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Black Horse
Combining concepts proposed for Black Horse (above) with another proposed spaceplane offers insights into suborbital docking and refueling. (credit: USAF)

A unified theory of suborbital docking and refueling

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In 1994, Mitchell Burnside Clapp briefly considered suborbital refueling as part of the development of the Black Horse, calling it “a speculative idea.” In 2004 and 2005, Allan Goff wrote two papers related to suborbital docking, proposing FLOC, for Fleet Launched Orbital Craft.

What is fascinating is that Goff and Clapp separately explored two sides of the same concept. Let’s call it “suborbital maneuvering,” an innovative option to solve the RLV and SSTO conundrums. What was lacking was a unified theory.

It is now possible to evenly compare suborbital refueling and suborbital docking through the same rocketplane. And the results are nothing short of astonishing.

My objective, based on my interest in the topic (see “Suborbital refueling: a path not taken”, The Space Review, February 3, 2020), was to create an Excel spreadsheet where the two could be compared, back to back. Indeed, a major difficulty is that the vehicles in the above papers are radically different: Black Horse and FLOC are like Laurel & Hardy. Imagine if instead the same identical rocketplane could be plugged into a unified docking and refueling spreadsheet. This would allow a direct comparison of suborbital docking and suborbital refueling.

But to do that, I realized that I had to be able to recalculate—re-proof—the numbers found in the two papers and validate that in Excel spreadsheet formulas. Not because I doubt Clapp and Goff, but instead more a matter of self-reliance. To me, this is akin to carving the two separate theories in granite. Well, I have managed to do this in the linked spreadsheet.

For the sake of honesty, I have included screenshots of the calculations as they appear in those papers. I recalculated Clapp’s two examples and Goff’s FLOC in individual spreadhseets. One can see that the Excel spreadsheet formulas (above) brings the exact same results as in the screenshots from their papers. As for the fourth sheet: it is the unified theory.

It is now possible to evenly compare Clapp and Goff: suborbital refueling and suborbital docking, through the same rocketplane. And the results are nothing short of astonishing.

The vehicle is called MUST: MUltipurpose Space Transport. Its concept of operations (CONOPS) would be as follows:

  • The MUST rocketplane features kerosene in the wings and liquid oxygen (LOX) and liquid hydrogen in the fuselage.
  • It takes off on classic turbofan power and kerosene fuel, except hydrogen is poured in the exhausts for a 400% thrust augmentation. That way the MUST vehicle accelerates, first to Mach 2, then to Mach 3. This airbreathing portion of the flight removes 2,000 meters per second out of 9,300 meters per second. Only 7,300 meters per second are done by the rockets.
  • It is time to light the tri-propellant rocket, which initially runs on kerosene fuel only. The spaceplane is now climbing at a 30-degree angle of attack, with the jets shutting down.
  • As MUST ascends, it gradually shifts its rocket fuel from kerosene to hydrogen, both burning LOX oxidizer.
  • Yet one last trick is needed to make orbit: a twin rocketplane has followed the first in its ascent trajectory. Now at a speed of 6,000 meters per second, the two vehicles make a liquid oxygen transfer between them: just enough to go into orbit. As for the tanker, it glides to reentry and a landing downrange. There, it takes some kerosene for its turbofans and return home.

This is the basic CONOPS for MUST: two vehicles, one refueling. It is the ultimate development of Clapp concept. Then there is Alan Goff’s own idea: multiple dockings. It doesn’t work with two or even three vehicles. But four, six, and eight “flocks” brings astonishing payload numbers to orbit.

In the end, the alliance of Clapp and Goff concepts around the same, large, tripropellant rocketplane would truly unlock the space frontier. And with two more refuelings—Earth orbit and the Earth-Moon L-1 Lagrange point—the same rocketplane could land on the Moon. Just think about it: from the surface of the Earth to the surface of the Moon with just one vehicle and three refuelings.

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