BASICS: SSTO stumbling blocks (was Re: SPACE: Roton in New

Michael M. Butler (
Thu, 06 Aug 1998 22:37:32 -0700

Rant on (please indulge me):

Having to bring virtually every nonpropellant ounce you launch (scant consumables and boiloff) back to your landing site causes many margins to be razor-thin. 'Tain't easy.

That decision to not rely on the rotor blades for takeoff lift probably really pained the designers. But that, and the decision to set a price point that doesn't send "industry experts" spinning in an anti-Roton tizzy, are signs that the company in question is (

  1. trying to play ball with the existing space launch cartel(s) _and_
  2. adopting something resembling prudent risk management.

It is possible that Hudson and company's calculations including safety factors suggested that the added mass of the longer rotors, stressed for the added forces, made benefit unclear; or increased boost-phase dynamic loads to a risky level, or simply presented too high an investment risk for the likely initial short-term Return On Investment (ROI).

I seem to see every (self-contained; laser launch doesn't count) SSTO design encountering the same set of problems. The semi-Roton (for such is what Hudson and company are actually describing now; as Lincoln famously observed, calling a tail a leg doesn't make it a leg) might have them to a lesser degree than a full Roton, built with available engineering and materials, would.

If I recall calculations I last made a decade ago, the two biggest problems for SSTOs are:

(1) sensitivity to small thrown mass changes (hence, overall mass ratio changes) and

(2) actual bring-down payload mass.
Boost-phase rotor deployment ought to improve both, or it ought not be done. Even a microscopic upward bump in thrown mass tends to have a negative impact on bring-down payload mass. If the latter goes negative, you're stuck with the old "Destination Moon" (or "Cold Equations") solution.

If you sell your system as just a delivery truck, bring-down mass can be finessed as not part of your business plan--somewhat. But a true space transportation system needs to be able to bring useful amounts of things down, too.

These matters have so far proven to be very tough on SSTOs, even those on the drawing boards. For decades, the only rocket technology thought to be up to the general SSTO surface-to-orbit, orbit-to-surface cargo job was high-mass-flow nuclear (Isp~=1000++). That's not a modern contender for all the usual NIMBY reasons. But modeling, materials and manufacturing technology unavailable in the 1950s make chemically boosted SSTOs tantalizing--the modern space launch golden apple, just barely out of reach.

Even the Shuttle puts atmospheric lift to use during boost. The numbers for true Rotons continue to be intriguing. Eventually, routinely available materials will advance to the point where chemical propulsion can reliably do some SSTO jobs.

Exactly what those jobs will be is still up in the air. I continue to watch Mr. Hudson's efforts with interest, and to wish him every success.

Rant off,


At 00:20 8/7/98 -0400, you wrote:
>On Thu, Aug 06, 1998 at 07:50:39PM -0700, Michelle Jones wrote:
>> mark the reason i am having such difficulty with this concept is that the
>> roton is a re-entry system. you still have to lift the thing into orbit
>> somehow to reenter the atmosphere. the reentry system of modern
>> government launchers is neither the cost driver nor the weight driver.
>Supposedly the Roton will cut launch weight by spinning the engine,
>pumping the fuel into the engine chambers and avoiding heavy pumps.
>The designers considered using the rotors to help lift the rocket,
>but considered the additional risk too great.
>Mike Linksvayer

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