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Old 05-09-2017, 07:22 PM   #1
Michael Thayne
 
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Default [Spaceships] Physics-respecting superscience

I've been thinking way too much recently about how to make superscience tech (specifically for space travel and space combat) that "respects" the laws of physics, in the sense of postulating loopholes in currently-known laws of physics rather than ignoring them outright, and making sure the loopholes aren't too abusable by clever PCs. I've come up with a number of proposals on how to do this—feel free to critique.

Newtonian Tractor Beams

Here "Newtonian" means "respecting Newton's three laws of motion". To get "Newtonian" tractor beams in this sense, modify the rules from Spaceships as follows:

The thrust given by the formula in Spaceships (using the smaller ship's mass for the mass part of the formula) becomes the "base" thrust. If the ships are equal in size, they both will experience thrust equal to about 70% of the base thrust, in opposite directions. If their SMs differ by 1, the smaller ship gets 90% of base thrust, while the larger ship gets around 30%. If their SMs differ by 2, the smaller ship gets 95% of base thrust, and the bigger ship gets 10%. (These are approximations to make the numbers easier to work with—I can post more exact formulas if anyone cares.)

For bigger differences in SM, the smaller ship's thrust can be rounded off to 100% of base thrust, while the "recoil" felt by the larger ship is (base thrust / ratio of the ships' masses). If the size difference is large enough that the GM feels the "recoil" is negligible (where "negligible" is context-dependent), normal rules can be used.

A side-effect of these rules is that tractor beams become usable as reactionless drives when close enough to a large object to push off from. Each major battery used for this purpose gives 0.3G of thrust within their range. I'm not entirely sure what that means, since I can't future out what 1/2D means for tractor beams, but it should make this approach more attractive for larger ships than smaller ones.

For example, going by Spaceships 3 an SM +10 ship can use this trick at 1500 miles (or possibly 5000 miles) from an appropriate "push off" mass, while an SM +5 ship will only be able to use it at 300 miles (or possibly 1000 miles). In delta-V terms, the SM +10 ship will get more than twice as much delta-V as the SM +5 ship (though even the SM +5 ship will get hundreds of mps of delta-V).

To be continued.

Last edited by Michael Thayne; 05-10-2017 at 05:19 AM.
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Old 05-09-2017, 08:38 PM   #2
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Default Re: [Spaceships] Physics-respecting superscience

A physics-respecting tractor beam has basically identical performance to throwing out a long, strong, stretchy rope to grab hold of the other object.
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Old 05-09-2017, 09:34 PM   #3
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Default Re: [Spaceships] Physics-respecting superscience

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Originally Posted by Anthony View Post
A physics-respecting tractor beam has basically identical performance to throwing out a long, strong, stretchy rope to grab hold of the other object.
Long but light-weight, so you don't waste energy on the "throwing" bit. And without superscience you might not be able to find any materials strong enough, stretchy enough, and lightweight enough.
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Old 05-09-2017, 10:10 PM   #4
Michael Thayne
 
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Default Re: [Spaceships] Physics-respecting superscience

Contragravity

For something that defies the laws of physics so blatantly, contragravity is, or at least can be, surprisingly non-physics-breaking in the sense of not breaking things you didn't mean to break. Similar to Anthony's point about physics-respecting tractor beams, above, contragravity can be treated like having indestructible, telescoping robot legs that can telescope to a very long length while having negligible weight. In other words, like having a space elevator wherever and whenever you need it.

When trying to exactly calculate travel times, the one thing you need to be careful about is not letting a contragravity booster "cheat" by getting energy both as kinetic energy and gravitational potential energy. For exact calculations of travel times, you can use the fact that escape velocity decreases with the square root of the distance from a planet's center.

