[Spaceships] Can't find the right propulsion system
 

Hi, all. I'm trying to find an appropriate propulsion system for little work pods and inter-ship lighters, for in the space near stations, in work areas, etc. These are not long-range vehicles so they don't need massive delta-V. But they probably do need acceleration measured in something larger than milligees, if only for time management concerns, so Ion and VASMIR drives are out.

In this setting (TL10) there is a limited superscience switch that makes antimatter extremely cheap and things like the antimatter plasma torch possible. But for small craft like this such systems wouldn't be used since every Tom, Dick, and Harry welder or taxi pilot can't be trusted with a drive that spews radioactive death.

So for the life of me the only option I seem to be able to find is a chemical rocket. But this seems like overkill at 3G acceleration. What I really need is just some sort of OMS, y'know? Have I missed some other option?

Re: [Spaceships] Can't find the right propulsion system
 

Yeah, 3g is way too much for something like the OMS. It was, erm, what was it:

Oh. Well never mind then.

--

More seriously, is Antimatter Thermal not acceptable for a lower power system?

Re: [Spaceships] Can't find the right propulsion system
 

If "oms" means Orbital Maneuvering System such as found on the Spece shuttle that _was_ a chemical rocket. In fact it was one using hydrazine and nitrogen tet (storable at room tempeature though hgihly toxic) for fuel. It would ahve had a lower isp than the SSMEs but not an order of magnitude lower.

The OMS were a small pair of engines compared to the SSMEs and you could use a smaller than the normal SM chemical rocket for your workpod. That would cut accelration down to 1 G.

Re: [Spaceships] Can't find the right propulsion system
 

For the purpose you're describing, I really like nuclear thermal rockets. Antimatter thermal rockets might be preferred if antimatter is cheap. Sure they have a non-zero radioactive death factor, but a much smaller one than a torchdrive, and in space where everyone needs shielding against micrometeors and cosmic background radiation anyway, it might not be that big a deal. Another option is HEDM rockets but the whole "fuel tends to explode" factor might scare some people off.

Re: [Spaceships] Can't find the right propulsion system
 

The problem you have is mostly that 5% of mass in rocket engines is a lot of rocket. Just use partial modules.

Re: [Spaceships] Can't find the right propulsion system
 

Yes, smaller systems would be a good idea. For example, a -1 SM HEDM rocket would provide 0.67g of acceleration and two -1 SM fuel tanks with HEDM reaction mass would provide 0.33 mps of delta-v (it actually has better performance than antimatter catalyzed water, which would end up being 0.2g/0.4 mps of delta-v).

Re: [Spaceships] Can't find the right propulsion system
 

I would avoid HEDM if not needed, seeing as it tends to be explosive. Systems with low delta-V requirements are typically willing to trade performance for safety.

Re: [Spaceships] Can't find the right propulsion system
 

They were, in fact, the basic same engine as used in the the Apollo Service Module, which is definitely smaller than the Shuttle. A later version is being used in the Orion spacecraft's service module: good rocket engine designs can stay around a long time.

Re: [Spaceships] Can't find the right propulsion system
 

You can use Smaller Systems to convert a single system into three systems of one size (SM) smaller: You can replace a 3G chemical rocket with a 1G rocket and two small fuel tanks, all one SM smaller than the ship. This gives 2/3rds the capacity of a full SM sized fuel tank. If you want to go really small, you can convert down by two SM levels and get 10 systems at SM-2 for a single full sized slot. Each rocket would provide 0.3G of thrust at that level.

Re: [Spaceships] Can't find the right propulsion system
 

Antimatter also tends to explode, and it seems to be ubiquitous in the setting. If you do not want HEDM, you can do chemical rockets for 1g and 0.1 mps.

