Earth Orbit to Lunar Surface
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Introduction to Translunar Orbits

Ted Hansen

A direct launch can put a vehicle on an intercept path to the moon, or the libration point, without requiring time spent in parking orbit. This method requires precise timing of the launch, with some variability in launch time (launch window) if the launch azimuth and the launch vehicle's pitch profile can be varied through the launch window of perhaps several minutes or tens of minutes.

Launching to a parking orbit gives the launch segment an easier job because the requirement to vary the pitch profile goes away. Instead, a variable time in parking orbit is required to absorb launch timing variation. Variable launch azimuth is still needed, as with the direct launch, to have a launch window.

If the vehicle can last an extended period of time in parking orbit, it can wait for either of the two times per month that the moon passes through the parking orbit plane, to perform the injection burn into translunar orbit. The parking orbit's inclination can be anything; however, an inclination equal to the moon orbit's (28.5 deg on the average) will require slightly less orbit transfer fuel. If the parking orbit's plane is not the same as the moon's, the translunar trajectory will be inclined to the moon's orbit but this is no problem. No plane change is required, and the only reason the fuel requirement increases slightly is that there is a slightly different cumulative perturbation from the moon's gravity acting on the trajectory.

A couple of interesting approaches should be mentioned. One is the GetAway Special (GAS) proposed by the Iowa State University Aerospace Engineering class several years ago for a Shuttle flight. It was to fly from the Shuttle bay to lunar orbit without expending fuel at all. The propulsion was the reactive effect of solar pressure impinging on solar sails which were to be held at carefully guided angles so as to always be increasing the orbital energy on the way to the libration point, then decreasing trajectory energy from there to lunar orbit. This approach needs to be launch to a parking orbit and the flight time is long (months or years).

Another approach uses extended periods of time in the zones of roughly equal gravitation (fuzzy boundary) between earth and sun and between earth and moon to achieve the trajectory energy change equivalent to orbit transfers. This also requires long flight time (months or years) and involves a close flyby of the trailing face of the moon. It ends in a very large orbit about the moon, near libration point distance.

Conventional orbit transfer burn flights require three of four days for the translunar portion.

Earth Orbit to Lunar Surface

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