Archive for the Traveller Category

Nukes in space!

Posted in Intercept, Rules, Traveller on April 6, 2015 by Mr Backman

Atomic blast

Humankind have detonated 17 nuclear devices in space during the cold war the largest being the US Starfish at 1.4 Megatons. From Wikipedia:

“Starfish Prime produced an artificial radiation belt in space which soon destroyed three satellites (Ariel,TRAAC, and Transit 4B all failed after traversing the radiation belt, while Cosmos V, Injun I and Telstar 1 suffered minor degradation, due to some radiation damage to solar cells, etc.).”

“The worst effects of a Soviet high-altitude test occurred on 22 October 1962, in the Soviet Project K nuclear tests (ABM System A proof tests) when a 300 kt missile-warhead detonated near Dzhezkazgan at 290-km altitude. The EMP fused 570 km of overhead telephone line with a measured current of 2,500 A, started a fire that burned down the Karaganda power plant, and shut down 1,000-km of shallow-buried power cables between Aqmola and Almaty.”

If you are playing in the Traveller setting nukes are only allowed by the Imperial Navy or System defense forces and can only be used in wartime. Mercenary slang for wars involving nukes is ‘Bad war’, all other wars are called ‘Good war’.

Missile nuke option

  • Large nuclear missile TL 6+
  • Medium nuclear missile TL 7+
  • Small nuclear missile TL 8+

Consult the tech chart above to see at what Tech Level each missile class gets the nuke option. Nuke option reduce thrust with -2G and have a price multiplier of x10. PEN & DAM +12 when directly impacting a target but they can also be proximity detonated for +3 DM to hit and PEN & DAM -6, proximity detonation is not a design option but a choice the missile operator can do when attacking.

Nuke attacks and defense
Nuke attacks work the same as for normal missiles but the target may defend using one laser battery and one nuclear damper battery. Ships with a functional Neutrino detector will know if a missile is a nuke or not, all others must guess. Firing a nuclear damper on non-nuke missiles has no effect, of course.

Proximity detonation
The nuke missile operator may elect to detonate the nuke some way off the target for a +3 DM and PEN & DAM -6. Damage from proximity detonations should use the Spray fire rules were the degrees of success give more hits rather than more damage at one hit.

  • VGood 3 Fair hits
  • Good 2 Fair hits
  • Fair 1 Fair hit

Nuke secondary effects
The X-Ray, neutrino and gravity burst from the detonation also affect nearby ships interfering with their sensors. Visual/IR and Radar lose any tracks unless they were popped down, Neutrino and Mass lose their tracks regardless. Max range for this effect depend on the size of the missile:

  • Small 1 square
  • Medium 3 squares
  • Large 5 squares

Fleet tacticians and nukes
Fleet tacticians allow their Ship tacticians to fly more dispersed formations. An Expert+ Fleet tactician allow his Ship tactician to have a wide enough separation from any nuke secondary effects!

Ripley: I say we take off and nuke the entire site from orbit. It’s the only way to be sure.

Hudson: Fuckin’ A!

Burke: Hold on a second. This installation has a substantial dollar value attached to it.

Ripley: They can *bill* me.

Intercept 3.3 update

Posted in Design system, Intercept, Rules, Traveller on November 10, 2013 by Mr Backman

Laser with mirror

Yeah, it is true, I finally managed to put it together and get it ready to download. there are lots of changes, rearrangements and tweaks, too many to mention all so I will just give you the major points. I have scattered some illustrations here and there in the rules to liven things up a bit and to exercise my limited artistic abilities.

Download the latest version of Intercept and designs from the Downloads page.

Design system

The design system has gotten some changes here and there but now for the first time there is actual documentation on how to design ships in the rulebook. This part is placed at the end, right before the tables and charts, so you can print out the Design rules in a separate booklet if you like.


There are many changes to how ships interact with planets and these changes are reflected in the new mapsheets. There is one sheet with a small planet in the middle, one with a large planet and also one where a large and small planet are placed far apart to play out scenarios in the Earth – Moon neighbourhood. The mapsheets are now included in the Intercept bundle.

Missile customization

Various missile customization options are tabulated in the rules but maybe not too deeply explained. Read about missile customization here.


