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.
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 ofdelta-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.
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.
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 theAtomic Rocketssite for immense help designing all of these drives.)