Sometimes the ideas I throw out are obscure and hard to communicate, and sometimes they are so blindingly obvious that they have been rehashed many times in the decades past. Since I have no feel for which is which, sometimes I throw an idea to the wolves (that’s you) to see which it might be, and also to see if some of the follow ons are equally obvious, or not as it may be.

In my last post on the small tetherocket, the idea was somewhere in the middle. This is one of the possible follow ons that I have thought about before, but only decided to write out as my reaction to some of the feedback in comments.

turbinozzleOnce again my cartoon may be a bit difficult to read. The black lines at 3,000 m/s are expansion nozzles spinning on a tether complex as in the last post. The small rocket inside the curves is fixed relative to the ship with a suggested exhaust velocity of 4,000 m/s retrograde to the ship. The 4,000 m/s exhaust encounters the spinning nozzles with a closure rate of 7,000 m/s. The exhaust bends nearly 180 degrees in the moving nozzle with the gasses retaining the 7,000 m/s velocity relative to the turbinozzle. The gasses exit the turbinozzle at 7,000 m/s nozzle relative which is 10,000 m/s ship relative.  Net Isp just over 1,000.

I suggested 4,000 m/s exhaust velocity for the H2/O2 rocket as it would likely have a low expansion ratio to fit the envelope. I suggest that the near 180 degree turn in the turbinozzle channels would cause shock losses that would cancel any expansion gains from the nozzles.  I believe that the thrust/weight ratio would remain in the 1 m/s range for the bare engine.

This would retain the capability of using any fluid reaction mass available in the solar system from CO2  to water to impure methane if that is available ISRU. The rocket engine would need changing out to a steam engine using whichever fluid is available for a likely Isp in the 500+ range if I see the reactions correctly.

One of the comments suggested beamed energy and hydrogen only as a superior alternative. Nothing any of us said would preclude using beamed energy to drive the reactions. It would solve a number of problems with onboard power if available. Hydrogen may or may not be the reaction mass of choice. The tankage mass and handling properties may well make it second best even if it happens to be available at a particular location.

On one end of the conceptual capabilities is the possibility that I am pessimistic in the capabilities suggested. A larger envelope for the fixed expansion nozzle may make it  possible to get 4,500 m/s exhaust velocity from the fixed rocket which would add 1,000 m/s to the final exhaust for a total of 11,000 m/s exhaust velocity. It may also be possible to recover the energy from the turbinozzle heating to a higher velocity exhaust which could possibly add another 1,000 m/s to the final gas velocity then totaling about 12,000 m/s.  Hopefully the speculative possibility of Isp=1,200 will have a qualified person of two running a few simulations for the entertainment value.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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5 Responses to Turbinozzle

  1. Chris says:

    Funnily enough, I thought about something like this, but integrating centrifugal turbopump with a combustion chamber and deflecting the exhaust. Unobtanium materials may be required. The thinking goes like this: fuel and oxidiser are injected into the inlet of a centrifugal turbopump and ignited. The accelerated exhaust is deflected to provide thrust (a static nozzle). The big question is: how on Earth would the combustion chamber/impeller be cooled?

    It may be easier to realize this concept as a liquid mass driver. I am wondering how alternative architectures, like e.g. sth. like a slingatron would work? Also, I was thinking: the main problem with railguns is erosion of the rails. How about we use a conductive ionic liquid instead? The liquid has to be conductive in order to avoid wasting energy on ionization.

  2. Derek R says:

    You will have a tough time getting to 3 km/s for the rotation speed. The world record is currently only 1.4 km/s. Centrifugal forces will rip the thing apart.

  3. johnhare johnhare says:

    This is tapered tether in microgravity and vacuum, which is capable of considerably more than a flywheel on Earth. Look up rotovator or skyhook tethers.

  4. George Turner says:

    A few minutes ago I was wondering whether it would really make much difference whether you deflected the combustion products before or after full expansion (high gas density at a low velocity or low density at a high velocity). Then I had another thought, related to some thoughts I had yesterday about having oxidizer in a normal orbit slamming into fuel in a retrograde orbit just as they entered a rocket engine that was happening by, converting their Earth relative kinetic energies into thermal energy. Low Earth orbit would add about 30 MJ/kg to the propellant’s specific energy, which is more than the chemical energy in most fuel and oxidizer combinations. Normally that orbital kinetic energy is inaccessible.

    But getting such a collision to work would be extremely difficult, unless it happened at the end of a pair of contra-rotating tethers where you could tightly control the trajectory. 3.5 km/sec tip speeds would add 6 MJ/kg to the propellant, but of course the tips can’t actually hit each other.

    So what if they were like a pair of shears or scissors, but with half a bell on the end. Basically line up the ends and drill a big nozzle aiming directly outward, thrusting directly toward the axis of rotation. In operation, right before they sweep past the intersection point, one tether ejects a quantify of fuel and the other ejects oxidizer, such that the fuel and oxidizer slam into each other with a closing velocity of 7 km/sec right as the tethers come together to complete the combustion chamber and nozzle geometry. It’s a mechanical analog to using microwaves to superheat the fuel far above normal combustion temperatures, and the extremely sharp fuel/oxidizer impact and explosion is naturally a pulsed engine.

    The downside is that it’s probably not going to be as efficient as the original tether concept for a given tip speed due to gas laws and whatnot. For example, if the tether was running at 100 m/sec, it would directly add 100 m/sec to the exhaust velocity, whereas a collision at that speed would only add 10 kJ/kg and in a rocket engine you probably wouldn’t even notice the difference. Examining the effects would require crunching numbers of specific heats, cp, cv, and all the rest. The intersection/opposite impact method would eliminate some of the torques inherent in the original concept.

    Secondary thought: If you mounted the rotating engines or nozzles in the original concept on a pivot that used a four-bar linkage to connect to the ship, they would always point in the same direction in space. However the gain against cosine losses might not offset the increased mechanical complexity.

  5. George Turner says:

    How about I add a new thought. Put the one engine a little outside the arc where the two arms will intersect, and have them release fuel and oxidizer about 45 degree prior to intersection. The propellant thus flies a pair of tangents, precisely flying through a pair of angled holes (or pipes) in the sides of the combustion chamber, where they make an oblique impact with each other. Part of the energy (normal to the impact) is converted to extra heat, and the rest remains as a rearward velocity component. I’m not sure if that component buys anything, though, because the velocity is still going to be sonic at the throat, in which case it will all get converted to extra heat.

    Anyway, it eliminates an open seam and gets rid of tremendous vibrations.

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