Planets have their own section now with rules on planetary line of sight, gravity etc in one place instead of scattered throughout the rules. The rules for stable orbits now cover polar orbits as well.  The planetary line of sight rules and sun shadow rules have changed considerably so make sure you read up on planets if you use them in the game.

Consolidated Pilot task

A fancy name for the section where various Pilot related stuff is located. You will find rules for docking, ramming, landing on planets, crashing into planets, voluntary and mandatory areobrakes etc. This section is a good place for a Traveller referee to get less ad-hoc and more believable rules for ship to ship and ship to planet interaction with more choices and participation by the players while still taking the characters skill into consideration.

Traveller integration

A page on how to integrate Intercept with Traveller has been added, to be filled with rules and tips whenever someone actually bothers to give me feedback. For me, Intercept is Traveller so I see no real point in writing conversions for various Traveller versions, the most likely one to get this treatment will be the Mongoose version of Traveller which I like in many ways, mostly for its adherence to the original LBB version. Just subtract 1 from your Mongoose or Classic Traveller skill level to get the D to use in Intercept, GURPS players should use (skill – 10) / 2.

Changes to Intercept

Posted in Design system, Intercept, Rules, Traveller on April 8, 2013 by Mr Backman

Some of you may not know it but I keep doing updates to the rules, maps, datacards and design system on a regular bases, just download the Intercept bundle here. Quite a lot of updates have accumulated without andy update posts but I will try to summarise the most recent changes below. To see what version you have simply look at the headings row of the design system for version date. I always update Ship.xls and Data.xls in tandem.  You can see the version of the Data.xls where you see the Ship.xls version number.  To update your designs you need to copy each column labeled Edit from your old design and paste* it into the new Ship.xls, then close the old design and save Ship.xls as the old designs name overwriting it. When you paste it is much safer to do the Paste->Paste values to ensure that you won’t overwrite and formatting.

Updating the Ship.xls sheet

All kinds of thrusters now correctly reports their actual thrust, the two values are Loaded / Unloaded performance. The Mtrl type field now goes from 0-3 with ever decreasing masses for most of a ships components. If you find your designs too low in acceleration and delta-V you should increase your material types to lighter but more expensive versions.

Editable Sun factor

Sun factor
There is now a light yellow field for the Sun factor. It should be easy to recalculate Vis(Hull) and IR(Hull) for other values of the Sun factor but now you can simply enter the value you want in the yellow box. Sun factor is +6 in the life zone of a solar system and goes up by 1 for each orbit inside and down by 1 for each orbit outside down to Sun factor 0. Mercury is +8, Venus is +7, Earth and Luna is of course +6, Mars is +5, Asteroid belt is +4, Jupiter and its moons are +3, Saturn and its moons are +2, Uranus and its moons are +1 and Neptune and its moons are 0. Pluto isn’t even a planet but as its orbit is never much closer than Neptune it is always 0 in Sun factor. Ships in Planet shadow subtract the Sun factor from Vis(Hull).

Living space
Living space and Life support determine the quality of life for the crew past the endurance of its crew stations. More living space per crew member means longer trips without physical or psychological effects. This is mostly here for roleplaying uses but if you design your own ships for competitive play make sure yiu stipulate the expected trip time requirements, ships using Jump drives should have at least one week endurance as this is the time the ship spends in hyperspace. Note that the Closed Lifesupport option requires lots of room for the green house but that entire volume is also treated as Living space and this is reflected on your ship designs.

Living space and crew comfort table

  • 1 m3 per crewmember Cramped 1 day cruise
  • 2 m3 per crewmember Normal 1 day cruise
  • 5 m3 per crewmember Roomy 1 day cruise
  • 10 m3 per crewmember Cramped 1 week cruise
  • 20 m3 per crewmember Normal 1 week cruise
  • 50 m3 per crewmember Normal 1 month cruise
  • 100 m3 per crewmember Cramped 1 year cruise
  • 200 m3 per crewmember Normal 1 year cruise
  • 500 m3 per crewmember Roomy 1 year cruise

Well, that is all for this update, more to come regarding the rules changes later (when I will also document the polar plane gravity sling stealth manuver sometimes called the Marre Red).

