Monday, January 4, 2016

The Starship Example: "Reactionless" Drive?

No, not really.  You all should know me better than that by now...

     That being said, RocketFans, the drive used by our example Hard SF Space Opera Frigate allows for constant acceleration without a net loss of energy or momentum.  And the reaction mass is not expended.

     I'll explain.

     First of all, the thing with this drive is that it uses negative matter.  Negative matter, if you're not hip to the latest fringe physics and whatnot, is the stuff that they say will make the Alcubierre Drive work, hold open wormholes to allow instantaneous interstellar travel, create gravity in spacecraft, remove gravity from flying cars, cure baldness, ED,  and a partridge in a pear tree.  The thing is that there is no such thing as negative matter, that we are aware of.  It's physically possible, however, that such a substance exists, so negative matter is Unobtainium, and not Handwavium, like Dilithium crystals.

Image result for Montgomery scott
Sorry, Scotty.
     Now, I became aware of using negative mass as a propulsion system, by way of Atomic Rockets, of course, via Dr. Robert Forward's Indistinguishable from Magic.  His simple diagram, shown below, shows how that if you put a wad of negative matter behind a spacecraft, and attach it to said spacecraft, the negative mass will push the spacecraft forward with a constant acceleration as if you put a fan on the quarterdeck of a sailboat and used it to fill the sails.

     Only, you know, actually work.

     Now, the esteemed keeper of Hard SF knowledge and wisdom, Winchell Chung, proposed the very important question: How do you stop?  That is where the design our example Frigate comes in.

    Here is an exterior silhouette of our frigate, perpendicular to the long axis. The forward spherical tanks on the nose of our craft hold negative matter  A nearly equal amount of negative matter is held aft, with a empty tanks just forward of that.  There is various pumping machinery and pipage that can carry negative matter between holding tanks.  How is an interesting question, since push on negative mass makes it push back, because it motion is, well, negative of what normal mass would do. But these Space Opera folks figured it out, and probably couldn't explain to us anyway.

     Now, the spacecraft is at rest in this distribution. To accelerate, the negative matter is pumped from the forward tanks to the after tanks.  Now the negative matter is almost all in the rear of the Frigate, and it pushes against mass of the ship.  As long as the all both after tanks are full, the Frigate will continue to accelerate.  At a constant rate.  Without expending any of the negative matter used.  There is a reaction going on here, even if the reaction mass is not spent. The reaction mass is the ship's mass, being repelled by the negative matter.

     If I understand the math right (which is by no means guaranteed),  when you want to stop, you pump the negative mass forward again, and you just stop. There is no net gain or loss of momentum, remember?  I pretty sure that means you don't get to keep the velocity you get when the negative matter's pushing on you.  I'm not sure how that will effect things like getting into planetary orbits and such.  Suggestions welcome.

    The next step for me to take involves purely arbitrary ex cathedra statements about performance of such a drive, and I used the Frigate to make these assumptions and hammer them out.  Here is the current list of Negative Matter Drive Abilities/Limitations:

  • The negative matter is a liquid, with the density (negative density) of water.  So, one m^3 of negative matter masses -1000 kg.
  • In order to propel a spacecraft at 10/sec^2 requires the equivalent of 10% of the spacecraft's mass in negative matter.
     The Example Frigate carries the equivalent of 30% of it's mass in negative matter, so it can accelerate at 30 m/sec^2.  Indefinitely.  Something about that seems off, don't it?

    Before too long, you'll be up to relativistic speeds, and that is a problem.  Not the physics of it; the ship will accelerate more and more slowly as the effects of relativity make the Frigate's mass increase.  But the crew would be long since dead, and all of the electronics fried by radiation as even normal average photons compress into gamma rays. 

   So the spacecraft's maximum velocity  duration of acceleration is dictated by the degree of particle shielding.  There is a bit of negative matter left in the forward tanks, and that will help repel some of the evil rays, but still, 5% lightspeed is a good number for a maximum virtual velocity for our Frigate.

   You may be asking why I think that 0.05c is a good speed limit?  Well, assuming an acceleration of roughly 2.8 days, a stop to rest, and another 2.8 day acceleration, you could get from Sol to Neptune, the radius of the main bulk of the Solar System, in about a week. So it's a good number from a meta-position, and justified by physics.

     Blocking radiation is not the only reason for the extra negative matter; you'll need it for FTL travel, after all.  But that's another post.
     The implication of such a drive are many and varied.  For one thing, being able to travel from one end of the Solar System to the other in week makes fighting interplanetay wars similar to the island hopping campaigns of WWII in terms of travel times.  Revolt of distant planetary colonies would be much more difficult, due to the shorter travel times.

