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According to a recent news announcement1, NASA is going to send a small helicopter to Mars for testing purposes. Future landers could carry more of these

future rotorcraft [that might] act as scouts and explore parts of Mars that rovers can't reach.

The article states the following about the initial deployment of the helicopter:

After landing, the rover will deposit its helicopter payload, then retreat to a safe distance while the rotorcraft take off.

If my understanding based upon what I have read is correct, the touchdown of landers is still a somewhat risky event, and new ways to protect landers from the impact on the surface are still an active topic of research. If so, could it be feasible to deploy the helicopter before the lander reaches the surface (thus evading the shock from the final impact), or does mid-air braking and stabilization pose an even greater problem for such a helicopter? (I assume putting some safety distance between the descending lander and the helicopter would be feasible by jettisoning the helicopter at a sufficiently divergent angle.)


1: I'm going to cite from the Washington Post article on the topic here, but many news outlets reported about this.

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  • $\begingroup$ If there is a problem when the helicopter tries to take off from the ground, the controllers have time to try to deal with it. Not so much if you pitch it out in mid-air. $\endgroup$ – Organic Marble May 16 '18 at 0:49
  • $\begingroup$ Consider the fact that nobody has attempted to do this on Earth, then think about how much more difficult it would be on Mars coming in from orbital speeds. $\endgroup$ – Phiteros May 16 '18 at 7:21
  • $\begingroup$ @Phiteros: "Consider the fact that nobody has attempted to do this on Earth" - I'd assume a major reason is our advantage that we don't have to land after coming in from orbital space before we reach the ground on Earth. $\endgroup$ – O. R. Mapper May 16 '18 at 8:41
  • $\begingroup$ What? Are you forgetting about all those trips up and down from the ISS? $\endgroup$ – Phiteros May 18 '18 at 6:37
  • $\begingroup$ @Phiteros: Not at all. What I meant is: If we want to let a helicopter fly here on Earth, there are more convenient ways than transporting it into orbit, then having it return from there in a spacecraft, and figure out when and how to deploy it from that spacecraft and how to protect it during the landing of that spacecraft. $\endgroup$ – O. R. Mapper May 18 '18 at 6:41
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It’s possible, but far too risky to be called feasible.

In theory, the concept is no different from previous missions that have used an aeroshell / parachute to slow to reasonable speeds then switched to a different system such as airbags or skycrane for landing. Rockets need decent stabilization for a successful landing too. However, there are a few practical issues.

  • No opportunity for system checks or troubleshooting, particularly important for a first experimental vehicle. Suppose the batteries turn out to be flat on deployment. If safely on the ground, you have the option to let them charge and continue the mission the next day. That is a little harder to fix in the ten seconds before you encounter the ground at high velocity.

  • Radios capable of interplanetary or even satellite communication tend to be large and power hungry, and a helicopter in thin air has a very limited payload. Some of the antennas on the rover are larger than the entire helicopter. Regardless of the landing system used, you’re going to need a safely landed rover so you can stick to the scale of parts found in consumer drones.

  • A small unpowered helicopter is quite similar to a parachute - both will float down slowly and be blown about by the wind. That doesn’t make it impossible to separate from a parachute, but it does significantly increase the risk of entanglement.

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As an addendum to the other answers, starting a helicopter or drone from a freefall can be a recipe for disaster as this is the easiest way to enter VRS or Vortex Ring State in which the propeller is surrounded by a torus of air and looses basically all thrust.

Entering VRS typically occurs when a helicopter decends in it's own downwash and is one of the most dangerous position for it to be in.

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In addition to the other answer:

The spacecraft approaches the landing site at high speed, and needs rocket engines (with far higher thrust than any helicopter rotor) to slow down for a safe landing.
A helicopter separating at high speed won't have enough thrust to slow to a hover before it hits the ground.

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