Subject: Space-tech Digest #82 Contents: Marc Ringuette energy-saver space drive? Chris Neufeld Re: energy-saver space drive? Chris Neufeld Re: energy-saver space drive? Joe Beckenbach Re: energy-saver space drive? Marc Ringuette Re: energy-saver space drive? Joe Beckenbach Re: energy-saver space drive? Chris Neufeld Re: energy-saver space drive? Joe Beckenbach Re: energy-saver space drive? ------------------------------------------------------------ Date: Sun, 19 May 1991 16:29-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: energy-saver space drive? It's been too long since we had an off-the-wall physics idea. I have one, and I can't decide whether it works or is just totally nuts. Y'know how you get more energy out of your fuel when you thrust at the closest point of an eccentric orbit? It happens because you get the same momentum out of your rockets, but that momentum is worth more energy. In general, if you gain X amount of velocity when you're already going velocity Y, the energy gain is 0.5m[(X+Y)^2 - Y^2] = 0.5m[2*X*Y + X^2]. So if we arrange somehow to thrust our rockets when we're going very fast, we get the same momentum but more energy. Maybe we can make use of that! How about this proposal, then, for thrusting a rocket in a straight line? Divide our rocket into two halves, with a really strong spring in between them, which begins in the charged-up position. Release the spring, and thrust the front half while it's going very fast; then let the spring bring the halves together with some extra energy stored in the spring, which is a bonus. Here are some diagrams with numbers: Start with a charged up spring, both halves at same velocity: 500kg 500kg _ _ |_|--/////--|_| --> --> 1 km/s 1 km/s When it's time to thrust, let the spring go, pushing the front half faster and the back half slower. _ _ |_|--/\/\/\/\/\--|_| -> ---> 0 km/s 2 km/s [strooong spring!] While the front half is going faster, apply your thrusters, which are located in the front. You get momentum and a lot of energy (because the front half is going so fast). thrust: eject 1kg backwards at 4 km/s. v v _ _ _ _ _ _ _ |_|--/ \_/ \_/ \_/ \_/ \--|_| -> ---> 0 km/s 2.008 km/s (energy gain = 8MJ) Let the spring bring the halves back together. You now have all the momentum from the firing of the rocket, but your spring is "extra charged" because of the extra velocity the rocket created between the two halves. _ _ |_|--/////--|_| --> --> (K.E. = 4MJ) 1.004 km/s 1.004 km/s (4MJ energy in the spring) Wild, huh? This is the same velocity we'd get by doing the thrust with the two halves connected...but we can use the 4MJ of extra energy in the spring to power our electrical generator for the next period of thrust, or whatever. We've saved half of the energy we would use if we didn't have the spring. I guess it only makes a big difference in situations where the exhaust velocity is some small multiple of spacecraft velocity. But still! It's a real head-scratcher. So am I halluninating, or what? ----------------- -------------------------- -------------------------------- | Marc Ringuette | Cucumber Science Dept. | What does a blonde say when | | mnr@cs.cmu.edu | Cranberry Melon Univ. | you blow in his/her ear? __ | | 412-268-3728 | Pittsburgh, PA 15213 | "Thanks for the refill." \/ | ----------------- -------------------------- -------------------------------- User: mnr@DAISY.LEARNING.CS.CMU.EDU Rabin-Modulus: UaCUDY+bd7EvAtkaZ9S1fRz7LO/MONro+pVvD/tNS52wf3tJZp4dcOKmEYNlhZh6 ----------------------------------------------------------------------------- | I use the "rpem" public domain public key email system, with the above key. | | Clip and save! For info on rpem, ask me, or riordanmr@clvax1.cl.msu.edu. | ----------------------------------------------------------------------------- ------------------------------ From: Christopher Neufeld To: space-tech@cs.cmu.edu Subject: Re: energy-saver space drive? Date: Mon, 20 May 1991 02:05:08 -0400 Well, it looks like this device might save energy, but it costs in reaction mass. If the spacecraft is fixed the final state includes 1 kg of matter moving to the left at 3 km/s, the bouncing spacecraft has that same kilogram moving to the left at 2 km/s, so you need 50% more reaction mass for a given delta-v in the example. I don't think this is practical. However, if you want to increase delta-v for a given nozzle velocity, you might run the bouncing ship in reverse, so that the engine fires when attached to the back of the spacecraft, and that part is moving backwards relative to the centre of mass. This requires that there be a system on board to inject energy into the spring system, so if your muzzle velocity is limited to some number you might gain by putting the rocket on a bungee cord and pumping the cord with your extra power. Looks like no free lunch after all, but I'll check again at a more sane hour when I have more confidence in the computational ability of my mind. Christopher Neufeld preferred: neufeld@aurora.physics.utoronto.ca also: cneufeld@pnet91.UUCP cneufeld@pro-cco.cts.com cneufeld@pro-micol.cts.com ------------------------------ From: Christopher Neufeld To: space-tech@cs.cmu.edu Subject: Re: energy-saver space drive? Date: Mon, 20 May 1991 11:20:45 -0400 I wrote: > Looks like no free lunch after all, but I'll check again at a more >sane hour when I have more confidence in the computational ability of my >mind. > Yes, there's no useful symmetric situation which lets you say "same effects but with