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Much has been made of how the reentry burns of Falcon 9 first stages occur at an altitude where the atmosphere is similar in density to the atmosphere of Mars, and so they demonstrate that supersonic retropropulsion will work for Mars entry by the Red Dragon.

However the Falcon 9 first stage slows from around 1.5 km/s to some velocity during that burn, but not to zero. If the stage is returning to Cape Canaveral, it slows to zero and then speeds up in the opposite direction, but that isn't really the same thing. It isn't spending time at low velocities in that atmosphere as a lander would coming in on Mars. As there is no GPS or any beacons on the ground, a lander will spend some time at low speeds making sure it knows exactly how far away the ground is. Plus it would enter the atmosphere at about 3.5 km/s. It would let drag slow it as much as possible but then would it be firing its engines at a speed similar to the F9 1st stage?

Red Dragon is a pretty different shape and mass than an F9 1st stage, and according to this presentation it will incorporate lift to come in pretty level, so it will have a different angle of attack. So, how is information from F9 retropropulsion helping?

My conclusion from this information in this answer by Mark Adler is that control of such a vehicle is challenging and not the same when parameters are changed, which is why I ask:

  1. Once in the supersonic flow, the aerodynamic effects of the messy business end of the rocket can be complicated, making control a bit of a challenge. You can have counterintuitive effects that redirect flow in unexpected directions at different angles of attack.

  2. Predicting the effect of the running engines on the drag is a challenge. The thrust plumes tend to reduce the drag, countering in part the intent of firing the engines to increase deceleration. With enough of a thrust to drag ratio, this is not a show stopper, but you need to be able to predict how large the effect is to know if you have enough fuel. This impact on drag also complicates what happens when you gimbal the engine, which is part of the challenge in #4. Again, high thrust to drag ratio can reduce the surprises here.

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    $\begingroup$ two things come to mind: they're learning how to start an engine while it's flying nozzle-first through the atmosphere, and they're learning how the exhaust plume works (shielding the rocket to a certain extent). $\endgroup$ – Hobbes Jul 30 '16 at 19:32
  • $\begingroup$ @Hobbes Sure, critical work to successful landing this way. But what i really wonder about is control of the craft. $\endgroup$ – kim holder Jul 30 '16 at 19:58
  • $\begingroup$ Curious to know what kind of data will be useful for Mars missions.I am guessing that comparing some elements in these two cases (1st stage landing to Earth and Red Dragon to Mars) maybe they find some similarities.For example more atmosphere drag or friction force at Earth, but also more gravity and a lot heavier vehicle(Falcon 9's 1st stage), so the speed at the landing procedure could be similar in both cases. Heavier veichle(1st stage) than Red Dragon, but also more thrust for decending. $\endgroup$ – Mark777 Jul 30 '16 at 21:54
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    $\begingroup$ One of the major issues with SRP on Mars is significant thrust losses from internal losses due to low ambient pressures. What SpaceX will have to figure out is how to maintain nominal pressure ratios so that they do not have internal losses in their nozzle. This is extremely difficult to design and account for. For reference, it took years to develop an ideal nozzle shape for terrestrial applications to minimize internal losses. These losses can add up on an interplanetary mission. Further analysis needs to be carried out, but this could be a deal breaker. $\endgroup$ – aaastro Aug 15 at 15:11

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