A unified theory of suborbital docking and refueling

Combining the concepts proposed for Black Horse (above) with another proposed spaceflight provides insights into suborbital docking and refueling. (Credit: USAF)


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In 1994, Mitchell Burnside Clapp briefly considered suborbital refueling as part of the Black Horse's development. Calling it „a speculative idea”. In 2004 And 2005Alan Goff proposed FLOC for fleet-launched orbital craft and wrote two papers on suborbital docking.

What is fascinating is that both Goff and Clapp separately explored two sides of the same concept. A novel option for solving the RLV and SSTO conundrums is what we call the „suborbital maneuver”. Lack of unified theory.

Suborbital refueling and suborbital docking can now be compared equally with the same rocketplane. And the results are nothing short of amazing.

Based on my interest in the topic (see “Suborbital Refueling: The Road Not Taken,” The Space Review, February 3, 2020), creating an Excel spreadsheet allows me to compare the two. In fact, a major difficulty is that the vehicles in the above documents are completely different: Black Horse and FLOC are like Laurel & Hardy. Imagine if a similar rocket plane could be inserted into an integrated docking and refueling spreadsheet instead. This will allow a direct comparison of suborbital docking and suborbital refueling.

But to do that, I realized that I had to recalculate—again proof—check the numbers found on the two sheets in Excel spreadsheet formulas. Not because I doubt Klopp and Goff, but more a matter of confidence. To me, this is tantamount to carving two separate doctrines into granite. Well, I managed to do this Attached spreadsheet.

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For fairness, I've included screenshots of the calculations that appear in those papers. I recalculated Klopp's two examples and Goff's FLOC in separate spreadsheets. One can see that the Excel spreadsheet formulas (above) bring the same results as the screenshots in their documentation. As for the fourth paper: It is integrated theory.

It is now possible to compare Clap and Goff equally: suborbital refueling and suborbital docking, with the same rocketplane. And the results are nothing short of amazing.

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

  • The MUST rocketplane consists of kerosene and liquid oxygen (LOX) and liquid hydrogen in the wings.
  • It uses classic turbofan power and kerosene fuel, except hydrogen is poured into the exhausts for a 400% thrust boost. That way the vehicle must accelerate, first to Mach 2 and then to Mach 3. Air breathing in this aircraft removes 2,000 meters per second out of 9,300 meters per second. Only 7,300 meters per second are made by rockets.
  • First it's time to launch a tri-propellant rocket that runs only on kerosene fuel. The spacecraft is now climbing at a 30 degree angle and the jets are stopped.
  • As it ascends, it gradually switches its rocket fuel from kerosene to hydrogen, both of which burn the LOX oxidizer.
  • One last trick was to make orbit: the twin rocket plane followed the first on its ascent trajectory. Now at 6,000 meters per second, the two vehicles exchange liquid oxygen between them: enough to go into orbit. For the tanker, it goes for re-entry and landing drops. There, it takes some kerosene for its turbofans and returns home.
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This is the basic CONOPS: two vehicles, one refueling. This is the final development of the Clapp concept. And then there's Alan Goff's own idea: multiple docking. It doesn't even work on two or three vehicles. But „flocks” of four, six and eight bring surprising payload numbers into orbit.

Ultimately, the combination of Clapp and Goff's ideas around the same large, trippropellant rocketplane would truly open up the space frontier. And with two more refuelings — Earth orbit and an Earth-Moon L-1 Lagrange point — the same rocket could land on the Moon. Just think: one vehicle from the surface of the Earth to the surface of the Moon and three refuelings.


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