is the plasma now? There's both direct heating by contact and radiative heating. This isn't a trivial question! – uhoh Jul 25 '16 at 03:32

1 Answers1

4

It is not the current heat shield technology that makes this challenging. The Galileo probe hit Jupiter's atmosphere at 47.8 km/s, almost three times faster than a return from LEtooO. The mass of the entire probe was below 1000kg.

There problem is something else, acceleration. Fighter plane pilots can experience up to 9 g's, and it is conjectured that it is possible to survive an acceleration up to 20 g's, using special equipment and ignoring terrible damage to the body. The Galileo probe decelerated at 230 g's ...

What technology you have to use is mostly dependent on how tough the tootronauts are, and not shield technology.

A regular reentry with the Soyuz vehicle on Earth is normally experiencing 4-5 g's. There have been cases where the crew has experienced more than 8 g's though.

If the tootronauts are anything like human cosmonauts, I will say a lifting body shape of the space craft is the best alternative. Then a spacecraft can counter gravity for a while using aero-dynamical lift, and wait for a descend into a layer of the atmosphere with a higher density until enough speed has been bleed off. Gliding ratios better than the space shuttle are pretty easy to achieve.

SE - stop firing the good guys
  • 43,384
  • 3
  • 143
  • 244
  • Wow!! That probe is amazing! So almost half the mass of the spacecraft was heat shield, and half of that was lost during that amazing atmospheric maneuver. g force is a real consideration I see now, but that doesn't just automatically make the heat shield not challenging. 200 seconds of 8 g braking has got to take some toll on a heat shield. I like your suggestion of using a shape with some lift and (hopefully) some ability to control the lift to regulate braking. Maybe that's what @RussellBorogove should've said here! – uhoh Jul 25 '16 at 14:03
  • @uhoh Link fixed, just an issue with auto completion. – SE - stop firing the good guys Jul 25 '16 at 14:45