Archive for May, 2010

100 diameters limit

Posted in Rules, Science, Science fiction, Traveller on May 30, 2010 by Mr Backman

Traveller has always had the rule that hyperspace jumps should be made beyond 100 diameters of the planet, gasgiant, ship, star or nearby massive object. When some kind of reason for this is mentioned it goes along the lines of  ‘too deep within the gravity well’ or other reference to gravity. Can ships jump inside nebulae (they’d certainly be inside 100 diameters of the nebula)? How can ships jump at all when they are always inside 100 diameters of the milky way galaxy? What about jumping near black holes or neutron stars (shouldn’t the density of objects be accounted for at all)?

We all know the real reason is to force ships to actually travel in space before jumping, without such a limit the ships could just as well jump directly from the ground and not much space travelling would occur. So let us all agree that wa want some kind of rule that forces ships to fly away from planets before jumping, preferrable such a rule should behave as the 100 diameter rule for planets yet still make some scientific sense. The rule should also dismiss the cases of nebulae and galaxies so ships can jump inside these while still abiding to the rule. If the rule is based on gravity instead of some weird new invented force all the better.

Gravity then, is proportional to the mass of the object and inversely proportional to the square of the distance. Gravitational force is not the only measure of gravity, we have gravitational potential and tidal force as well. These two are effects derived out of gravity but they behave differently range wise:

  • Gravitational potential falls off as M/R, where M is the mass of the planet and R is the distance from the planet. It is a measure of the energy needed to reach the distance R.
  • Gravitational acceleration falls off as M/R^2, where M is the mass of the planet and R is the distance from the planet. It is a measure of the gravitational acceleration exerted on an object at the distance R.
  • Gravitational tidal force falls off as  M/R^3, where M is the mass of the planet and R is the distance from the planet. It measures the fall-off rate of gravitational acceleration. It is the force that causes ebb and flood on Earth as well as what causes the moon to always show the same face towards Earth.

The mass of a planet is proportional to its volume (given the same density), that means that it rises with D^3. Twice the diameter and the planet becomes 2^3 = 8 times as massive. The 100 diameter rules states that a planet twice as large must be jumped from twice as far away and as mass scales with D^3 we need something that scales as 1/R^3 and the only gravity effect that fit the bill is tidal force. Using tidal force as a limiter for when a safe jump can be performed makes a lot of sense; it is a measure of fast gravity changes near the ship. If jumdrives need a uniform gravity field to work properly the tidal force tells us how much gravity differs in different parts of the ship. If jumpdrives need to know the exact gravity pull when jumping the tidal force tell us how much error we get from our positional error. 

Safe jump distance (taught to Imperial school children to be 100 x the diameter of the object) is really calculated like this (x^(1/3) means the cubic root of x):

  • Planet safe jump Rj = 1 000 000 km x (Traveller Size / 8 ), multiply by the cube root of Earth density if you want that level of detail (Earth has density 1.0)
  • Planet safe jump Rj = 1 000 000 km x (M) ^(1/3), M is measured in Earth masses (Earth has a mass of 1.0)
  • Star safe jump Rj = 0.5 AU x (M) ^(1/3), M is the stars mass in Solar masses (Sol has a mass of 1.0)

What does all this give us? The referee can tell its players that they must travel out 100 diameters from a planet to “where the tidal force is weak enough to safely engage the jump drive”. If one wants the detail one can calculate the actual safe jump distance from any object. When scientifically versed players asked how one can jump inside the 100 diameters of the milky way the referee can tell them it is because the tidal force from the galactic centre is way too weak to cause any problems, the same goes for jumping inside nebulae.

Note: I have taken the liberty to round off figures in the formulae above, it should really be 1 280 000 km but I find one million kilometers easier to remember.

Relativistic rock? Is that a sub-genre of Space rock? You know, Hawkwind, Ufomammut and the like?

Wilderness refuelling

Posted in Intercept, Rules, Traveller on May 29, 2010 by Mr Backman

The assault scout Anacron have detected the gravity waves of a large fleet of Zhodani warships entering the system, they must leave the system and warn the navy of the impending attack.

“We need jump fuel to flee the system before the Zhodani fleet arrives, what options do we have?”

“We could skim the gasgiant but the Zhos will certainly picket the gasgiant”

“We could match course with a comet and do some ice mining”

“We could land on the outermost moon of the gasgiant and fill up on methane”

“Set course for the outermost moon of the gasgiant then, make sure the moon is between us and the gasgiant as we approach. We don’t want the Zhos catching us on the planet”

“Roger that Sir”

This post has been updated 2018-07-12 to reflect changes in the rules.