Re: [Spaceships] Can't find the right propulsion system
 

I was hesitant to suggest "smaller systems" because the OP mentioned "little work pods", which might imply SM+4. It made me think of the Kobold from Spaceships 6 (an SM+4 design that uses nuclear thermal rockets as I've suggested). But smaller systems is definitely a valid choice for SM+5 designs.

Re: the danger of antimatter, antimatter-catalyzed fuel (as opposed to antimatter boosted) fuel is not listed on p. 62 of Spaceships as a volatile fuel type. This surprised me when it was first pointed out to me, but it's true, and a major reason I suggested antimatter thermal rockets.

Re: [Spaceships] Can't find the right propulsion system
 

The volatile systems section is rather generous; rocket fuel and antimatter-catalyzed hydrogen or water should be added to the volatile list, and antimatter-boosted or matter/antimatter should be noted as hilariously explosive (the maximum REF of volatile fuel is roughly 124 * (mps/tank)^2; Nuclear Saltwater will probably go subcritical after destroying the ship, limiting the destruction, but other volatile fuels will probably manage at least a pretty high fraction of that max).

Re: [Spaceships] Can't find the right propulsion system
 

Since many systems are easy enough to extrapolate, I'm perfectly happy to allow smaller modules even on SM+4 vehicles. And there are certainly tiny rockets. Capsules use SM+0 or smaller chemical rockets for attitude control.

Re: [Spaceships] Can't find the right propulsion system
 

This looks like the answer I was seeking.

But, yes, I'm making a lot of little SM+4 pods. But I see no reason I can't still do this. Right?

Yes, initially I designed the pods with antimatter thermal rockets. They were pretty perfect. Except for the spewing radioactive death thing.

Thanks, all.

Re: [Spaceships] Can't find the right propulsion system
 

I've actually done this with small "Work Bee" yard ships. They are SM4 and have SM2 smaller systems to represent tiny thrusters and some tools and other equipment. Given that you can make palm-sized chemical rockets, SM2 is still downright large. Here are some fan rules for tiny systems: http://gsuc.roto.nu/doku.php?id=sm_3

Re: [Spaceships] Can't find the right propulsion system
 

For the mission you describe, 3G sounds like more acceleration than you'd likely want anyway. Why do work pods and intership taxi vehicles need that kind of acceleration?

Re: [Spaceships] Can't find the right propulsion system
 

Where are the rules for using smaller systems on larger craft?

Re: [Spaceships] Can't find the right propulsion system
 

Spaceships 7, p4-5.

Also, if you have guesses about how it works, they're probably right. It's not very complicated or tricky.

Re: [Spaceships] Can't find the right propulsion system
 

Because they don't; you failed your Reading check. :) I said that 3G rockets were overkill. You're thinking that I said the exact opposite of what I actually said.

Thank you.

Re: [Spaceships] Can't find the right propulsion system
 

I find the 3G rocket systems are handy for Tug designs that might end up having to clamp to and move a larger vessel. For small ships that just have to go to a nearby location in a shipyard though, an SM-2 system at 0.3G is plenty. Just remember if you need to drag around a big hull plate or something the extra mass might actually justify the full 3G. For instance an SM5 tug can shove around an SM6 ship at 0.75G if the tug has a full 3G rocket system installed. Just make sure not to use the rocket at full thrust when the tug isn't actually pulling anything.

Re: [Spaceships] Can't find the right propulsion system
 

I would honestly go with a higher delta-v system. A nuclear thermal rocket provides 0.5g and 0.45 mps per fuel tanks at TL9+. It could even be used for shuttle taxis from the surface to orbit and back.

Re: [Spaceships] Can't find the right propulsion system
 

In general the more delta-V per tank, the more destructive the exhaust. Depending on the details of how it's to be used, it might be better to not use a reaction drive at all (use something like tethers, or something that interacts with EM fields created by a larger vehicle).