Intercept 3.2 update

Posted in Intercept, Rules, Traveller with tags , on November 18, 2012 by Mr Backman

Intercept 3.2 jay!

Oh, has it been this long since my last post? Why? Well, I switched from working at Starbreeze on Syndicate  and joined Machine Games to work on an as of yet undisclosed title.

Lately I have gone through the Interecpt rules, simplifying it and making it easier to play. The result is the all new Interecpt 3.2 available here and my ready-made ship designs based on Traveller available here. The most important changes have to do with the Sensor rules and how planet LOS is handled but there are small changes here and there throughout the rules. The aerobrake rules are also simplified and expanded to cover crashing into planets as well, however unlikely that may be.

Sensors in 3.2

Performing Scans work more or less the same as in 3.2; scans are normally done in boxes but the option to do really narrow scans on 1×1 and 3×3 squares remain. If your signal is 0-5 you have a Contact and if it goes to 6+ the Contact becomes Tracked. Contacts always tell you the position so narrowing down the Scan to get the much coveted 6+ Signal has become much easier. Consecutive Contact Scans have the Signal requirements for Tracked lowered as follows:

  • First Contact: Tracked on 6+
  • Second consecutive Contact: Tracked on 5+
  • Third consecutive Contact: Tracked on 4+
  • Fourth or more consecutive Contact: Tracked on 3+

Tracked Signal requirement goes back to 6+ as soon as the sensing ships fail to get a 0+ Signal out of it. This leads to interesting choices for the sensing ships: Should they reduce Scan area to increase Signal and risk missing the ship completely and thus nullifying the cumulative bonus?

Radar Scans treat Contact as Tracked so any Radar Signal of 0+ becomes a Tracked result. The flip side of Radar is of course that any ship getting a Tracked result of a target (0+ Signal) must tell everyone its position, ships therefore tend not to use their Radar until after they have already become Tracked.  Think of Radar as a flashlight searching for people at night and you’ll understand why you cannot find someone with your flaslight without them also finding you.

Planet Line Of Sight

One problem with the old planet LOS rules was that it was entirely up to the sensing player to make sure the Scan was correct, it was also quite hard to do Scans near the edge of the LOS blocked arc because Scans are always square while the edges of arcs are always jaggy. The new rules let the sensing player put his Scans wherever he wants them ignoring planet LOS, he can even put them on top of the planet itself. It is then up to the other player to check LOS using the following procedure: Target player checks what arcs are touched by the Scan. He then asks the sensing player what opposite arc, if any, his sensing ship is in. The sensing player must tell him whether he is inside one of those opposite arcs or not and the target player can simply ignore the Scan for targets inside the opposite Scan. If the sensing player did a Scan including the planet all arcs are touched so the sensing player then must tell the player in what arc his sensing ship is in that case.


The Sunfactor represents the strong light, microwaves and neutrino emanating from the star. This factor is typically 6, +1 per orbit inside the hospitable zone, -1 per orbit outside the hospitable zone down to 0. Earth is 6, Mars is 5, Venus is 7 etc. The Sun factor is subtracted from Scans towards the sun as per the Sunblinding rules. Ships in the planetary shadow subtract the Sunfactor from their Visual(Hull). Optional: Streamlined and Airframe ships not in shadow and directly facing the sun subtract Sunfactor / 2, rounded up.


Scans toward the sun are harder as he sensors are blinded by the sun. If your ship is below or south of your scan the Scan factor may be reduced by the Sunfactor. Mass scans ignore Sunblinding and any sensing ship in planetary shadow also avoid Sunblinding except Neutrino scans who suffer Sunblinding even when in shadow. Complicated?
Sunblinding subtract the Sunfactor from Scan.
Mass Scans ignore Sunblinding.
Neutrino Scans cannot use planetary shadow.

God must love vacuum, why else would he create so much of it?

