Suborbital Refueling

Till now I’ve been writing about concepts that have been proposed elsewhere. My contributions have been in doing some detailed calculations and making the calculation methods available. And perhaps thinking on a larger scale. This time, we’re off on some new territory. I’ve seem one figure on a website that looks similar, but now I cannot find that reference.

Suborbital refuel is a concept that combines a conventional rocket launch, with a gun-launch system. The idea is to place a “delicate” payload on top of a conventional rocket. The rocket boosts to about 1/2 of orbital velocity and then follows a ballistic, suborbital trajectory. For mass efficiency, the vehicle may drop the empty propellant tanks. Meanwhile, an earth-based accelerator (or “gun”) launches a projectile that contains more propellant and a docking mechanism. The gun launch is also for 1/2 of orbital velocity and along the identical trajectory. The two vehicles quickly dock while well above the atmosphere. The docking mechanism includes fuel lines, so the combined modules can reignite the rocket engine and continue to orbit. The advantage of refueling is that the mass fraction (final mass to initial mass) of the rocket is about four times better than a single-stage-to-orbit rocket. There are many other combinations possible. For example, the gun launched module could include a rocket engine so that hooking up propellant lines can be avoided.

It is important to realize that the vehicles are above the atmosphere during docking, and they are following what is really an orbit. It’s just that the orbit eventually intersects the earth. The docking is therefore similar to a conventional orbit docking, except that everything has to be accomplished in a few minutes.

There is an alternative version of this system that uses oxygen coming down from orbit instead of propellants being launched from a gun. The idea is to use the atmospheric harvesting idea to gather oxygen in orbit. A propellant laden vehicle then partially reenters the atmosphere, bleeds velocity, and skips back up to match trajectory with the rocket from earth.

Schematics of the concept and an initial study are available on the main website. The only study to-date is a calculation of the time available for docking for various initial trajectory assumptions. We also look at the required gun launch angle. I had held off publishing this information because I had meant to do a study of the vehicle masses to demonstrate the potential cost savings. But it’s been so long since I posted I thought it best to put up what I’ve got and work incrementally. Too many fall projects on the old house.

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