     But that's just the begining - Ships with a negative mass drive expend no reaction mass. They keep it, dragging along behind them trough a loophole in physics.  Small craft, even star-fighter sized craft, have the same "range" as dreadnoughts - limited only by power generation and supplies.  As for acceleration, it would be a lot easier to give a 10-ton fighter ten gees worth of negative matter than a mile-long, million-ton battle ship.  And if such small craft accelerate long enough, they could easily become Relitivistic nighmare missiles.  Since I don't want that, I will have to make a new addition to my list:
  • Negative Matter is too expensive to waste on disposible spacecraaddition
     Hey, since ships lose all momentum when you move the negative matter around, Disposible attack vehicles and recoverable star fighters use the same amount of remass!  And since NegMat is so expensive, you want to recover any craft using it.  So...Star Fighters are back on the table.  Huh.
     But this rule leads to even more consequenses, and they must be thought out and dealt with.  For on thing, if NegMat is too pricey for use on missiles, it's too expensive to use on things like cargo crates, hover boards, and even the classic Air/raft of Traveller fame.  Fortunately, I just read a post by fellow RocketFan Rob Garrita on the topic of Air/rafts that actually inspired a solution to this conumdrum.

    Remember the fun thing about NegMat, how it is not consumed?  That means that we can pump the stuff from our ship into holding tanks in cars, crates (or, at least, a hover-dolly) and whatever, and return it later.  It also means that if you're tooling around in an Air/raft, and lose it, get it stolen, blowed up or any of the things that can happen to Players in a RPG, your mother ship has at the very least lost some of its accelration.  Worst case, you can't raise ship.

    But it also means that said players can steal other people's Air/rafts, and take their NegMat... as treasure.

    And that, RocketFans, is how worldbuilding leads to plots and ideas based on the technology you develop, Hard SF style.


  1. Ooh, interesting! I'm not sure how negmatter can prevent you from drifting... that implies a privileged frame of reference. But it does explain the sort of 'antigrav' we see in Star Wars hover vehicles. They carry just enough negmatter to offset the pull of gravity, held below the vehicle. Pod racers use it for lift, but various jet turbines etc for thrust. Hmmm... turning a negmatter-propelled vehicle? I guess you just have different amounts in each tank...?

  2. I don't understand why you'd stop immediately if you pumped the negative mass forward. Wouldn't you just induce an acceleration force from the front (instead of the back)?

    1. Like the diagram shows above, at rest the NegMat is divided; half aft and half pumped back forward. This balances it out.

  3. I love the post, but decidedly must fix something. While the neg matter is pushing matter wouldn't the matter push back with equal force? The only way this works is if neg matter does not have personal inertia and newtons laws of energy conservation a broken. Sure it is negative gravity, but still energy conservation should apply as it is a mass albeit a strange on.
    Do please let me know what you think. Seriously interested, not trolling.

  4. As for turning, I'd use Control surfaces like aircraft in atmosphere and an RCS in space. Varying the amound of NegMat in the tanks is possible, but I would imagine fiendishly complex...

  5. Michael: Part of the weirdness of Negative Matter is that while NegMat repels positive matter, Positive Matter is still attracted to NegMat! That's how the drive works: Negative matter pushes the spacecraft away, while the spacecraft pulls the NegMat along with it.


    1. If 1 kg of negamatter acelerates upward at 1G using one kgf while in a 1G field, the strength of the field of the air-raft becomes interestingly negative...
      And trivial. The gravitational field is about 4.03e-37 per kg @ 1m. Since the Air-raft is a rough box of 3x3x6m..., the median distance is √(3²+1.5²)=√(9+2.25)=√(11.25)≅3.354m, but the upward thrust there is at a slope of 1/2, so only about half of it's force matters, so F=sin(60°)*4.03e-37•M/11.25=(0.866•4.03•14/11.25)e(3-37)=48.86e-34 kg/m2² or converting to m/s², 3.5e-28m/h², or 2e-23m/d²....

      and is using 14 tons of negamatter.
      You're not going anywhere with it in anything resembling a reasonable time.

    2. Aramis....I did NOT follow that at all. I'm not ashamed to admit it. I have come to the conclusion that NegMat needs to have 10 times the density of water to do what I want. So...10% of an Air/raft's VOLUME in NegMat would be enough to to cancel it's weight in a 1G field. That jut get it hovering; neutrally boyant - like you said, it ain't going anywhere. The easist way to get the thing to move would be to have ducted fans, hybrid engines or some actual propulsion system. It would no longe be analogous to the Traveller Air/raft that way, but MWM made those up a decade before Dr. Forward published Indistinguishable from Magic, so there you go.

      BTW, I love your stuff on COTI!

  6. I didn't follow either, but my understanding is that the gravitational attraction between positive and negative matter isn't terribly useful for small craft (you would have to make spacecraft the size of moons to get appreciable accelerations that way).
    Another major issue is the nature of negative matter. The Wiki page says that -ve and +ve matter, should they touch will simply cancel out - not annihilate as antimatter is wont to, but simply disappear without a trace (leaving behind nothing but their combined momentum, which is somewhat awkward, since that makes no physical sense). Thus if you use -ve matter, you have to keep it suspended away from your +ve matter hull, possibly via electromagnetic/electrostatic means.

    I've been thinking about Hard SF Space Opera as well, and the best idea I've got so far is to use negative matter as ballast. A craft with near equal amounts of +ve and -ve matter would be able to accelerate at tremendous rates, even with a very weak drive system. In fact performance for such a system would be driven not by the power of your drive, but by the engineering considerations that drive how accurately you can measure and control the mass of your spaceraft.


Questions, comments, criticisms? All non-Trolls welcome!