Air-raft to orbiting ship

Posted in Science, Traveller on December 29, 2010 by Mr Backman

Various canon Traveller sources state that Air-rafts can reach orbit and in my Traveller campaign precisely that situation arose during my weekend session with my kids. I assume here that the ship we want to match orbit with is in Low Earth Orbit (LEO). The problem is much simpler if the ship is hovering on its contragrav above the planet but that is not what the canon sources say; ‘orbit’ does not mean outside the atmosphere, it means outside the atmosphere with enough speed for centripetal forces to match gravity.

If you dig into the problem there are lots of complications that crop up:

Problems with the air-raft
An open topped vehicle is hardly built for vacuum as this costs a lot extra, so I guess the instrumentation, upholstery etc will break in vacuum. Another problem is that an air-raft produces something like 0.1 G thrust for propulsion which mean (ballpark calculations here) that to reach say 5 km/s orbital velocity they must accelerate for over an hour (ca 5000 seconds).

Problems with the calculations
To match the orbit of a ship the air-raft driver must eyeball the ship and vector (yes, LEO ships can be seen at dusk or dawn by the human eye) and then match that orbit by hand with the air-raft over a more than an hour long acceleration phase. The air-raft will have no instrumentation for orbit matching and the like, just an accelerometer based (Traveller vehicles does not rely on the crude GPS system we use) absolute positional instrument that also indicate height as well as speed gauges. Calculating the orbital mechanics and driving the air-raft to comply is in my opinion a really hard problem for a spaceship pilot and impossible for mere grav-jockeys. If you think orbit matching is a piece of cake try it yourself with the free PC space simulator Orbiter.

IMTU (In My Traveller Universe)
My TL progression differs from canon and GURPS Traveller and this causes even more problems:
(I don’t add gravtech until TL 10, so I can have cultures with jumpdrives without grav and floorfield, ‘Hard-SF with jump’ if you will)
Jumpdrives TL 9
Floaters TL 10
Floorfield TL 11
Gravthrust TL 12
Floater gravbelts TL 13
Reactionless drives TL 13
Gravbelts TL 14
Tractor beams TL 15
Pressor beams TL 16
Rattlers (high freq tractor weapons) TL 17

Floaters are grav ‘thrusters’ that can only negate gravity, they can never create upwards or lateral thrust, just negate the downward pull of gravity. Floaters and gravthrust have ‘thrust’ proportional to local gravity so a 1G (Thrust = mass) floater will negate gravity on all planets, regardless of gravitation (simplifies designing gravvehícles and ‘explains’ why gravthrust is useless for interplanetary travel). Floaters come at TL 10, are much cheaper and require much less power per ‘thrust’ than regular gravthrust. Regular gravthrusters produce floating at the cost of x1/10 thrust (a 1G gravthrust would use 0.1 G for floating and 0.9 G for propulsion for example).
My air-rafts are so cheap they use floaters powered by a fuelcell for lift and turbojet for thrust (both the fuelcell and turbojet are hydrogen powered and need an atmosphere with oxygen to work).

So IMTU the air-rafts cannot reach orbit at all, they cannot even operate in anything near vacuum, fitted with compressors they can work in Very thin atmospheres, but that’s it.

Edit: I have updated the Intercept design system to reflect the TL progression (and no, there are no tractor, pressor or rattlers yet).

Hyperspace for Dummies

Posted in Intercept, Traveller on September 21, 2010 by Mr Backman



Hyperspace is so central to Traveller that one wonders why there is so little hard ‘facts’ about it. I will give my take on how hyperspace and hyperjumps work as well as the reasons why I choose to do it this way.

The main reason for having a detailed explanation how jumping works has to do with roleplaying in general. In actual roleplaying situations the GM must come up with consistent answers to ‘What if someone shuts down the jumpdrive when already in hyperspace?’, ‘How can a ship jump at all as it is inside the 100 d limit of our galaxy?’, ‘If drop tanks are ejected prior to jump, doesn’t that mean that jumdrives consume its fuel prior to entering jump?’, ‘Is jumpfuel burned as normal fuel similarly to a fusion plant?’, ‘Why does a jump take exactly one week?’, ‘What purpose serve the jump-dimming when the lighting power-draw is insignificant to what a typical starship use?’, ‘Can you detect a ship exiting hyperspace?’ etc.