Re: [Spaceships] Can't find the right propulsion system
 

Well, destructive potential is a related to thrust and delta-v. Doubling thrust usually means doubling mass flow or doubling exhaust velocity. Increasing mass flow will decrease delta-v proportionally to the increase in thrust while increasing exhaust velocity will increase delta-v proportionally to the increase in thrust. Energy is proportional to the square of exhaust increase.

For example, compare the TL9 HEDM rocket (2g/0.5 mps) to the TL9 fission thermal rocket (0.5g/0.45 mps). The former possesses ~2x the exhaust velocity, burns twice as much reaction mass per second, so it is putting out ~8x the energy of latter. While a TL9+ HEDM rocket has exhaust with a temperature of over 26,000 K, the fission rocket is only running at around 6,500 K. While dangerous to unprotected systems, a TL9+ fission thermal rocket is much safer than the TL9 HEDM rocket, dealing ~1/3 as much damage to anything caught in its exhaust flow.

Re: [Spaceships] Can't find the right propulsion system
 

No it doesn't. The TL9 HEDM rocket has 11% higher exhaust velocity and 4x greater thrust, and thus is putting out 4.44x as much energy, but since you only have to burn it for 1/4 as long the net hazard is 11% higher. It's also running at about a 23% higher temperature (temperature varies with the square of exhaust velocity and not correlated with thrust). In any case, HEDM is also a bad choice, most thrusters intended for purposes like this are things like cold gas thrusters with exhaust velocities up to maybe 1 km/sec and thus delta-V per tank of .03mps or so.

Re: [Spaceships] Can't find the right propulsion system
 

Of the rockets available in Spaceships, the ones that seem to produce decent thrust without too much burning doom or issues with casual anti-matter ownership seem to me to be the basic chemical rocket and a nuclear thermal rocket using water as reaction mass (or hydrogen if delta-vee is more important than thrust). One means having highly volatile fuel and oxidant sitting round, the other having fissile materials readily available to all and sundry.

Re: [Spaceships] Can't find the right propulsion system
 

Might want to just tune down your concept of decent thrust. For comparison, a NASA MMU has a total acceleration of about 0.009G with a total burn duration of 450s for a total delta-V of about 40 meters/sec. You might want a tad more performance, but reaching a safe walking speed of 1 yard per second takes 0.09s at 1G, so 0.05G is probably plenty.

Re: [Spaceships] Can't find the right propulsion system
 

Even 0.05G if you don't want a large fraction of the module's mass to be rocket gives fairly limited choices using Spaceships. Chemical, HEDM, NTR, light-bulb, fusion torch if available. I'm not counting nuclear pulse drives or nuclear salt-water because if anti-matter isn't desirable I very much doubt they would be either.

A smaller-system (1/3 size) NTR with a single 1/3rd size fuel tank would give 0.167G and 0.15 mps DV using hydrogen or 0.5G and 0.05 mps DV with water. I think you could probably make a pretty simple, reliable, and failsafe NTR unit at TL10, especially if getting optimal thrust/weight and specific impulse out of it wasn't the priority.

Re: [Spaceships] Can't find the right propulsion system
 

The basic power output of a rocket is 0.5 * thrust * exhaust velocity; for a 1 ton ship managing 0.167G, the thrust is 1480N, at 0.45mps/tank exhaust velocity is 14,400m/s, and thus your 'small' system is still belching out more than ten megawatts.

If you want something that isn't ridiculously dangerous, accept low performance; something like 0.05g at 0.09mps/tank is within the reach of storable liquid rockets.

Re: [Spaceships] Can't find the right propulsion system
 

Hmm... suggestion: Spaceships... 9?: Work Pods and Smallcraft?

Re: [Spaceships] Can't find the right propulsion system
 

Hm... Low thrust rockets:

Remote Heated Hydrogen: Hydrogen heated by laser/solar power to 5800 K. 0.3 mps per tank/0.01g
Microwave Water Rocket: Water heated using microwaves to 3100 K. High Power System. 0.3 mps per tank/0.06g