I have recently added som new fuel options to the Intercept design system and some explanations might be in order. These refuelling options does not affect the combat capabilities of a ship so those who use Intercept strictly for battles may want to skip. The new design system is available here.

Hydrogen fuel
Ships use fuel for two things; reaction mass for rockets and jumpfuel, the hydrogen must in both cases come in the form of liquid hydrogen or LHyd.

The fuel used by the fission or fusion powerplant of the ship will not be considered here as it is built into the powerplants and will keep the powerplant running for a year (6 months for fission) before replacement. Powerplants need this ‘refuelling’ regardless of whether they are run or not. Both fission and fusion plants have fuel that decay over time and this decay make the fuel less efficient and harder to ‘burn’ (Tritium for fusion plants, Uranium or similar for fission plants). Carrying extra fuel wouldn’t help either as that fuel would decay as well. Fusion powerplant refuelling is covered by the annual maintenance fee.

Liquid hydrogen has a density of less than 10% that of water and as volume is at a premium on starships, a lot of effort has been spent on how to increase the density of hydrogen storage. Hydrogen also happens to be the most common element of the Universe, there is plenty of hydrogen in water, ammonia and methane, in fact there is more hydrogen per cubic meter of those substances than there is in pure liquid hydrogen, these compounds are also very common on planets, rings, comets and asteroids. These two facts have led to the development of a number of alternative fuel storage technologies.

Hydrogen storage
A ship can store hydrogen in four different forms:

  • Liquid hydrogen or LHyd is the only form useable by jumpdrives or reaction engines, all other forms must be converted into LHyd before use. Jumpdrives are very sensitive to impurities in the fuel so a ship using wilderness fuel can add a fuel purifier to filter out Deuterium, Tritium, helium and other impurities from the jumpfuel. There is no need to purify reaction mass.
  • Water or H2O holds 50% more hydrogen than LHyd but is ten times as dense. Water must be processed by a water cracker before it can be used as jumpfuel or reaction mass.
  • Ammonia or NH3 holds twice as much hydrogen as LHyd and has the same density as water. Ammonia must be processed by a NH3/CH4-converter before it can be used as jumpfuel or reaction mass, this converter work for both ammonia and methane but not for cracking water.
  • Methane or CH4 holds three times as much hydrogen as LHyd and has the same density as water. Methane must be processed by a NH3/CH4-converter before it can be used as jumpfuel or reaction mass, this converter work for both ammonia and methane but not for cracking water.

Water crackers ammonia converter and methane converters are rated in hours per hull percentage converted, this is the output percentage and not the input. A 1 hour per % water cracker would convert 0.67% of water into 1% of LHyd per hour, an equally rated ammonia converter would convert 0.5% ammonia into 1% LHyd per hour  and the methane converter would convert 0.33% of methane into 1% LHyd. The fuel purifier mentioned above, is also rated in hours per fuel percent purified.

Tanks and converters
Aside from reaction mass and jumpfuel you can add tankage for water/ammonia/methane, this tankage cannot be used directly, it needs to be converted into LHyd by an appropriate converter (NH3 converter, CH4 converter or H2O cracker).

Add a purifier if you want your LHyd clean and free of impurities. The purifier removes any Deuterium, Tritium, Helium or other traces from the LHyd, purifying your jumpfuel decreases the risk of misjumps and J-drive damage when using wilderness fuel. Some starports sell unpurified LHyd at a lower price.

Add fuelscoops to your ship if you want to skim gas giants for hydrogen, adding the aforementioned purifier will help you filter out the impurities from gasgiants. Note that not all gasgiants give you hydrogen when skimming, some will give you ammonia instead. The fuelscoops will convert your fission and fusion rockets into air breathers which will reduce fuel use and lessen radioactive waste when flying in an atmosphere.

Fuel skimming in Intercept
Skimming fuel from gasgiants is probably the most dangerous form of wilderness refuelling, how dangerous is up to each referee using whatever rules system he prefers. If you want to do fuel skimming during an Intercept battle you can use these rules:
Fuel skimming consists of repeatedly performing aerobrake manuevers on a gasgiant. As gasgiants are huge planets all skimming should be done with the large-scale rules (1 square equals 100 000 km, one turn equals one hour). How much fuel you get from each aerobrake pass depends on your speed prior to aerobraking; you get 20% fuel per brake G. Roll for aerobrake damage as outlined in the Intercept rulebook. If the ship is stationary in a gasgiant voluntary aerobrake square it can skim 5%.

