Re: SPACE: Roton in New Scientist

Philip Witham (
Fri, 14 Aug 1998 13:52:11 -0700

At 10:58 PM 8/13/98 -0700, Michelle Jones wrote:
>Michael M. Butler wrote:
>> >... To get into orbit cheaply, you need to build your rockets Big and Dumb...
>yup. looks like there are three showstoppers for well funded amateur groups.
I think we are talking about things that are beyond amateur groups here.

>1.regardless of how big and how dumb a liquid rocket is, one still must pump a
>high volume of cryogenic liquids to combustion chamber pressure.
Selection of the pumping and thrust chamber cooling design is basic to liquid motor design. There are two main schools: Pump fed, and pressure fed. Pressure fed systems are simplest, but the tanks are necessarily heavier. This increases the empty mass fraction relative to a pump fed system. Seeking performance despite the cost, the industry went to pumps. Carbon fiber tanks reduce the pressure-fed weight penalty, and this changes the tradeoff from what it used to be. Truax/Aerojet's second stage designs run at only a 75PSI chamber pressure, taking advantage of the vacuum to allow this without sacrificing expansion ratio (ISP). This also reduces the tank weight penalty.

>3. guidance and control: this is a very complicated problem generally, requires
>a sophisticated feedback mechanism with thrust vector control by means of a
>steerable nozzle for instance.
But it's a well understood problem, with (partially) off-the-shelf solutions. No surprises.

>there are a lot of different concepts all going under the name roton, some
>workable, others not. i have only recently come to appreciate this. the beauty
>of the simplest roton system is that it solves all three of the above, sort of. >centrifugal force pumps the cryogens. you need not have high l/d, since you can >give away some performance. the inherent stability spinning about an axis with >high moment of inertia obviates a flight computer. there are still some difficult >problems, such as how to turn the vehicle, but i have some ideas. a roton could be >conceive which has very few moving parts.
Hmmn. I doubt you can get away without thrust vector control, especially for the second stage. A rocket doing an orbital insertion wants to fly a precise arc, optimizing various losses and winding up in the desired orbit. As for the aspect ratio of the rocket, two points. One, pressure fed rockets have very stiff, strong tank walls, and don't suffer structural problems of pump fed designs. Two, as you make a rocket bigger, the scaling laws say that aerodynamic forces matter less, and so the shape optimizes towards a short, wide one as the size goes up.

>either way, the roton is within the reach of sophisticated amateurs and
>underfunded entrepreneurs.
Only in the sense that any design is, in my opinion: One can afford to play with them, but to field a successful launcher... Just how deep are your pockets?