Another consideration is the health of the player characters. We want hyperspace to work in such a way as to not needlessly kill off the characters. If any malfunction of a jumpdrive while in hyperspace results in the guaranteed destruction of the ship we’ll spend a lot more time designing new characters than we really want to. This is probably the most important consideration as long as it doesn’t get too silly; we don’t want all misjumps dropping you into space battles, ancient installations or mother-lode asteroids.

I will refrain from the Trekkie way of inventing new particles, fields and sciences as much as possible and instead try to come up with simple real world explanations, but please note that I do not actually know how jumpdrives work nor how to build one (how could I, Grandfather planted the jumpdrives for each major race to find, nobody but him really invented the jumpdrive).

‘Known facts’

  • Traveller books depicts the jumpdrive and hyperspace as something that is not quite understood by scientists, they know how to build a jumpdrive but cannot fully explain how the physics behind them work.
  • Jumpdrives punch a hole into another dimension, spend a week there, and return somewhere else, without actually travelling the distance.
  • Jumpdrives create a jump-bubble prior to jumping and that bubble surrounds the ship during the jump and isolate it from hyperspace while there.
  • Ships require 10% of jumpfuel per jump-number, regardless of tech level. Ships with jumpdrives cannot be made smaller than 100 displacement tonnes.
  • Astrogators presumably do their thing in hyperspace, after jumping, as there would otherwise be no need for astrogator crew on X-boats. As X-boats don’t have maneuver drives one could calculate jump from the outside, set the jumpdrive to jump and pick the ship up at the destination, with no need for crew at all.
  • Jumping requires energy and this energy must be delivered within a certain timeframe for the jump to work at all.
  • If you misjump into a hex (one parsec wide) with a star system you always end up inside the star system (near a planet, gas giant or star). The likelihood of that happening by chance is in reality minimal, so jump exiting somehow tend to occur near masses. Voluntarily jumping to deep space is rare and dangerous, this explains why so few J1 ships have extra jumpfuel and also the importance of  the ‘mains’. The fact that ships can cross the great rift tells us that deep space jumping can be done, but only at great risk and effort.
  • Some sources say that ships retain their vector relative the planet they jumped from but there is no mention of how the typically huge differences in relative velocity among star systems are handled.

Q: Why does jumpfuel take up 10% per jump number regardless of tech level/ship size and why can you not make smaller jump capable ship than 100 dTon?

A: The jumpdrive uses hydrogen to create a jump bubble around the ship prior to jumping. The jump bubble envelops the entire ship and is created from hydrogen plasma. The thickness of the bubble is proportional to the curvature radius at each point on the surface, the larger the radius the thicker the plasma need to be. When the curvature radius become too small the jump bubble becomes too thin to function properly. Average curvature radius for a ship increases with the scale of the ship while the surface area of the ship increases with the square of the scale. The volume of the jump bubble increases with the scale of the ship as scale x scale^2 = scale^3 or put simpler; the volume of the jump bubble is proportional to volume of the ship. Because of the minimum curvature radius the volume that the jump bubble encompass cannot be too small. Higher jump numbers require higher plasma densities which leads to a corresponding increase in jump fuel requirement.

The above reasoning ‘explains’ why jumpfuel is proportional to ship volume and why there are no ships of less than 100 dTons displacement. If you want to put a jumpdrive inside a 50 dTon ship you still need to give it a J-drive and jump fuel for a 100 dTon ship, to keep the curvature radius above the limit. Instead of forbidding jump ships smaller than 100 dTon they become increasingly expensive. A 50 dTon J-1 ship would need 20% jump fuel per parsec and the J-drive would cost twice as much per ton of ship. The latest Intercept design system incorporates this feature, get it here.

Q: Why is hydrogen used for jump fuel? Do ships really burn all that in a week?

A: Traveller state that only hydrogen will do as jump fuel and should preferably be purified in some manner to work reliably. Sources state that the jump drive is not a fusion reactor so there must be some other quality of hydrogen that is used for the jump fuel. One unique property of hydrogen that no other element has is a perfectly charge-symmetrical nucleus. Add a neutron (as in Deuterium) this symmetry is lost, even more so in Tritium. Elements with more than one proton cannot have a charge symmetrical nucleus, adding neutrons will only make matters worse. Let’s assume that this is what sets hydrogen lacking neutrons the only working element for jump bubble creation. Fuel purification then, is the process of removing hydrogen isotopes as well as any traces of the other elements. Naturally occurring hydrogen has about one Deuterium for every 6000 Hydrogen atoms, enough to destabilize the jumpfield somewhat but not enough to make jumping impossible.

