Space Travel

If you come from Traveller, forget Jump Drives, Maneuver Drives and Power Plants. In-System is a Solar System setting. Spacecraft are specialized, with most designed around one or two specific jobs. A vehicle that is excellent at launching from Earth is usually a poor choice for crossing the Solar System, and vice versa.

Spacecraft by Function

Launch Vehicles

Launch vehicles exist for one purpose: escaping a planetary gravity well. They produce enormous thrust for a very short period of time and consume tremendous amounts of fuel in the process.

Examples include the Apollo Saturn V, modern SpaceX launch vehicles, and the MAV from The Martian.

Typical characteristics include:

immense thrust
enormous fuel consumption
short operational life
rarely travel beyond Low Earth Orbit

Once their job is finished, launch vehicles have usually delivered either cargo or another spacecraft into orbit.

Orbital Vehicles

Orbital vehicles move people and cargo between stations, moons and colonies. Since they never have to fight Earth's gravity, they can carry enough propellant for repeated trips without becoming excessively large.

Typical characteristics include:

chemical propulsion
moderate fuel capacity
relatively inexpensive
the workhorses of the Earth-Moon system

The Long Beach Transfer Shuttle is a typical orbital vehicle. It routinely carries passengers between Luna and the Lagrange colonies.

Fans of The Expanse can think of Uncle Mateo's ice miner or the crowded Belt Transit Ferry that Detective Miller uses between Ceres and Eros. Those vessels are somewhat more advanced than those found in the In-System setting, but they fill a similar role.

Deep Space Vehicles (DSVs)

Deep Space Vehicles are designed for journeys lasting weeks or months. They are capable of operating anywhere in the Solar System and represent the largest and most advanced civilian spacecraft of the period.

Examples include the Hermes from The Martian and Discovery One from 2001: A Space Odyssey.

DSVs are uncommon. Only a few dozen exist during the early years of the setting.

Propulsion Systems

Chemical Reaction Drives

Chemical reaction drives work much like modern rockets. A fuel is mixed with an oxidizer, ignited, and expelled at high speed through a nozzle to create thrust.

The efficiency of a rocket fuel is measured by its Specific Impulse (usually abbreviated Isp). Higher Isp means more thrust for a given amount of propellant.

By 2158, three fuel combinations dominate:

Methalox (liquid methane and liquid oxygen). Reliable, inexpensive and widely available.
Hydrolox (liquid hydrogen and liquid oxygen). Higher performance but considerably more difficult to store and handle.
Alulox (aluminum and liquid oxygen). Lower performance than either methalox or hydrolox, but well suited to lunar industry because aluminum is abundant on Luna and the lower gravity makes extreme thrust less important.

Every launch vehicle uses chemical propulsion. Orbital vehicles almost always do as well. The technology is mature, dependable, and supported by nearly two centuries of industrial development.

Nuclear Thermal Rockets (NTRs)

Nuclear Thermal Rockets represent the next step in propulsion.

Unlike chemical engines, an NTR does not burn fuel with oxygen. Instead, a nuclear reactor heats liquid hydrogen to extremely high temperatures. The expanding hydrogen is then expelled through a rocket nozzle to generate thrust.

The reactor itself is not the engine. It is simply an extremely powerful heat source.

NTRs are much more fuel-efficient than chemical rockets, making them practical for journeys throughout the Solar System. They also introduce significant engineering problems. The reactor produces intense radiation, requiring heavy shielding between the engine and crew. Large radiators are needed to reject waste heat, and hydrogen fuel tanks occupy considerable volume.

For these reasons, NTR-powered ships are large. During the period covered by In-System, there are no small Nuclear Thermal Rocket spacecraft.

Burns

Rather than tracking fuel by the kilogram, In-System measures fuel in Burns.

A Burn represents a standard expenditure of propellant for a particular spacecraft. Larger ships consume much more fuel than smaller ones, but each still measures its available maneuvering budget in Burns.

A ship carrying many Burns has greater flexibility. It can make course corrections, rescue another vessel, or recover from navigational mistakes. The tradeoff is reduced cargo capacity.

Commercial freighters usually carry only the Burns required to complete their assigned route with a reasonable reserve. Exploration vessels often sacrifice cargo space for additional maneuvering capability.

Advanced engines can expend multiple Burns simultaneously, producing greater acceleration when required. This may be useful when towing another vessel, making an emergency maneuver, or reducing travel time.

Typical Burn Costs

Maneuver	Burn Cost
Leave orbit for a trip to another planet	4
Enter orbit around a planet	4
Leave orbit for a local moon	1
Enter orbit around a moon	1
Descend to the surface of a Size 0–3 moon	1
Ascend from the surface of a Size 0–3 moon	1
Major orbital change	1
Deorbit for planetary entry	1

Minor orbital adjustments normally require no Burn if the pilot succeeds at an Average Pilot check. Failure costs one Burn.

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