guest blogger john hare
When posting about capturing an asteroid using a Lunar gravity turn, I realized that there is a very useful orbit there that I had not heard of. A station in retrograde orbit at altitudes similar to the moon but just different enough not to impact has a number of very attractive features.
A vehicle coming in from Mars, Venus, an NEO, or any other point in the inner solar system can use a Lunar gravity turn to rendezvous with the station without burning propellant. Clever use of the incoming direction and angle of gravity turn can capture a vehicle with an Earth relative velocity between 1 and 3km/sec inbound. A vehicle coming in at 3km/sec times the approach to chase the moon in its’ orbit. The closing rate on the moon becomes 2 km/sec. With a 180 degree gravity turn, the vehicle is in a 1km/sec orbit Earth relative at Lunar altitude.
A vehicle coming in at 2km/sec Earth relative approaches the moon at 90 degrees to its’ direction of travel and uses a 90 degree Lunar gravity turn to reach the same retrograde orbit. Other incoming velocities will need other angles of approach to get the same result. The big penalty is that Earth approach must be done at exactly the right time of the Lunar month with very precise angles of approach.
Earth to Retro Station is a matter of using a TLI that will allow a 300m/s burn at perilune to put it into the retrograde orbit with the station.
The advantages to returning this way are that no heat shield is needed for the returning vehicles, the vehicles never see the high gee environment of a reentry, and that the complete vehicles are in a parking orbit that retains much of the energy for another mission. Heat shields can be attached at the station if the vehicle is to be sent on to Earth anyway, or just the crew and samples can be returned in a dedicated taxi vehicle.
The heat shield that is left behind is a massive payload boost when you do the numbers. 15% seems to be a common number thrown out for heatshield mass. A ton of vehicle returning from Mars will have 150 kg of heat shield. Propellant must be provided in Mars orbit to put the vehicle on TEI. Depending on the propellants and trajectories chosen, that propellant can mass twice that of the article being accelerated. In this case, that is 300 kg of propellant just to push the heat shield. That propellant must come from Mars, or even Earth itself if in situ propellants are not available. If from Earth, the heatshield and its’ Mars departure propellant mass 45% as much as the ship. Give any mission an additional 45% mass budget and serious value has been added. Or a launch tonnage 70% of the baseline mission for the same job.
For NEO sample return missions the gain is much higher. They must use propellant to rendezvous with the NEO as well as the Earth return boost. If it is a storable propellant and 5km/sec delta V, then a 65% gain is possible, or a ship 60% of the initial mass.
Since the Earth return vehicle need never see high gees, it can be built much lighter for zero gee work only, or have a tether based artificial gravity that doesn’t have to be discarded before reentry. If manned, the vehicle doesn’t need to be able to protect the crew from acceleration with special couches and whatnot.
With the vehicle stopped in a parking orbit with the station, it can be refueled and retasked and sent again. There is no need to build another vehicle and lift it out of Earths’ gravity well for the next job. These vehicles could fly dozens of operational missions for decades with a little maintenance, propellant, and upgrades as the technology requires. Voyagers and Pioneers have lasted a while with no hands on in space.
Leaving the station has some interesting characteristics also. A small burn sends a vehicle toward the Lunar intercept point where a gravity turn can send the vehicle to Earth or an interplanetary trajectory with only stationkeeping propellant. The interplanetary trajectory starts with 3km/sec Earth relative, which should get the vehicle to Mars, Venus, or several other interesting places, again with just stationkeeping propellant.
If these numbers are right, then this would be a path more useful for an interplanetary cycler for convenience and ability to support it than the orbits normally suggested.