Example The 60 000 dTom Azhanti High Lightning cruiser relies on its fuel shuttles for gasgiant skimming but in an emergency it can perform the skimming itself, at quite some risk. The hull of the Azhanti has a safe speed of 0.5 for aerobraking so each point of speeds adds +2 on its aerobraking damage rolls. Skimming at speed 1 down to 0 would give it 20% fuel per pass, three such passes and it will have replenished its jumpfuel. Each pass the Azhanti must roll at +2 for hull damage on the damage table ie a 4+ would cause Light hull damage. Yes, only in extreme emergencies will the Azhanti do the skimming on its own. You may wonder why they didn’t simply give the Azhanti Streamlined or better hull and the answer would be surface area. A warship needs lots of surface area to mount all their weapons and sensors. Tradeoffs I keep telling you, tradeoffs.

Relativistic rocks don’t kill people – People with relativistic rocks kill people.

Gravity in boardgames

Posted in Boardgames, Design system, Intercept, Other vector movemet systems, Scenarios on May 13, 2010 by Mr Backman



Space combat games are typically played on black hexgrids with no terrain features at all. Some games add asteroid fields, planets, nebulas, electric storms etc and some even try to incorporate the gravity field around planets. This is at least something we know, real planets do have gravity fields around them and climbing up the gravity well takes serious effort; just look at how big the Saturn V needed to be in order to send three guys to the moon and back.

What can we say about gravity then? Well, it pulls you back toward planets, a ship can remain indefinitely in orbit without thrusting, the higher above the planet a ship orbits the longer it takes; 1.5 hours in low orbit, 24 hours in Clarke orbit and a whole month per lap when you are as far away as the moon. We also know that if a ship has enough speed it can escape the gravity of a planet, this speed is called escape velocity for obvious reasons. Orbits don’t have to be circular either, they can be elliptical with one part getting real close to the planet and the other part taking it back further out, these orbits are also stable and require no thrust to maintain.

Prior examples

The first boardgame I came across that used vector movement was also the first that tried to depict gravity in a sensible manner, that game was Mayday. Mayday borrowed its gravity rules from Triplanetary and the mechanics where simple: If your vector, including its endpoint but excluding its startpoint, intersected one or more hex adjacent to a planet its future position would be affected. Another game with gravity rules was a game called Orbit war that was published in the Space gamer and then became a full blown boardgame.

Intercept version

In Intercept we want to do more than just being in orbit or not. Having several stable orbits with different periods allow us to model low tech orbital warfare with limited endurance fission/fusion rockets and spotting limited by the horizon. We can do elliptical orbits but that is something that just happen to work, free chrome one could say.

How do you do gravity in Intercept then? If your ship is inside a planet’s gravity well (6 squares for Earth) check what arc of the
planet you are in and adjust your drift in the direction of your current position towards the planet. Yes, gravity pull is based on the
position of your ship versus the planet but applied on the drift of your ship. If your ship is on the planet itself you do not adjust for
gravity (what direction would that be?).

That is all folks; if the ship is inside the gravity well but not on the planet you note the direction towards the planet and move your drift in that direction.

Scenario: Fission duel

This is a simple scenario with two equal ships battling it out in orbit above a planet. the ships start in the same orbit on opposite sides of the planet knowing where the opponent is but they cannot track him because the planet blocks LOS. The ships are 1G fission thrusters with 8 turns of thrust endurance and they are armed with a single small missile turret. You must carefully maneuver your ship close enough for your missiles 2G range single turn range. The ship has a crew of two; 1 pilot and 1 gunner/sensor op, there is no repair crew so there can be no repairs. Use the orbit from the image, ship A starts at x=0, y=-1, ship B starts at x=0, y=1 with the drift positions as shown. As I said earlier, this scenario is especially suited for deterministic play.

Gentlemen, start your fission drives, let the duel commence!

Major victory: Your opponent is a mission kill (incapable of firing and incapable of maneuvering) and you manage to land on the planet.

Minor victory: Your opponent is a mission kill and you are not.

Draw: Both ships incapable of maneuvering  and in such orbits that they will never get within 2G of a missile shot.

Make your own TL 8 Fission thruster equipped ships, equal or custom designed by each player. 100 MCr each is my suggestion for price.