3 ms·

If the capsule/rocketplane has some lift & preferably steerable aerosurfaces then you can compensate the purely ballistic deceleration somewhat. But yeah, if i

by m4rtink 5mo ago

If the capsule/rocketplane has some lift & preferably steerable aerosurfaces then you can compensate the purely ballistic deceleration somewhat. But yeah, if it is going down almost vertically then this will not be enough.

pfdietz 5mo ago
And all but rather short ballistic trajectories (well below orbital speed) will come in at a steep angle. Unless one has seriously variable aerodynamics, the vehicle will have to swerve to nearly horizontal over a distance of about 1 scale height of the atmosphere, which is about 10 km. The exponentially thinning atmosphere goes from "too thin to matter" to "brick wall" over a short distance. The acceleration for turning is v^2/r; for v = 5000 m/s and r = 10 km this is 250 g. Acceleration also limits how rapidly one can reenter from beyond Earth orbit. At > LEO velocity, the vehicle has to use (downward) lift to stay in the atmosphere, and if v is too high the required acceleration is too high.
- seabass-labrax 5mo ago
  It would still be possible to fly a predominantly ballistic trajectory, yet use rocket propulsion to decelerate whilst still outside the atmosphere. It would require a huge amount of extra fuel compared to a purely ballistic trajectory, but perhaps still less than achieving a full orbit and de-orbiting again for some destinations.
  - pfdietz 5mo ago
    At global distances a fraction orbital trajectory would use less total rocket delta-V. More practical would be a trajectory that was a series of small suborbital arcs with skipping off the atmosphere (perhaps with some airbreathing propulsion during the skips.) The thermal protection can cool by radiation between the skips.
    - seabass-labrax 5mo ago
      That would be very cool to witness! SABRE sounds like it would be an ideal engine for that application.

3 ms·

If the capsule/rocketplane has some lift & preferably steerable aerosurfaces then you can compensate the purely ballistic deceleration somewhat. But yeah, if i

by m4rtink 5mo ago

pfdietz 5mo ago
And all but rather short ballistic trajectories (well below orbital speed) will come in at a steep angle. Unless one has seriously variable aerodynamics, the vehicle will have to swerve to nearly horizontal over a distance of about 1 scale height of the atmosphere, which is about 10 km. The exponentially thinning atmosphere goes from "too thin to matter" to "brick wall" over a short distance. The acceleration for turning is v^2/r; for v = 5000 m/s and r = 10 km this is 250 g. Acceleration also limits how rapidly one can reenter from beyond Earth orbit. At > LEO velocity, the vehicle has to use (downward) lift to stay in the atmosphere, and if v is too high the required acceleration is too high.
- seabass-labrax 5mo ago
  It would still be possible to fly a predominantly ballistic trajectory, yet use rocket propulsion to decelerate whilst still outside the atmosphere. It would require a huge amount of extra fuel compared to a purely ballistic trajectory, but perhaps still less than achieving a full orbit and de-orbiting again for some destinations.
  - pfdietz 5mo ago
    At global distances a fraction orbital trajectory would use less total rocket delta-V. More practical would be a trajectory that was a series of small suborbital arcs with skipping off the atmosphere (perhaps with some airbreathing propulsion during the skips.) The thermal protection can cool by radiation between the skips.
    - seabass-labrax 5mo ago
      That would be very cool to witness! SABRE sounds like it would be an ideal engine for that application.