Ad Astra: Shift Drive

Ludicrous speed... engage!

For some reason, I love space opera but hate reactionless drives. The two are usually part-and-parcel, since space opera tends to have spacecraft that travel at the speed of plot, but there are some exceptions. But reaction mass limited spaceships creates a problem, in that non-FTL travel inside a star system takes, at absolute best, days. If you want your players to be able to hurry to their ship, burn for escape velocity and dash towards the gas giant moon to save the stranded scientists... reaction drive travel means there's a period of a week or two where they're twiddling their thumbs. Kinda saps the tension out of the scene.

This isn't the end of it – there are plenty of ways to make this kind of timescale work in your setting. But if you want to use more conventional plots and timescales, you need a way for people to travel fast inside a solar system, and that means FTL. But this takes all the fun out of reaction drives! If all they're used for is getting to and from orbit, then they cease to be interesting.

So what if FTL travel requires deltaV too?

The Shift Drive
"Shift drive" is a generic term to refer to this kind of FTL. In GURPS Spaceships terms, it's a stardrive engine (p. 25) that allows travel at speeds exceeding the speed of light. How it works can be explained in a variety of ways. Perhaps a shift drive drags a bubble of hyperspace into realspace around the ship. Perhaps it locally increases the speed of light, with everything else speeding up in proportion. It might convert the spaceship into exotic matter, like negative mass or tachyons.

However it works, the shift drive allows the spaceship to travel at faster-than-light speeds, but it has to use its conventional engines to accelerate and manoeuvre in FTL! Without any kind of motive thrust, a shift drive is useless.

When a shift drive is activated, the ship's deltaV is measured in parsecs per day (ppd) instead of miles per second (mps). Multiply a spaceship's total ppd by it's FTL rating. Fuel is used to accelerate up to cruising speed, then back down to normal speeds just like for conventional space travel. If it's important, the acceleration of any spaceship with an active shift drive is multiplied by 220,000,000.

With a shift drive, any normal manoeuvres can be done in FTL, as long as the the drive remains powered. A spaceship can alter its course or speed mid-flight, spaceships with plenty of fuel will use the flip-and-burn to travel at maximum speed, and two spaceships might try to dodge and out-manoeuvre each other in FTL!

A shift drive can also be used for FTL travel within a star system. Any speed at or below 12×(FTL rating) times the speed of light requires a negligible amount of fuel*.

  Example: A spaceship with a shift drive with FTL-1 has 20mps of deltaV. After engaging the shift drive, it uses 10mps of fuel to accelerate up to an FTL speed of 10 parsecs per day. It cruises for two days, travelling twenty parsecs, then uses 10mps to decelerate back down to sublight speeds, and disengages the shift drive. If the same spaceship had a shift drive with FTL-2, it could accelerate up to 10×2 = 20 parsecs per day. Inside a star system, the spaceship with FTL-2 can go a maximum of 12×2 = 24 times the speed of light without using fuel, travelling 1AU in 20 seconds!

What happens if the shift drive is disengaged at FTL speeds is up to the GM. It may harmlessly drop the ship back to sublight speeds, removing the deltaV cost to slow down at the end of a trip. Or it might cause catastrophic damage, up to and including total vaporisation!

* – With an FTL-1 shift drive, accelerating to 13c requires 0.01mps of deltaV. So I picked one c lower, to get a nice round number for "zero" fuel consumption.

Travel With Shift Drives

Interplanetary Travel

1. Time to Accelerate
To determine the time taken to accelerate to the desired lightspeed factor, use this:

  T = Vc × 1.4/A

T is time in seconds.
Vc is the desired velocity in multiples of lightspeed.
A is sublight acceleration in G.

The distance travelled during the acceleration is tiny, so you can ignore it.

2. Cruise Time
The cruise time needed is worked out with:

  T = 500 × D/Vc

T is the travel time in seconds.
D is the distance in AU.
Vc is the velocity in multiples of lightspeed.

  Example: The UNS Tempest is a military frigate in orbit of Earth. She has a basic shift drive (FTL-1) and an antimatter plasma torch, giving 1G acceleration. The captain receives a communique that space pirates are attacking a ship in orbit of Saturn, and gives the order to go to FTL. Earth and Saturn are aligned right now, so the total distance is 8.5 AU. The Tempest engages her shift drive and boosts up to 12c using her antimatter torch. This takes 12 × 1.4/1 = 16.8 seconds. The remaining distance is 8.5 AU, travelling at 12c. This takes 500 × 8.5/12 = 354.16 seconds, just under six minutes. The Tempest spends six minutes in FTL, which the crew use to ready themselves for combat, then the ship turns about, burns down to sublight speed and drops out of FTL right on top of the pirates!

Interstellar Travel

1. Time to Accelerate
To determine the time taken to accelerate to the desired lightspeed factor, use this:

  T = dV × 0.0462/A

T is time in hours.
dV is the desired velocity in parsecs per day.
A is sublight acceleration in G.

2. Distance Travelled During Boost
Work out the distance travelled during acceleration using this:

  Dpc = T² × A × 0.00042

Dpc is distance in parsecs.
T is the acceleration time calculated in step 1.
A is the sublight acceleration in G.

3. Cruise Time
The cruise time needed is worked out with:

  T = D/(dV × F)

T is the travel time in days.
D is the distance in parsecs.
dV is the cruising deltaV in parsecs per day.
F is the FTL rating of the spaceship.

  Example: The UNS Tempest is in orbit of Saturn and has been ordered to report to Tau Ceti for docking, crew relief and redeployment. Tau Ceti is 3.65 parsecs away and the Tempest's single fuel tank gives it 120mps of deltaV. The captain is in no rush, so she decides upon a leisurely pace of 2 parsecs per day to save on the expensive antimatter fuel. The Tempest engages her shift drive and burns 2mps of deltaV, which accelerates her to 2 parsecs per day. This takes 2 × 0.0462/1 = 0.0924 hours, or about 5.6 minutes. During the boost, the Tempest travels 0.0924² × 1 × 0.00042 = 0.000003586 parsecs. This is so small we can ignore it. This time is doubled to account for deceleration, so the total so far is 0.185 hours, or about 11 minutes. The cruise then has to cover 3.65 parsecs. This takes 3.65÷2 = 1.825 days (or 43.8 hours). Adding it all together, the Tempest takes 43.985 hours to travel to Tau Ceti, so a little under two days.

Nota Bene
I'm aware this doesn't cover the flip-and-burn case, which is especially important when using large amounts of deltaV! I've got basic calculations for this but need to double-check them and don't have the time as of writing. I'll add them with an edit later!