“A fully fuelled power plant will enable a starship an effectively unlimited number of accelerations (at least 288) if necessary to use the maneuver drive during the trip (as when miniatures combat is used to resolve a ship to ship encounter).” – Traveller, Book 2
Judging by the ideas I post about, I am most interested in the implications of Book 2: Starships; and fuel has been bugging me for years.
It’s easy to calculate the usage for starships; 10% of the hull tonnage for jump fuel, and 10 tons per power plant number for power and manoeuvre drives for two weeks. Notice that in this edition of the rules, power plant number is only indirectly related to ship tonnage, and not at all related to manoeuvre drive capability.
For small craft, fuel is consumed at the rate of 10 kg per G of thrust per ten minute turn, regardless of what kind of small craft, how much mass it has, or how much cargo it carries; non-starships have life-support for 30 days, but the book is explicit that this is driven by air, food, water, and recycling machinery – nothing to do with power or fuel.
So starships have at least 288 turns of fuel regardless of Gs, lifeboats have 500 G-turns of acceleration (500 turns at full burn), ship’s boats 900 (150 turns at full burn), pinnaces 1200 (240 turns), cutters 1500 (375 turns), and shuttles 900 (300 turns). Nobody is going to run out of fuel in combat, which as I’ve calculated earlier is not likely to last more than 45 turns for the typical engagement.
Small craft are unable to land on worlds of size 8+, and most ships have 1G drives so arguably can’t lift off from worlds of size 9+, but I’ve ignored that for over 40 years and I’m not going to start worrying about it now.
No, what bugs me about all this are the implications for the interstellar society’s technology.
All we know about these is that they consume vast amounts of hydrogen and don’t work reliably within 100 diameters of a planet. Since in the 1977 edition all worlds have the same density, this is tantamount to saying they don’t work reliably if local gravity is more than a certain value – a theme the authors would return to in 2300AD.
(In my Traveller universe, this was due to the Lens-Thirring Effect or gravimetric frame-dragging disturbing the jump field as it formed, but nobody cared, not even the players who were actual physicists.)
If you’re interested, the drives operate reliably once local gravity drops below about one five-thousandth of a G, an acceleration of roughly 0.2 millimetres per second squared.
The rules imply that ships make a zero-zero intercept with the 100 diameter limit and thus are at rest when they jump. Since the destination system will have a different proper motion in the galaxy than the starting point, this can’t be to set the correct velocity for the target system, so my working assumption is that the jump drive is also very sensitive to ship movement.
Manoeuvre drives consume the same amount of fuel regardless of the mass of the thing they are accelerating, and the amount of fuel consumed is directly related to the acceleration.
In metagame terms, this is so that you don’t have to solve Tsiolkovsky’s rocket equation in your head every time you change course, and a good thing too.
In game terms, this simplification has certain implications for the setting. If the fuel were being used as rocket fuel, as the 1979 edition of Book 5 suggests, then since the mass flow rate and the standard acceleration due to gravity are the same, the fuel’s specific impulse must change from moment to moment according to the mass of the ship. I can’t imagine how that could work, so I assume that M-drives are some sort of reactionless anti-gravity propulsion, in effect banks of air/raft grav modules, which is more in line with the 1980 edition of Book 5.
One implication of that is that air/rafts have interplanetary capability, although the open top means you’re spending a long time in a vacc suit on that trip. Do what I do and take some Fast Drug with you.
(As an aside, IMTU manoeuvre drives have vectored thrust; the thrust axis is parallel to the deck in atmosphere, and perpendicular to it in vacuum. So popular deck plans are still usable regardless of grav plates, and there is no need to redraw them as tail-sitters.)
I am going to steer well clear of inertialless drives, as they would mean a ship running into something bounces off it like a balloon; although that would explain why nobody in the game universe worries about free traders ramming the planetary surface at 90 kilometres per second, it does make you wonder how ship weapons would work, and what happens if you fly into the solar wind.
