Controlled Aerobraking

[Enthalpy]

Hello dear friends!

Here I plan to describe automatic spacecraft and manned spaceships that aerobrake at celestial bodies like Earth and Mars, and have big movable control surfaces in order to adjust much the braking force, provide stability, and when needed provide downlift.

They look roughly like a capsule fitted with big movable petals, which can be behind the capsule for weaker braking, or extended for stronger braking.

The same petals are useful as a parachute at lower altitude and speed, and possibly as legs to land and to move the spacecraft. This may save some mass, and limits the risks of unwanted interactions.

Drawings are in the pipe and should make it clearer.

Marc Schaefer, aka Enthalpy

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Here's how the petals' position meet various needs, here to aerobrake and land; other missions like an aerocapture would follow a different sequence.

 

 

The petals have (strong) actuators for several movements. Downlift needs it, and also the roll control (which can require additional control surfaces), walking on the soil as well. A taller spacecraft that requires downlift might have a tiltable heat shield.

A numerical example is to follow.

Marc Schaefer, aka Enthalpy

[Enthalpy]

As an example, an automatic probe shall aerobrake and land on Mars.

The body has 4m diameter or 12.6m², the eight 1.5m*8m petals cumulate 98m². This fits in Ariane 5's fairing, Atlas V and Delta IV as well. Atlas V 551 achieves 8900kg in tilted Gto, so is shall inject 6227kg at 3262m/s above Earth's gravity; Ariane 5 Me achieves a bit more. The slightly accelerated transfer (roughly 200 days) arrives with 4235m/s above Mars' gravity and plunges to 6559m/s little near Mars' surface.

Aerobraking over straight 2*190km (no downlift) needs only 40m/s² (smoother than the launch) or 249kN, beginning and ending at 30km above datum, with the middle at 24.7km. Fine, that's higher than Olympus Mons, and the mean free path is still <1mm at 30km. Here's my estimate of the atmosphere's density, fitted at 0km and 21km; adjust the braking height if you have real data - this may need real-time measures because of Mars' variability.

 

 

I take the hypersonic drag as density*area*tilt*C_h*V² with a C_h~0.8.

• 6559m/s and 0.22g/m³ at 30km need area*tilt=33m²: petals 1/5 active.
  Efficient braking could hence begin earlier.
• 5051m/s and 0.78g/m³ at 24.7km correspond to the body: petals just 1/30 active.
• 3542m/s and 0.22g/m³ at 30km need 113m²: petals fully active - but keep some angle.
  This is the speed of a circular orbit, though a capture would have sufficed.

The wide tunability permits a softer brake and, together with some up- or downlift, can compensate some variation in Mars' atmosphere. The petals keep the probe stable. All this is easier than with a lifting body. The probe can even adjust its path to the landing site.

The petals open to 30° short of flat for the descent, so the probe shows 97m²; with transsonic C<sub>x~0.9, the speed drops to 192m/s near the ground (14.4g/m³ and 3.7m/s²). No parachute threatens to fall on the probe.

A rocket shall brake to the soft touchdown. If pushing 3.7+10m/s² it costs 263m/s performance. For comparison, hovering and manoeuvring over the ground at 5m/s² during 60s also costs 300m/s.

One simple autonomous touchdown algorithm could be:</sub>

• _{With all petals-feet moderately down, descend gently until one touches, then stabilize the height.}
• _{Lower the other petals-feet until all touch, then stop the rocket.}
• _{But if the feet don't touch after a reasonable stroke, then take off and try again 300m farther.}

<sub>One 150+150m hop with 3.7+10m/s² push costs 91m/s performance, one 30s descent attempt 120m/s. Braking plus three landing trials cumulate 805m/s. At isp=340s it takes 1336kg propellants. If the tanks and engines weigh 140kg it leaves 4751kg.

The dirty used heat shields are dropped on the ground and the probe walks away, with 20m span giving a nice clearing capability. But if dropping the shields isn't clean enough, the petals and all shields plus the rocket propulsion can belong to a crane that lowers a rover before separating itself and flying away, as the Mars Science Laboratory did.

Marc Schaefer, aka Enthalpy</sub>

Edited August 4, 2013 by Enthalpy

[Enthalpy]

The masses in my last message are wrong.

Atlas V 551 can inject only some 5.1t in transit to Mars.

And as an excellent practice, the unsterilized launcher shall not collide with Mars. Consequently, the probe must correct its path, which cost Mars Science Laboratory 14% of its mass. So a direct comparison wouldn't be fair.

MSL innovated with its "guided entry" which used an unbalanced capsule (like Apollo) that rolled to adjust its path, to compensate for atmospheric fluctuations, for wind, and other perturbations, in order to land precisely. The petals I propose give stronger means of action, and their stability looks more reassuring. They weigh a lot, but MSL ejected 150kg to unbalance the capsule before aerobraking, later 150kg again to recover balance. The parachute weighs also. This makes the petals more aceptable.