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Old 07-11-2022, 05:25 AM   #1
scc
 
Join Date: Mar 2013
Default [Spaceships] Getting A Few Thousand Extra MPS

So I'm playing around with something and I need/want to send manned missions to the star nearest Sol with more or less hard science TL9-10 ships. The crew will be gene-engineered with the ability to hibernate, but still.

Yes, I know, a tall order. I can get to about 4% light speed, the the journey is just over 100 years, but I want to know if options other then just more fuel tanks would work, something like laser sails (Lasers hitting a solar sail) or maybe a similar idea involving Mag-Sails, maybe particle beams?

Anyone got any suggestions/guidelines/rules?
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Old 07-11-2022, 05:53 AM   #2
DemiBenson
 
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Default Re: [Spaceships] Getting A Few Thousand Extra MPS

Quote:
Originally Posted by scc View Post
So I'm playing around with something and I need/want to send manned missions to the star nearest Sol with more or less hard science TL9-10 ships. The crew will be gene-engineered with the ability to hibernate, but still.

Yes, I know, a tall order. I can get to about 4% light speed, the the journey is just over 100 years, but I want to know if options other then just more fuel tanks would work, something like laser sails (Lasers hitting a solar sail) or maybe a similar idea involving Mag-Sails, maybe particle beams?

Anyone got any suggestions/guidelines/rules?
Yeah, you can multi-stage more than once. In fact, there’s no upper limit to the number of times you can multi-stage, so all those MPS stack.
And the final stage only needs a magsail to decelerate, since that will slow you down if relative velocity is faster than (I think it was) 375 mps.
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Old 07-11-2022, 06:57 AM   #3
Aldric
 
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Default Re: [Spaceships] Getting A Few Thousand Extra MPS

Or you can strap your ship to an ice rock with an external clamp and use it to power your choice of fusion drives that support it. Initial acceleration is quite bad due to the additional mass, but it will get better over time.
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Old 07-11-2022, 07:38 AM   #4
Varyon
 
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Default Re: [Spaceships] Getting A Few Thousand Extra MPS

Is a Ramscoop (SS1:21) an option? That's TL10 and gives infinite delta-v, provided your drive uses hydrogen for its reaction mass.

Realistically, given you need to speed up the grabbed hydrogen to the velocity of your vessel, there's a practical limit to how fast you can go, but I think it's in excess of 0.04c. Spaceships doesn't comment on this anywhere I'm aware of, however - as far as it's concerned, once you hit minimum velocity for the ramscoop, it turns your drive into a reactionless engine.
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Old 07-11-2022, 07:56 AM   #5
malloyd
 
Join Date: Jun 2006
Default Re: [Spaceships] Getting A Few Thousand Extra MPS

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Originally Posted by scc View Post
Yes, I know, a tall order. I can get to about 4% light speed, the the journey is just over 100 years, but I want to know if options other then just more fuel tanks would work, something like laser sails (Lasers hitting a solar sail) or maybe a similar idea involving Mag-Sails, maybe particle beams?
The magsails are limited to the velocity of the solar wind (more or less) so that's not going to help. Laser pushed lightsails are possible, but imply *insanely* powerful lasers you may not want in your campaign. Realistically any engine that will get you to a fraction of the speed of light implies world-changing levels of energy are available more easily than a starship, but it's easier to ignore for onboard drives than for arrays of exawatt lasers.

The higher the specific impulse of your engine the faster you can ultimately get, so you want the highest Isp engine in your rule set includes - I believe that's Fusion Pulse for the "realistic" TL10 or less engines in Spaceships, and realistically is going to be some sort of fusion engine using the reaction products as the exhaust directly. Acceleration is almost irrelevant for these sorts of time scales so you might as well go with the smallest engine allowed with that high Isp.

In all probability the answer is going to turn out to be you can't do it within the rules. The simple fact is interstellar travel is *hard*, and sending something as inefficiently massive as actual crews is very possibly always going to be impossible in reality. Expect to need to compromise on the realism of the engine (I mean, even more than by ignoring the massive energy generation implications we already do) to get this to work.
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Old 07-11-2022, 10:41 AM   #6
mlangsdorf
 
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Default Re: [Spaceships] Getting A Few Thousand Extra MPS

Going by the Spaceships rules, you can put 14 systems in a detachable lower stage, 1 of which must be an engine and 1 of which should probably be an armor system to represent the ship's hull structures. A fusion rocket (SS1 p 23) gets 60 mps per tank at TL 10, and a spaceship gets a 40% bonus to delta-V for having 9-12 tanks. So 12 tanks will get you to 1008 mps, which is a little more than half of what you need for a ramscoop to work.

So you're looking at a 5 stage design. Assuming a not-crazy 3,000 ton survey craft at the destination, you have a stack:
Stage 5: SM +9 Survey craft with ram scoop which is the upper stage for:
Stage 4: SM +10 collection of initially empty fuel tanks which are the upper stage for:
Stage 3: SM +11 collection of initially empty fuel tanks which are the upper stage for:
Stage 2: SM +12 collection of full fuel tanks which are the upper stage for:
Stage 1: SM +13 collection of full fuel tanks.