But, if the drives are reactionless, why does fuel consumption increase with acceleration but not with mass? And why can’t ship’s boats lift off a size 8 planet although they have 6G of acceleration?
Even the smallest power plant consumes five tons of hydrogen per week; the density of liquid hydrogen is 70.8 kg per cubic metre, which is where Traveller’s 14 cubic metres per displacement ton comes from; five tons of liquid hydrogen occupies 70 cubic metres.
Apart from a brief flirtation with black holes in the late 1970s, I’ve assumed power plants are fusion units. (Why no more black holes? Orbital disturbances on the destinations, and the question of how you stop them orbiting each other when you have several ships close together.)
Current fusion power designs suggest a one gigawatt reactor would need about 5 kg per week of mixed deuterium and lithium, so even allowing for hydrogen fuel being less efficient, starship power plants are in the terawatt range – a terawatt is 500 times the maximum power generated by the Hoover Dam or by the Almaraz nuclear power plant in Spain, which is the biggest one currently active; 2,000 times the output of the fission reactors which operate large nuclear-powered aircraft carriers. To put that in perspective, a terawatt is enough to run all of China and most of the USA at the same time; if this line of reasoning is correct, a free trader’s power plant would generate enough power to run a sizeable fraction of contemporary Earth. (So maybe the frontier worlds are littered with obsolete ships hooked into the power grid? Another scenario for the PCs – “Look, you’ve got a ship’s engineer, right? If this blackout goes on much longer the mayor will have my ass!”)
What are they doing with all that power? And how are they dumping the waste heat?
We have no idea what the power consumption of the various ship components are, but it’s reasonable to assume that ships wouldn’t generate that much power unless they had to; otherwise they would have more expensive power plants than they need. It’s unlikely that life-support, avionics and so on need much more power than they do on contemporary nuclear submarines, so it’s probably needed for the jump and manoeuvre drives; the rules state that the power plant has to at least match the jump drive, while in 1977 there is no connection with the manoeuvre drive (and small craft don’t even have power plants), so we can infer that almost all that power is necessary to initiate or maintain jump, and the power plant is idling most of the time.
When the power plant spools up to power the jump drive, I would expect it to get really hot, really fast, so I suspect a lot of the shielding around the drive compartment referred to in the rules is against heat (and possibly magnetic fields) rather than anything more exotic. Even so, with nowhere to dump the waste heat, I’d guess we’re looking at a sudden multi-terajoule spike to initiate jump rather than a steady one terawatt drain to keep it running; Almaraz Nuclear Power Plant has a reservoir and a river to keep it cool, and unless jumpspace is very cold and full of some sort of conducting fluid, there’s nowhere for the heat to go aboard ship. Radiation is the only option and the least effective way to transfer heat, and if the ship’s skin is radiating any significant fraction of a terawatt of waste heat it’s going to be a bit warm in the passenger staterooms.
This is probably why the back end of the ship glows blue-white; nothing to do with the manoeuvre drive, it’s frantically trying to radiate gigawatts of heat without melting the main compartment or blinding the people looking out of the front end.
If so, one has to wonder what that does to the weather near the starport, and insurance premiums for ground crews.
My head hurts now, and I already know none of my players care. So I’m going to ignore this and hope it goes away, but there are a few things that will probably creep into the setting for me now – just as an author can invest great effort in working out background material which is only implied in the finished novel, so I spend a lot of time thinking stuff through which only appears as casual throw-away comments in game. For this exercise:
- You must have zero velocity relative to your planet of origin at jump, and local gravity must be very small. This explains both the 100 diameter limit and the deceleration to zero on arrival there.
- The shielding around the engine compartment is to protect you from heat and magnetic fields, nothing more exotic. It’s safe to enter unless you’ve just initiated a jump, in which case it’s hot enough to need vacc suits or similar protective gear.
- Somewhere in the campaign is a colony world using a crashed free trader as a power plant for its main city. Heartless PCs could of course repair it and make off with it, leaving the colony in the dark; more heroic ones could thwart such a plan by a band of ruffians.