If a combination contragravity / ion-drive ship wants to rendezvous with a ship in low Earth orbit—let's say 150 miles above Earth's surface, to be precise—it needs to expend approximately (1 - sqrt[4000/4150]) times Earth's escape velocity, or around 0.13 mps, which would take under 12 hours with 0.0005G of thrust. At the end of the trip, it would have no vertical velocity, and its tangential velocity would only be what it started out with on Earth's surface, less than 0.3 mps. As such, it would "fall behind" Earth's rotation as if it were above geostationary orbit, and would need to expend additional delta-V to rendezvous with a ship orbiting without the benefit of contragravity.
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Old 05-09-2017, 10:11 PM   #5
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Default Re: [Spaceships] Physics-respecting superscience

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Originally Posted by Michael Thayne View Post

A side-effect of these rules is that tractor beams become usable as reactionless drives when close enough to a large object to push off from.
That's a reaction drive, in itself.
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Old 05-09-2017, 11:13 PM   #6
Michael Thayne
 
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Default Re: [Spaceships] Physics-respecting superscience

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Originally Posted by Johnny1A.2 View Post
That's a reaction drive, in itself.
Technically, yes. Just one that has many of the convenient features people expect from reactionless drives.

EDIT: Also, in the sense you're using jet engines, space elevators, ramscoops, and legs are all reaction drives. What makes those four things different than rockets is not needing to carry reaction mass. In fact, what makes reactionless drives so tempting is that we're so used to having a massive object to push off (since we're all standing on one). Letting space ships push off from massive objects at a greater distance than normally possible seems like a much more logical way to fulfill that urge than deciding to ignore physics altogether.

Last edited by Michael Thayne; 05-10-2017 at 05:18 AM.
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Old 05-10-2017, 05:28 AM   #7
Michael Thayne
 
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Default Re: [Spaceships] Physics-respecting superscience

Two simple new systems for building spaceships:

Grav Driver

If gravity control technology exists, and it obeys Newton's three laws, it will be usable to build a better mass driver. Multiply the mass driver's thrust and delta-V by 5, cost and other rules are unchanged.

Gravitic Generator

Above, I talked about how to avoid making contragravity a source of free energy. But what if you embraced that feature of contragravity technology? A gravitic generator uses the stats for the perpetual motion machine in Spaceships 7, but needs a gravity field of at least 0.5G to operate.

GMs who want to pay lip-service to conservation of energy might rule that gravitic generators work by siphoning a planet's rotational energy, in which case they probably won't work on a planet that's tidally locked to its parent star.
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Old 05-10-2017, 06:07 AM   #8
Michael Thayne
 
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Default Re: [Spaceships] Physics-respecting superscience

Stardrive Limitations

Stardrives raise many awkward physics questions. Here are some proposed limitations that constrain the awkward questions:

Hyperdrives and jump drives: One way to limit hyperdrives (and jump drives) is as follows:

1) Such drives require first being in orbit around a star, and that furthermore ships always come out the other side of the jump in orbit around a star. GMs may wish to limit the eccentricity of the orbits that jump drives work for, but this isn't strictly a requirement.

2) The ratio of the mass of the starting star to the radius of the starting orbit must be equal to the ratio of the mass of the endpoint star to the radius of the endpoint orbit. For example, since Proxima Centauri is 0.123 solar masses, a ship jumping from, say, the Earth-Sun L4 point to Proxima Centauri would have to emerge 0.123 AU from Proxima Centauri. A mission to Proxima Centauri b would have to travel the last 0.05 AU using slower-than-light engines.

The "initial" star and the "endpoint" star can be the same, but rule 2 means in-system use of drives following these rules can only get you around within an orbit—say high Earth orblt to a station at Earth-Sun L4. If the drives work with highly elliptical orbits, they'll speed up elliptical transfer orbits without reducing the delta-V requirements. If they require low-eccentricity orbits, most within-system travel will be remain fairly slow, though pilots won't have to worry about launch windows.

Furthermore, ships don't have to emerge from hyperspace orbiting in the same direction as other nearby objects. This means a warship making a kamikaze or hit-and-run attack on Earth could emerge from hyperspace moving at 37 mps relative to Earth (i.e. twice Earth's orbital speed, which is 18.5 mps).