So you have an initial ship that is roughly the size of the largest real world oil tankers (300,000 tons) which accelerates at .01 G to 924 mps, which takes 6 months. It then drops Stage 1, slimming down to the size of a nuclear aircraft carrier, and accelerates at 0.01 G for another 6 months to a total 1848 mps. Second stage is discarded and the ramscoop opens up, allowing continual acceleration for the next 20 years or so, reaching a top speed of about 0.2c, well below relativistic effects.

The next bit is tricky - the engines need to do a retrograde burn while the ship the ram scoop is still pointed in the direction of travel. Maybe there's a rear mounted ram scoop for this? Regardless, another 20 years of deceleration drops the speed down to just under 0.01c and the ram scoop shuts down. Burn all the fuel in Stage 3 to get down to 924 MPS and discard it, then keep burning fuel. Assuming that the speed of the target system is only a few MPS relative to the departure system, the 3,000 ton survey craft has arrived in only 40 years or so.

Most of that is back of the envelope math, and Spaceships is an approximation of a design process. But if you accept the Spaceships numbers as being more-or-less realistic, then you can travel 4 light years in 40 years or so at TL 10 without using superscience, though it will cost you over 10 billion GURPS dollars, and that's just the cost of engines and fuel tanks that you're discarding along the way.
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Old 07-11-2022, 04:55 PM   #7
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Default Re: [Spaceships] Getting A Few Thousand Extra MPS

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Originally Posted by mlangsdorf View Post
So you have an initial ship that is roughly the size of the largest real world oil tankers (300,000 tons) which accelerates at .01 G to 924 mps, which takes 6 months. It then drops Stage 1, slimming down to the size of a nuclear aircraft carrier, and accelerates at 0.01 G for another 6 months to a total 1848 mps. Second stage is discarded and the ramscoop opens up, allowing continual acceleration for the next 20 years or so, reaching a top speed of about 0.2c, well below relativistic effects.



Most of that is back of the envelope math, and Spaceships is an approximation of a design process. But if you accept the Spaceships numbers as being more-or-less realistic, then you can travel 4 light years in 40 years or so at TL 10 without using superscience, though it will cost you over 10 billion GURPS dollars, and that's just the cost of engines and fuel tanks that you're discarding along the way.
Impressive! Of course, you can’t get back so easily.

To the OP: There was a book by Robert L. Forward called Indistinguishable From Magic that contained some articles on interstellar travel with real physics, and that I recommend. Calculating the power requirement of his laser-pumped light sail proposals raises some qualms (especially in light of the kzinti lesson), but there’s plenty of fuel for thought.
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Old 07-11-2022, 05:08 PM   #8
mlangsdorf
 
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Default Re: [Spaceships] Getting A Few Thousand Extra MPS

I realized a way to get there with only 3 stages - instead of making the next stage take up the forward 6 systems of a spacecraft, use a 1/3rd size upper stage that only takes up the first two systems. That means you can devote 16 systems per stage to reaction tanks, and use the more favorable x2 modifier (SS1 p 17). So an SM+13 first stage gets to 60*16*2 1820 mps by itself, launches an SM+11 second stage for deceleration, and deposits a SM+9 survey craft. Not that much of an improvement, but it saves a couple of reaction drive assemblies on the intermediate stages and brings the cost down closer to 6 billion GURPS $ or so.

And yeah, getting back in a problem. In theory, you can add two more stages, but that brings you up to SM+17 for the initial craft, which is 30 million tons and 1,500 yards long. Better hope you find the secret to FTL travel on the other side of the trip and can retrofit it.
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Old 07-11-2022, 05:17 PM   #9
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Default Re: [Spaceships] Getting A Few Thousand Extra MPS

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Originally Posted by mlangsdorf View Post

And yeah, getting back in a problem. In theory, you can add two more stages, but that brings you up to SM+17 for the initial craft, which is 30 million tons and 1,500 yards long. Better hope you find the secret to FTL travel on the other side of the trip and can retrofit it.
It’s a great plan! I’m proud to be part of it!

Another possibility is that if a similar but smaller project has previously delivered a robotic surveyor with a good science array you might know there is a shirtsleeve-habitable planet at the destination and manage to cram a small colony into a 3,000-ton ship. The First Fleet that established the colony of New South Wales amounted to about 4,000 tons (burthen) of shipping — they weren’t able to survive in Australia, let alone on an alien planet, but that was mostly down to incompetence, not material inadequacy.
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Last edited by Agemegos; 07-11-2022 at 05:25 PM.
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Old 07-11-2022, 05:44 PM   #10
Anthony
 
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Default Re: [Spaceships] Getting A Few Thousand Extra MPS

If you're allowing a ramscoop to work (most attempts to model them result in a producing more drag than thrust) you can reach 1,800 mps with a two stage (advanced fusion pulse rocket, 13 tanks, total 2080 mps; actually applying the rocket equation actually says 12 tanks is adequate).
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