Chair Force Engineer

Monday, April 18, 2011

Polar Express

One of the most surprising aspects about the recent Falcon heavy announcement was SpaceX's decision to make Vandenberg Air Force Base the initial launch site for their new heavy-lift rocket. The selection of Vandenberg indicates that Falcon Heavy will be launching to the south and south-southwest, putting payloads into retrograde and highly-inclined orbits like the ever-useful sun-synchronous orbit.

Highly-inclined orbits have many advantages, particularly for sensor missions. They can observe a large fraction of the earth's surface due to the wide range of latitudes they travel. Sun-synchronous orbits provide constant coverage by passing over the same point on the earth's surface at the same time every day.

Most satellites that have been launched into highly-inclined orbits have been military in nature. So it stands to reason that the Pentagon may have some uses in mind for the Falcon Heavy. In the past, the Pentagon has expressed little interest in rockets bigger than the Titan IV (which replaced the shuttle's military mission) or Delta IV Heavy. The only proposed payload requiring a bigger rocket was the Zenith Star space laser.

I have always viewed Falcon Heavy as a launcher to support manned spaceflight, able to lift components and propellant for space stations, or for spacecraft designed for flight beyond earth's orbit. Yet those payloads are best assembled in low-inclination orbits and launched to the east from Cape Canaveral. This puts the spacecraft in an inclination that will not requiring the burning of too much propellant as it departs on a plane connecting the earth with the intended destination (moon, Mars, asteroid or other celestial body.)

Thus far, no human has ever launched into a near-polar orbit. Originally the space shuttle was supposed to fly some missions from the "cursed" SLC-6 launch pad at Vandenberg, a plan that was cancelled after Challenger was lost. The Vandenberg missions necessitated the shuttle's big wings, so the orbiter would have enough range to glide cross-track back to its California landing site after one orbit. (After all, the earth processes roughly 45 degrees after one orbit period for a shuttle in low earth orbit.) Because the Van Allen radiation belts emanate from an axis passing near the earth's poles, the astronauts would be exposed to a much higher radiation dose than on missions with lower inclinations. For winged spacecraft like the shuttle, the near-polar trajectories greatly limit launch aborts similar to the Trans-Atlantic Abort for easterly launches, which would allow a shuttle to land downrange (no luck landing a shuttle in Antarctica!) This is less of a problem for capsule-type spacecraft, which can set down anywhere in the ocean as long as a recovery ship is within a reasonable distance.

So what does this mean for Falcon Heavy's potential payloads? At this point it's more of a mystery than the Falcon Heavy upper stage or the first-stage crossfeed system (which will be the subject of an upcoming post.)