Warp Drives: Rather than being treated as reactionless drives, warp drives can act as a multiplier on reactionless drives. When turned on, they instantaneously add pseudovelocity equal to [(warp multiplier - 1) x ship's real velocity]. For this approach, a ship's velocity always means "velocity to whatever celestial body is exerting the strongest gravitational force on the ship" (using a weighted average of two celestial bodies' frames of reference may be appropriate at a Lagrange point, or in other special situations).

There's a downside to this type of warp drive, however: gravitational forces on the ship are increased by the warp multiplier. This mean warp drives don't reduce a ship's delta-V requirements for any given maneuver, they just reduce the amount of time the transfer orbit or whatever takes to execute.

The choice of maximum possible warp factor is an important decision for GMs using this rule. I recommend a maximum warp multiplier of 1200, regardless of number of warp drives installed (i.e. multiple warp drives provide no benefits other than redundancy). This means that "near c speed" becomes "one parsec per day".

Given a maximum warp multiplier of 1200, and if we assume ships far away from any star use the frame of reference of the Milky Way as a whole, time travel with warp drives becomes possible, but difficult—it requires the use of unusually fast-moving stars, which could then become important plot devices in a campaign.
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Old 05-10-2017, 10:11 AM   #9
Fred Brackin
 
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Default Re: [Spaceships] Physics-respecting superscience

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Originally Posted by Michael Thayne View Post
A side-effect of these rules is that tractor beams become usable as reactionless drives when close enough to a large object to push off from..
It would only be limited in this fashion if it really did work like hands at arm's length. You couldn't attract (or move towards anything) that would slip through the tractor's "fingers".

If it isn't that arbitrary and does affect the local hydrogen atoms it's a reactionless drive at all times. Space isn't ever quite empty. It would also seem to be a much superior ramscoop.

Oh, and tractor beams in atmosphere work more like long-distance vacuum cleaners than anything else.
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Old 05-10-2017, 10:17 AM   #10
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Default Re: [Spaceships] Physics-respecting superscience

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Originally Posted by Michael Thayne View Post
Stardrive Limitations

Stardrives raise many awkward physics questions.
If you want your stardrive to really be physics-respecting, then you must consider that quantum mechanics (generally) and general relativity (in asymptotically flat space-time, like just about anywhere you will be leaving from or going to) conserves electric charge, energy, momentum, and angular momentum locally. That means if you "warp out" or "hyperjump" you need to leave your mass (and charge) behind, moving with your last velocity at the spot where you jumped/warped/whatever. When you reach your destination, there needs to be pre-existing mass there moving with the right velocity for you to appropriate (unless you want to deal with the weirdness of isolated negative energy/mass, with the resulting runaway instabilities that will probably destroy your game world if not your universe in short order). For example, you might have "transfer booths", one at each star system. Step into one, and the people at the other end throw your mass of gravel into their side. Then you are interchanged with the gravel, energy and momentum and whatnot are all conserved locally, and physics is happy. This is similar to how wormholes would work (which are valid solutions to the Einstein equations in general relativity).

Alternately, you could use the loophole that these conservation laws only apply in asymptotically flat space-time. If there is a path of highly curved (twisted, contorted, convoluted) space-time between two star systems, then conservation of <blah> only applies when considering the path as a whole, not within the path (although there may be other conservation laws that apply, depending on the symmetry of the path). So if you find one end, you could warp to the other end and still satisfy physics. This is similar to the concept of a Krasnikov tube (a valid solution to the Einstein equations in general relativity, and which make a sort of rail-way to the stars).

However, if you have a "start anywhere, go anywhere" style of FTL, you don't have physics-respecting superscience. Even if it is "start anywhere within this region of this star system, go anywhere within that region of that star system" - since areas around stars are pretty close to asymptotically flat (or else we wouldn't have nice Keplerian orbits and tidal forces would rip us to pieces and all sorts of other strangeness that we don't observe).

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