The Duna Express Mission

First visit to the Red Planet (by an unmanned probe)

Having conquered all celestial bodies within Kerbin's sphere of influence, the Kerbal Space Program was ready to finally visit other planets. The most obvious choice was Duna, the Red Planet. But before a manned mission could be sent, the Program needed to learn the specifics about Duna's atmosphere and to develop systems and gain experience in interplanetary travel. The Duna Express multicomponent robotic mission was to be the first milestone.

Interplanetary travel presents its own new challenges. For instance, because Duna's orbital period around the sun is similar to Kerbin's, efficient transfer trajectories occur only within certain windows separated by hundreds of days. Given the spaced-apart launch windows and the long travel time, it is important that any mission sent to Duna did as much as possible.

The Duna Express robotic mission was thus designed with multiple components: two reconaissance satelites, one to be left in low Duna orbit to perform terrain mapping and gravimetric measurements, the other to be sent to an orbit around Ike, Duna's moon. The third and final component was a robotic lander, which would perform a powered descent to Duna's surface with the goal of taking crucial measurements of Duna's atmosphere and provide the first images of its surface. With these robotic explorers, the Program would have a complete overview of the Duna system, a necessary step before manned landings.

Another important aspect of the mission was testing the feasibility of interplanetary aerocapture, something which had never been attempted before. The presence of an atmosphere brings a unique opportunity to cut down on the high delta-v cost required for orbital insertion coming from an interplanetary trajectory. In theory, if a spacecraft coming along an escape hyperbolic trajectory passes low enough within the atmosphere, aerodynamic drag will slow it down enough to be captured by Duna's gravity without a fuel expenditure.

However, the maneuver needs to be very precise: if the path is too shallow, the craft will ricochet off the top of the atmosphere, not slowing down enough and missing its orbital capture; if it's too steep, the atmosphere will slow it down too much and the craft will crash into the surface. The imprecise knowledge of Duna's atmosphere rendered any calculations uncertain at best. This was definitely something to be attempted by a robotic mission.

These, then, were the stated objectives of the mission:

Because the aerocapture is a secondary objective and must not compromise the achievement of the primary objectives, the spacecraft was designed with more than enough delta-v to achieve orbital insertion around Duna, should the aerocapture fail.

Here then, finally, is the Duna Express spacecraft:

The lower portion is the lander, designed to be able to conduct a powered descent to Duna's surface and have fuel to spare, since Duna's aerodynamics are poorly understood. Mounted on top of it are the twin reconaissance satelites destined for Duna and Ike orbits:

The interplanetary booster stage, shown below, is a simple rocket with enough delta-v to take the spacecraft to Duna and perform the orbital insertion burn, in case the aerocapture fails. A total of about 1800 m/s is required for the full trip from low Kerbin orbit, including the insertion burn. It is expected that about 400 m/s can be saved if the aerocapture maneuver is successful.

Finally, a new launch rocket was designed in order to put the spacecraft in low Kerbin orbit. The design deviates from more traditional designs in that it is more lightweight for the amount of payload it can lift to orbit.

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