more available energy." The clue to this is that the concept of "faster exhaust velocity" is not invariant under a galilean velocity transformation. Something's got to give. In this case you can inject more energy into the spring for fixed delta-v of the spacecraft, but at the expense of more reaction mass than in the fixed-ship design. Reversing the concept you get more delta-v for a given reaction mass, but you have to inject additional energy into the spring system since the motor actively damps it. This second scenario might be useful if you've got a surplus of energy and are limited in nozzle velocity of reaction mass. I can't think of a practical spacecraft design to take advantage of the increased reaction mass efficiency of backward propagating thrusters. I have visions of spacecraft with big conveyor belts running over two opposite faces and two thrusters zipping along the surface first from bow to stern when they are on opposite sides of the craft and both firing, then from stern to bow when they are off and big electric engines are running the conveyor belts back up to speed after the energy they lost from the reaction force of the engines. Another scheme has two monstrous counter-rotating spars each with two engines (for mass balance) and the ship mounted at the midpoint of the two spars. As each engine comes to the orientation such that it is pointing in the direction in which the reaction mass is supposed to be fired, they fire up briefly, while the craft at the hub spins up the spars again with electric motors. '\ /` upper left stick going counter-clockwise \ / upper right going clockwise v --> X when both sticks are straight up, top thruster on / \ CCW spar and bottom thruster on CW spar fire `/ \' spacecraft at the 'X' One pair of motors could be replaced with dummy masses, but who wants to bring ballast as heavy as an engine into space? The spars could be shortened but then the dummy masses would have to be larger to compensate. As I said, neither of these designs looks practical. The first because it would be hard to get the velocity of the conveyor belts up high enough to improve the reaction mass efficiency much, and the second because those spars would wrap like string about the central hub if they were any reasonable length. Aluminum spans a kilometre long are very flexible unless made very heavy. Maybe if the engines have a low thrust but have to be kept a long way from the crew compartments? The real problem to the spinning wheel design is the problem of applying the restoring torque. You would rather not do it at the hub because of the small lever arm, since that would tend to wrap up your spars around the ship. You obviously don't want to apply a restoring torque with reaction mass thrusters on the ends of the spars! I can't think of an obvious way to couple between the hub and the ends of the spars electromagnetically, and a torquer which circulates a current at the tips of the spars in the ambient magnetic field has to work with fields of about 5 nanoTesla, the typical interplanetary magnetic field. Ahhhh, maybe this. Sorry for the stream of consciousness letter, but it just occured to me. When those two spars are running past each other they can, if they pass close together, push off each other electromagnetically, spinning each other up with energy supplied through wires from the power plant (which we have assumed exceeds the power requirements of the thrusters). Comments? Christopher Neufeld preferred: neufeld@aurora.physics.utoronto.ca also: cneufeld@pnet91.UUCP cneufeld@pro-cco.cts.com cneufeld@pro-micol.cts.com ------------------------------ To: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU Cc: space-tech@cs.cmu.edu Subject: Re: energy-saver space drive? Reply-To: jerbil@ultra.com Date: Mon, 20 May 91 09:29:14 -0700 From: Joseph Beckenbach Marc, I've cc:'ed space-tech in an attempt to keep people's minds at rest that physics hasn't been pulling our legs all these years. [Tossed metaphors for lunch, yum! :-] TANSTAAFL. You've neglected the initial energy in the spring in your calculations, _among_other_things_. Let's look at center-of-mass view, which is moving rightward compared to your original drawings: 500kg 500kg _ _ |_|--/////--|_| spring: 500 kg km^2 s^-2 = 500 MJ Release spring, using stored energy, starting _oscillation_. Assume for simplicity that the separation is 100m and that thrusters are fired instantly at the greatest velocity of the front mass relative to center of mass: 500kg 500kg _ _ |_|--/\/\/\/\/--|_| spring: 0 MJ 1 km/s <-- --> 1 km/s separation 100m 500kg 499kg _ _ |_|--/\/\/\/\/--|_| spring: 0 MJ 1 km/s <-- --> 1.008 km/s separation 100m Finish vibration cycle: 500kg 499kg _ _ _ _ _ _ |_|--/ \_/ \_/ \_/ \_/--|_| spring: -500 MJ 0 km/s -> 0.008 km/s separation 200m 500kg 499kg _ _ |_|--/\/\/\/\/--|_| spring: 0.488 MJ 1.004 km/s --> <-- 0.996 km/s separation 100m 500kg 499kg _ _ |_|--/////--|_| spring: 499.988 MJ 0.004 km/s --> --> 0.004 km/s separation 0m Due to the precision used, this implies a very small net LOSS. If I had the time and some scratch paper, I could show that the spring neither gains nor loses. Using center-of-mass view is valid under the special principle of relativity: change to another frame of reference with constant velocity difference does not change physical laws. Note that you get exactly the same effect no matter when you fire from whichever mass; the energy in the spring merely keeps the two masses together. Joseph Beckenbach --- Joseph Beckenbach jerbil@ultra.com 408-922-0100 x246 An artist is not a special type of person; rather, each person is a special type of artist. ------------------------------ Date: Mon, 20 May 1991 14:04-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Re: energy-saver space drive? To sum up: I was hallucinating. I forgot in my example that the front mass would end up with a mass of 499 kg. The new result is: After thrust: 500kg 499kg _ _ _ _ _ _ _ |_|--/ \_/ \_/ \_/ \_/ \--|_| -> ---> 0 km/s 2.008 km/s (energy gain = 8MJ) After they're back together: _ _ |_|--/////--|_| --> --> (added K.E. = 2.5 MJ) 1.003 km/s 1.003 km/s (1MJ energy in the spring) (2MJ energy in the exhaust) --------------------------- 5.5 MJ total, 1 MJ recoverable We could have achieved this by ejecting the 1kg exhaust at 3 km/s instead of 4 km/s, and it would cost us the same 4.5MJ of net energy. -------- What happened to the analogy with the "thrust close to the planet" case, where you get real gains? I think the problem is, in the planet case the extra velocity you end up with comes from the planet not getting to pull on you for so long. In the rocket case, once you've ejected the reaction mass, the system is closed and you can't play those games. Marc ------------------------------ To: Christopher Neufeld Cc: space-tech@cs.cmu.edu Subject: Re: energy-saver space drive? Reply-To: jerbil@ultra.com Date: Mon, 20 May 91 09:55:21 -0700 From: Joseph Beckenbach In his letter, Christopher Neufeld writes with two craft designs with backward-moving thrusters. This is no win over fixed thrusters. The heart of the problem is transferring the energy of the thrusters to the ship. The energy needed to provide that coupling would be comparable to the output of the engine itself. The scissors structure would need to have a much more powerful motor at the fulcrum than at the engine end, otherwise the thruster starts to forward-propagate. The belt has similar problems in order to prevent it from being a treadmill on which to test engines. TANSTAAFL. While having windmills in space would be neat :-) maneuvering and control concerns tie us to more compact structures when the entire propulsion complex is part of the craft. Maybe ramjets at the end of the scissors, but these would forward-propagate for efficiency and still cause structural problems. Joseph Beckenbach --- Joseph Beckenbach jerbil@ultra.com 408-922-0100 x246 An artist is not a special type of person; rather, each person is a special type of artist. ------------------------------ From: Christopher Neufeld To: space-tech@cs.cmu.edu Subject: Re: energy-saver space drive? Date: Mon, 20 May 1991 14:56:16 -0400 In his letter, Joseph Beckenbach seems to misunderstand my assumptions in my gedanken designs. I state quite clearly (I thought) that the scenario is one in which there is more energy available than can be used by the thrusters, and those thrusters are limited in exhaust velocity. For instance, consider a pellet gun as the engine, firing reaction mass backward at 10 km/s. By the design of the pellet gun, it simply can't manage a higher muzzle velocity no matter how much energy you put into it. Well, if you've got energy to burn, is there some way of improving the muzzle velocity? Yes, you can do as I suggested. In inertial space the separation velocity of the pellets and the centre of mass of the spacecraft is higher in the windmill design, giving an effective increase in muzzle velocity. This means a given amount of reaction mass can supply a larger delta-v. The same considerations apply for the conveyor belt design. So, explicitly, here are the assumptions which went into my analysis again: exhaust velocity limited to a value not tremendously higher than a reasonable windmill tangential velocity; energy to burn; limited reaction mass. In this setup it just means that the engine burns more energy and achieves a higher exhaust velocity. I never claimed this gives "a free lunch," merely that it improves the reaction mass efficiency of a motor whose intrinsic limitations prevent it from achieving a very high exhaust velocity. Is this clearer? I doubt that this would be a useful design approach, but I thought it would be interesting to point it out. Christopher Neufeld preferred: neufeld@aurora.physics.utoronto.ca also: cneufeld@pnet91.UUCP cneufeld@pro-cco.cts.com cneufeld@pro-micol.cts.com ------------------------------ To: Christopher Neufeld Cc: space-tech@DAISY.LEARNING.CS.CMU.EDU Subject: Re: energy-saver space drive? Reply-To: jerbil@ultra.com Date: Mon, 20 May 91 12:39:13 -0700 From: Joseph Beckenbach Chris, Ah, I see now, you're transferring an excess of energy from the central pivots to the reaction mass as it leaves the engine. This is reasonable. As you mention, this is not likely to be a useful approach: if there's so much energy hanging around in the first place, why not go with a different engine which can make use of that energy more directly? Part of my confusion was that your article fell hard on the heels of Marc's posting with the mis-analysed spring. I was still in "heavy critique" mode. 1/2 :-) Now. the spring idea might be useful in a steep gravity well, especially if the low end can selectively be sent into the black hole. :-) Joseph Beckenbach --- Joseph Beckenbach jerbil@ultra.com 408-922-0100 x246 An artist is not a special type of person; rather, each person is a special type of artist. ------------------------------ End of Space-tech Digest #82 *******************