Spacecraft – Low Tech

Ad Astra space travellers are forced to use somewhat more realistic space drives than in “traditional” Traveller. Even with antimatter, high-tech interstellar craft typically take a week to travel to or from the jump point. Without antimatter, travel times stretch into months.

I’ve updated my Traveller ship design spreadsheet to reflect this new reality. This article describes the primitive drive technologies available at TL6 – TL8, together with some examples.

(This is the first part of my article on Ad Astra space drives. Part 2 is here. Today, TL6 – TL8.)

Tech Level 6 – Low orbit, via multi-stage rockets.

The dawn of the space age is dominated by primitive chemical rockets. These beasts are too inefficient to reach orbit, so they use huge multi-stage boosters to launch tiny payloads into low orbit. On the plus side, the huge propellant flow serves to cool the engines, so there is no need for dedicated radiators.

Chemical rockets need liquid oxygen, and liquid hydrogen fuel. Hydrogen is widely available, but oxygen is rarely sold as fuel, except on low tech worlds.

Ion drives are available, but they are so slow that even tiny probes take years to reach targets in the inner system. These primitive space craft have tiny power requirements, so fuel cells usually suffice.

Space Shuttle

TL6 low-orbit shuttle 200t, MCr43, 36t cargo.

200t streamlined wedge configuration, with chemical rockets. Tanks fill 60% of the hull, which is enough for 4.6km/s of delta-V – sufficient for orbital manoeuvres and landing. Launch is achieved by using a large external drop tank, and two detachable booster rockets. The shuttle cannot fly higher than low orbit.

Tech Level 7 – Interplanetary flight barely possible.

Solid-core fission (NERVA) rockets can fly higher, and are far safer than chemical rockets. They still require multiple stages to reach orbit. VASIMR thrusters, powered by nuclear power plants, make interplanetary travel just barely possible.

NERVA rockets use tonnes of common hydrogen as a propellant. Their solid radioactive cores require frequent replacement too, and those can usually only be found at A- & B-class starports, on relatively primitive worlds. Many governments restrict the sale of highly enriched uranium, since it is a component in simple fission bombs.

Note the distinction between propellant, and fuel. Propellant is an inert substance – usually hydrogen – that is accelerated out of the drive’s nozzle, in order to throw the craft forward, in accordance with Newton’s third law. Fuel is the energy source that powers the drive. This can be fissile material such as enriched uranium, or at higher tech levels, fusion fuels such as deuterium, lithium, or helium-3, or even antimatter.

Dream Chaser

TL7 orbital shuttle 200t, MCr47, 56t cargo.

200t streamlined wedge configuration, with solid-core fission rockets. Propellant tanks fill 40% of the hull, which is enough for 5.1km/s of delta-V – sufficient for orbital manoeuvres and landing. Launch is achieved by using a large external drop tank. The shuttle can lift large cargoes to high orbit.


TL7 Mars ship 200t, MCr26, 3 years, 132t cargo

200t dispersed structure, with VASIMR thrusters capable of 0.00001G. Hydrogen propellant tanks fill 14% of the hull, which is enough for 12.1km/s of delta-V. A round trip to Mars (closest approach, no landing) takes 3 years.

Tech Level 8 – Routine Lunar transfer.

The apex of the fission age. Gas-core fission rockets are finally able to power single-stage-to-orbit shuttles. These flexible workhorses can make routine journeys between gas-giant moons, or even make interplanetary trips, when equipped with drop tanks. Fission shuttles are commonly used even today – as they are simple, robust and reliable.

Fission rockets require their own specific fuel. This is pretty much always available at A-class starports, and on lower-tech worlds, but can be hard to come by elsewhere.

VASIMR thrusters continue to improve, now capable of hauling bulk freight interplanetary distances in months rather than years. The power usually comes from advanced fission plants.


TL8 orbital shuttle 200t, MCr49, 37t cargo.

200t streamlined wedge configuration, with gas-core fission rockets. Hydrogen propellant tanks fill 55% of the hull, which is enough for 16km/s of delta-V. That is sufficient for launch, orbital maneuvres and landing, without needing external boosters or tanks. If it’s refuelled in orbit, the shuttle can fly to Luna in 2 days, or Mars in 12 months – land and return.

Mission 2 Mars

TL8 Mars ship 200t, MCr26, 18 months, 130t cargo

200t dispersed structure, with VASIMR thrusters capable of 0.00004G. An advanced fission plant generates the necessary 7MW power. Hydrogen propellant tanks fill 15% of the hull, which is enough for 26km/s of delta-V. A round trip to Mars (closest approach, no landing) takes 18 months.