Q: What purpose serves the jump dimming performed during the jump entry? The minuscule draw from ship lighting can be of no consequence to the jump?

A: Jump dimming is often explained that way and yes, it is just a tradition today but the original reason served a purpose once. When the jump-plasma forms it’s emissions are read by sensors on the hull to control its shape. It was once thought that light from windows would distort these readings, so much that the light from a camera flash could cause a ship to misjump. This has since proven to be false but traditions die hard, especially among the superstitious spacer culture.

Q: What happens if you turn off, damage or destroy the jumpdrive while in jump?

A: A misjump is guaranteed as the astrogator will get no readings from the jump-drive when exiting hyperspace but aside from that nothing bad will really happen. The jump drive is actually not used except at jump entry and jump exit, the rest of the time in hyperspace it is shut down. Yes, you can do maintenance on the jump-drive while in hyperspace, if you like to gamble.

Please note that ships in hyperspace do not actually travel during the week, the ‘decision’ on where to exit will be decided by the astrogator during exit. The fact that they do not travel means that jump masking will never occur, if a ship jumps 100 diameters from a planet but 57 diameters from a star the ship is considered jumping from 57 diameters when rolling for entry as well as how far away from the destination it exits. Always use the lowest diameters figure. The ship will ‘pick’ a mass point to exit at, at the same number of radii as when entering hyperspace. If a ship exits into deep space it will enter nearby a comet, asteroid, brown dwarf or even a Pellegrino pancake.

Q: What does hyperspace look like?

A: It is entirely black, zero Kelvin black. The glowing hydrogen plasma jump bubble is left behind in normal space so you see nothing but blackness out there. All this black is a good thing because that means you can radiate heat from your radiators normally even when in hyperspace.

Q: How does droptanks work?

A: Drop tanks tell us that the jump fuel is used up before jumping as the tanks supposedly remain outside the ship and need not be taken into account for jump displacement (according to book 5 High Guard). Jumping also requires a fair amount of energy (also according to book 5 High guard). High Guard states that a computer model equal or above the jump number of the ship is required, this is regardless of astrogator skill. It seems that the astrogator must remain aboard during a jump; if not, there would be much cheaper and more humane to use unmanned X-boats. Below I will outline my ideas on how the process of entering hyperspace works:

The jumpdrive is starting to dump hydrogen plasma to create the jump bubble. This will take anywhere from 15 minutes to 1 hour depending on the power applied. The bubble creation is controlled by the computer, higher jump numbers require faster computers to cope. Any drop tanks are still attached to the ship, they will be jettisoned just moments before the jump.

Q: How much power is needed to jump?

A: As the Jumpdrive itself is not a powerplant it needs external power to operate the plasma that creates the jump bubble. Power consumption and jump prep duration in Intercept is as follows:

Regular 14.5 m3 dTon

  • 0.5 EP x Jn / 100 dTon = Enter jump takes 1 hour, which is the longest time and lowest power for a jump to be possible.
  • 1 EP x Jn / 100 dTon = Enter jump takes 30 min or two regular Intercept turns.
  • 2 EP x Jn / 100 dTon = Enter jump takes 15 min, which is the shortest jump time possible. 

Custom 5 m3 dTon

  • 12.5 MW x Jn / 500 m3 = Enter jump takes 1 hour, which is the longest time and lowest power for a jump to be possible.
  • 25 MW x Jn / 500 m3 = Enter jump takes 30 min or two regular Intercept turns.
  • 50 MW x Jn / 500 m3 = Enter jump takes 15 min, which is the shortest jump time possible.

Q: Why is safe jump distance based on planetary diameter?

A: Hyper space jumping is affected by the local gravity gradient or in layman terms; how much the gravity changes at the jump point. Change in gravity is called tidal force and happens to fall off at the cube of distance and as mass increases with the cube of planetary diameter the tidal force is proportional to planetary diameter. A more thorough explanation to this can  be found here.

