5 ms·

90 minutes is a low earth orbit period. A suborbital craft won’t be travelling at that speed.

m4rtink 5mo ago
Like, you could do a partial orbit & then drop down over the destination. But it would need much more delta-v & an orbital class heat shield. It was proposed as nuclear warhead delivery method though: https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment_System https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment...
pfdietz 5mo ago
Unless a suborbital trip is nearly at orbital velocity, it will involve a high, arcing trajectory. This will make the deceleration at the end unacceptably (lethally) high for all but short arcs. Some of the Mercury suborbital missions involved deceleration of 15 gees, if I recall correctly.
- 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.
- phire 5mo ago
  That was only an issue because they were fired pretty much straight up; They only went 500km down range. You can also reduce peek deceleration forces by using aerodynamic lift to stretch out the reentry over a longer period.
  - pfdietz 5mo ago
    No, it's an issue for most arcing trajectories. Lift doesn't help much if you're coming in at a steep angle. Reentry from orbit only works well because the entry is almost flat; there even a little lift helps a lot.

Like, you could do a partial orbit & then drop down over the destination. But it would need much more delta-v & an orbital class heat shield. It was proposed as nuclear warhead delivery method though: https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment_System https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment...

pfdietz 5mo ago

Unless a suborbital trip is nearly at orbital velocity, it will involve a high, arcing trajectory. This will make the deceleration at the end unacceptably (lethally) high for all but short arcs. Some of the Mercury suborbital missions involved deceleration of 15 gees, if I recall correctly.

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.
phire 5mo ago
That was only an issue because they were fired pretty much straight up; They only went 500km down range. You can also reduce peek deceleration forces by using aerodynamic lift to stretch out the reentry over a longer period.
- pfdietz 5mo ago
  No, it's an issue for most arcing trajectories. Lift doesn't help much if you're coming in at a steep angle. Reentry from orbit only works well because the entry is almost flat; there even a little lift helps a lot.