Aerocapture
At arrival into Duna's gravitational sphere of influence, the trajectory of Duna Express was a hyperbolic escape trajectory.
With no correction, Duna's gravity would barely deflect the spacecraft's path. However, instead of performing a retrograde capture burn that would require about 400 m/s of delta-v, a much smaller radial burn was scheduled to lower the periapsis from about 1,000 km to 13 km — deep into Duna's atmosphere.
The best calculations had provided a narrow aerocapture altitude window: the spacecraft had to pass between 12 and 15 km of altitude. Such high precision maneuver is difficult to achieve. Even with very small thrusts at minimum power from the spacecraft's engine, the final trajectory was close but not exactly as planned: the final periapsis was 12.7 km.
Would it be too low? Mission planners had intended to be conservative and rather have a periapsis higher than optimal, since then engine thrust could be applied to assist the aerocapture. But too low a periapsis was harder to compensate, since planetary atmospheres are exponential and density increases rapidly as one dips lower into the atmosphere.
But 12.7 km it was. Duna Express rotated to face the atmosphere tail-first ... as Duna got bigger and bigger.
The spacecraft soon crossed Ike's orbit.
Then, finally, with a control room full of tense, over-caffeinated Kerbal engineers, Duna Express crossed the atmospheric interface. For the first time in more than two months, the spacecraft's sensors detected the presence of an atmosphere — minute, but undeniable. Aerocapture had begun.
As it plunged into the slowly thickening atmosphere, at first nothing happened, no change in orbital energy. The craft was still accelerating towards its periapsis, propelled by the force of gravity alone. But one must remember that Duna's atmosphere is much shallower than Kerbin's, so its density becomes important only at much lower altitudes.
Would the maneuver work? If it didn't, and the spacecraft simply flew by Duna and continued on escape trajectory, it would activate its engine and complete the insertion as it is usually done. As it approached its projected periapsis of 12.7 km, the atmospheric density started getting higher and higher. First from micrograms per cubic meter, to miligrams, and then to grams.
In no more than 30 seconds, atmospheric drag increased from negligibility to an acceleration comparable to local gravity. At the 13 km altitude mark, drag reached 5.9 m/s2 — around twice Duna's surface gravity. Duna Express was definitely braking!
The flight computer recalculated the orbit second by second. Because aerobraking started before the original periapsis altitude, the orbit's periapsis shifted to lower altitudes. The spacraft soon passed its original 12.7 km periapsis and was still losing altitude. Was it going too steep? Would it crash into Duna?
Then, the flight computer indicated the first key event everyone was waiting for: the orbital energy of the spacecraft was now negative — Duna Express was no longer on an escape trajectory! The first half of the capture was complete!
But would it slow down too much? Although the craft was already gaining altitude again, atmospheric drag was still significant. Engineers watched the telemetry nervously, knowing there was nothing they could do — they were only watching a replay of events that had already transcured, the information of them needing several minutes to reach Kerbin at lightspeed.
The apoapsis decreased very quickly.
When Duna Express finally reached the other side of Duna's night side ...
... the second key moment finally ocurred: sensors indicated that atmospheric density was below scale minimum — Duna Express had cleared the atmosphere! With a final apoapsis of only 518 km, the spacecraft had accomplished the first aerocapture in the history of spaceflight!
It was now on an elliptical orbit around Duna. The last step of the maneuver was raising the periapsis, which had dipped from the original 12.7 km to 11.1, km, above the atmosphere, putting the spacecraft —finally— into a stable elliptical orbit around Duna.
Duna Express had reached its destination — and with a greatly reduced fuel expenditure.
Continue to Satelites ...
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