Q: Why does misjumps so often end up near planets?

A: Obviously to keep the players entertained but alive. The in-game explanation goes like this:

Hyperspace jumps must be performed inside 1000 planetary diameters and preferably outside 100 planetary diameters. Jump exiting always occurs at the same diameter multiple as the entry. Jump at 57 diameters and you’ll end up at 57 diameters of the target. Entering or exiting hyperspace beyond 1000 diameters is impossible. When a ship is about to exit hyperspace the jumpfield ‘selects’ a mass at least as large as the ship at a distance from the entry based on jump number. The astrogator guides the selection to his intended target at those frantic last minutes prior to exit. Roll astrogation when exiting, not entering hyperspace.

Use whatever method your rule set dictates for determining where you exit but if you should exit an empty hex just decide that the ship exits near a Oort comet, rogue planet, escaped asteroid, brown dwarf etc. This explanation also gives the referee the opportunity to have deep space misjumping players discovering hidden Zhodani fuel caches, derelict spaceships crashed on a comet etc etc.

Q: What is jump masking and how does it work?

A: GURPS Traveller added some rules about how planets and stars 100 diameter sphere would block jump travel. This made calculating jump times more complicated without adding anything to gameplay. It also broke the assumption that hyperspace travel doesn’t propel the ship in our universe; how can a planet or star block a ship that is no longer in our universe? The ship enters hyperspace, stays there for a week and precipitate out at a destination that depends on the astrogator controlling’ the jump drive at the end of the week.

Those who like jump masking can of course use it but I see no point as it breaks the established fiction of hyperspace without giving anything back aside from complication.

Q: Does ships retain their vector when exiting jumpspace?

A referee that allows ships to retain their relative vector when jumping must also account for the relative velocities of the two star systems, and as such velocities are typically very large we get all kinds of problems including perfectly good jumps smashing into planets. We need a system where the ships exit jumpspace at rest visavi the jump target (yes, that means they will eventually fall into the planet if their maneuver drives are dead, X-boat pilots lead interesting lives). The Mach conjecture described below bear little resemblance to the real one posited by Ernst Mach.

Inertial mass is created by the gravity of surrounding matter according to the Mach conjecture. Closer mass has a stronger influence over inertial mass as the mass further away takes longer to ‘react’ to changes in momentum by our ship. The velocity of a ship relative the gravity well it jumps inside is canceled when entering hyperspace, the velocity distributed among the surrounding mass from the Mach wave created by the jumping ship. The Mach wave travels at the speed of light so conservation of momentum is never broken inside the light cone. Ships exiting hyperspace always exit at zero velocity relative the jump target (there are no deep space jumps as explained above).

This may sound as if relative velocity has no significance when entering jumpspace but as the positional uncertainty is larger with higher velocity, hyperspace entry become more difficult in a manner similar to being deeper in the gravity well.

Hyperspace game mechanic sketch

  1. Travel to somewhere inside 1000 diameters of the planet (ideally one should travel to 100 diameters exactly, closer than that and the entry will be more difficult).
  2. Spend 15 minutes to 1 hour using up all the jump fuel (from internal tanks or external droptanks), this phase require constant power as explained above.
  3. Roll a task that depends on Computer, how deep in the gravity well, velocity relative to the central planet, quality of fuel (purified or not), jumpdrive damage. This roll does not depend on the astrogator skill.
  4. The ship has entered hyperspace, the hyperdrive is switched off and the ship will remain in hyperspace for one week.
  5. At the end of the week the astrogator must help the jumpdrive ‘select’ a mass to exit at. The difficulty of the astrogator task depends on the result of the jump entry task above.

The ship will always exit at zero relative velocity and at the same number of diameters from a mass-point more massive than the ship itself. It is possible to exit at the entry point, randomly or on purpose. The astrogator decides on the destination at exit, never at entry. Having no astrogator or a broken jumpdrive means the ship will automatically misjump.

Entering hyperspace may very well look like the effect in Star wars, but hyperspace itself is black.

Well, that’s it folks.

Conserve space – dump in jump.

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?