(Credit goes to the Atomic Rockets site for immense help designing all of these drives.)

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  1. alex said,

    18 May, 2015 @ 09:14

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    Alexander Tingle Note: I’ve changed the way jump drives works so that traversing a jump wormhole is now instantaneous. That means that at least you don’t have to wait around in jump space for ages on top of the long journey to the jump point.

    This has the interesting implication that interstellar communication is now much faster than travel. A small ship or probe can just flick forward and backwards through the jump-chord, relaying information very quickly. I’m not sure how I feel about that.

    Samuel Haldane Alex, what rules set are you using? (I recall that various versions of Traveller had entirely different rules sets. MegaTraveller is the one I know, but I think there’s one similar to CoC as well).

    Alexander Tingle I’ll be basing it on MegaTraveller, since that’s what I know. I’ve modified the combat system, because MT’s was stupid. And of course, the tech-levels are different too. In truth, I’m building my own system from the skeleton of MT.

    Samuel Haldane Ah, MegaTraveller has a lot of good ideas and a lot of bad execution. One thing that always bothered me was that stats have almost no effect on the outcome of actions. With a minimum stat (2) you roll 2D6, with average (7) you roll 2D6+1, and with maximum (12) you roll 2D6+2. This is a pretty minor difference. A genius is only 2 points on 2D6 more likely than a total idiot to solve a mental problem. By contrast, doubling the time you take over the task makes it 4 points easier, and halving the time you take makes it 4 points harder.

    Alexander Tingle The stats go up to 15, so you can get a +3, but that nit-pick doesn’t really undermine your point. On the other hand, in Cthulhu, your stats play no part whatsoever in task resolution, so the genius has no advantage over the fool.

    Andy Miles Intelligence and Education do have an effect in MegaTraveller but in an indirect way. You cannot have more skill levels than the sum of your Int & Edu. What I always found stranger, and I think this applies to all editions of Traveller, was that you could have an Army or Marine character with physical stats of 222.

    Alexander Tingle They’ll be in the office.

    Matt Fitzgerald You do have to love any system where your character can die during the creation process though. DEX, INT & EDU covering 90+% of skills was a little irksome.

    Samuel Haldane Cthulhu – I love the concept, but I think the CoC rules system is one of the worst on the market. Attributes having almost no influence on skills in CoC is one of my big objections to the game.

    Samuel Haldane MegaTraveller – If you fail your survival roll, you don’t die (unless you choose that as an optional rule). Instead, you’re injured in the line of duty. This has no effect on your stats, but for some reason you’re forced to leave the service. Why? It’s nonsense. Even bureaucrats have a survival roll. How do they injure themselves – staple their hands to the desk? You might argue that the survival roll deals with just losing your job, but that’s separately handled by the re-enlistment roll.

    Samuel Haldane The MegaTraveller character creation system has some great concepts, but the way they’re executed adds up to a huge mass of senseless nonsense. The same is broadly true of the MT rules in general.

    Andy Miles In the original (Classic) Traveller rules the survival roll was just that – to see if you survived. All of the career options at that time were military or para-military so it made a certain degree of sense. The reasoning was to make players think about the trade off between the risk of gaining new skills and mustering out benefits vs the risk of death in the line of duty. For later civilian career options it can be seen as being forced out of your career either through injury or in disgrace. Shifting back to CoC there was an optional system at one time that linked the number of available skill points to stats. You would get a multiple of each stat to spend on skills related to that stat – I always thought that that was quite a good method. Remember though that both CoC and (Classic) Traveller were quite early on in the development of RPGs and that some, such as D&D, almost completely ignored skills.

    Alexander Tingle My usual approach is to use about 20% of the rules, and ignore the rest. Take MT as an example – I’ve never even read the ship design rules. I skimmed them enough to realise that they were a half arsed attempt to extend the (already overcomplicated) Striker vehicle design rules to all vehicle ever – from scooters to battlecruisers. Plus added incoherence, poor playtesting, and typos!

    Cthulhu was the only system I ever ran where I think I used all of the rules. That’s high praise – they were simple yet complete. I’ll take that any day.

    Matt Fitzgerald Did they just say “Terrify your players and their characters, then drive them both insane” ? smile emoticon

    Alexander Tingle That’s the pocket version.

    Andrew Davies I am feeling really nostagic for traveller. It was the first roleplaying game I played and have been enjoying your posts.

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