The spacraft soon passed its original 12.7 km periapsis and was still losing altitude. Because aerobraking started before the original periapsis altitude, the orbit's periapsis shifted to lower altitudes. The flight computer recalculated the orbit second by second. At the 13 km altitude mark, drag reached 5.9 m/s 2 - around twice Duna's surface gravity. In no more than 30 seconds, atmospheric drag increased from negligibility to an acceleration comparable to local gravity. First from micrograms per cubic meter, to miligrams, and then to grams. As it approached its projected periapsis of 12.7 km, the atmospheric density started getting higher and higher. 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.
But one must remember that Duna's atmosphere is much shallower than Kerbin's, so its density becomes important only at much lower altitudes. The craft was still accelerating towards its periapsis, propelled by the force of gravity alone. Aerocapture had begun.Īs it plunged into the slowly thickening atmosphere, at first nothing happened, no change in orbital energy. For the first time in more than two months, the spacecraft's sensors detected the presence of an atmosphere - minute, but undeniable.
KERBAL SPACE PROGRAM ESCAPE TRAJECTORY FULL
Then, finally, with a control room full of tense, over-caffeinated Kerbal engineers, Duna Express crossed the atmospheric interface. Duna Express rotated to face the atmosphere tail-first.
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.īut 12.7 km it was. 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. 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. Such high precision maneuver is difficult to achieve. The best calculations had provided a narrow aerocapture altitude window: the spacecraft had to pass between 12 and 15 km of altitude. 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. With no correction, Duna's gravity would barely deflect the spacecraft's path. At arrival into Duna's gravitational sphere of influence, the trajectory of Duna Express was a hyperbolic escape trajectory.
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