IR movie of falcon 9 boost back and reentry by NASA

[Lars-J]

I think part of the problem with modeling hypersonic retro-propulsion is that there has been so little data to use as a basis for a model. Wind tunnels have a hard time simulating the exact conditions.

At these speeds, the outcome might not always be what you expect. (for example a low thrust creates a bubble that lowers the friction of the craft, thus making it fall *faster* than it would have otherwise)

There is a thread here that discusses hypersonic retro-propulsion issues: http://forum.nasaspaceflight.com/index.php?topic=33006.0

If as stated, the exhaust stream creates a buble that reduces friction, this may actually be advantageous during reentry as it would reduce heating of the skin and shear forces, until the stage slows down enough so it can use friction to slow down further without burning up.

This is not necessarily better. Ideally you want to bleed if your velocity during a longer time. If you reduce your friction and then hit the lower (and denser) atmosphere faster, you peak G loads and heating will likely be higher.

[Jcc]

I think part of the problem with modeling hypersonic retro-propulsion is that there has been so little data to use as a basis for a model. Wind tunnels have a hard time simulating the exact conditions.

At these speeds, the outcome might not always be what you expect. (for example a low thrust creates a bubble that lowers the friction of the craft, thus making it fall *faster* than it would have otherwise)

There is a thread here that discusses hypersonic retro-propulsion issues: http://forum.nasaspaceflight.com/index.php?topic=33006.0

If as stated, the exhaust stream creates a buble that reduces friction, this may actually be advantageous during reentry as it would reduce heating of the skin and shear forces, until the stage slows down enough so it can use friction to slow down further without burning up.

This is not necessarily better. Ideally you want to bleed if your velocity during a longer time. If you reduce your friction and then hit the lower (and denser) atmosphere faster, you peak G loads and heating will likely be higher.

Maybe ideally but that obviously doesn't work for the first stage flight profile, otherwise they would do it. If the stage re-enters at Mach 6 with no TPS and no retro propulsion it would burn up (or break up). With retro propulsion it doesn't break up. Ideally you would want the maximum friction the stage can take in addition to propulsive braking, but not breaking up is a good second choice.

[Torbjorn Larsson, OM]

There was speculation a few years ago, on the Red Dragon thread I believe, that having the superdraco plumes pointed outward along the rim would cause meaningful atmospheric concentration below the heat-shield as it passes through; increasing the atmospheric friction rather than trying to slip through it.  Adding surface area (ballutes, tabs, expandable heat shields, etc. would increase the friction further, as more atmosphere interacts with greater surface area. 

I'll speculate here; that the main challenge on Mars entry is maximizing the atmospheric drag while the main challenge on Earth is minimizing the friction on initial atmospheric impact.                             

Larry Lemke, lead on the Red Dragon project, described RD EDL and retrorocketry @ SETI Talks in June.



The detailed retrorocket segment is 50:00 - 55:00, with references.

The short of it is that:

- Low retrorocket thrust will blow out the shock front and increase drag, whether central or peripheral. Peripheral thrust will maintain that at higher thrust.

- For the RD and F9R S1 EDL, the thrust levels blow a hole in the shock front. They turn off the drag in the simulations since its effect is insignificant.

- RD will do an aerobrake at 5 km above the surface where the atmosphere is densest. (Segment starts at about 30:00.) I checked as best I could at the time, and IIRC that atmosphere height is roughly equivalent to the pressure/drag regime Apollo used when returning from the Moon.

To do that efficiently RD will use attack angle to dive as fast as possible.  Then use that control to maintain height (no Apollo skips, so more like 2-3 min at low g's I think) while drag slows RD until loss of loft at about Mach 3 (IIRC; you may want to check that). Finally do a supersonic brake to hover landing with 10 mt of fuel.

The brake trajectory is ~ 2500 km long, so you may want to start at Vastitas Borealis until you know what you are doing. VB is 8000 km+, so in the beginning drag coefficient is once again not much of an issue. [  http://en.wikipedia.org/wiki/North_Polar_Basin_(Mars) ]

[guckyfan]

Very interesting. While watching it an idea entered my mind. As there were pepople much smarter and knowledgeable than me at it they likely have discounted it already. But anyway here it is. Since it is Red Dragon specific I post it in the appropriate thread.

http://forum.nasaspaceflight.com/index.php?topic=33596.msg1274793#msg1274793