Archive for the Vector movement Category

Floater thrust

Posted in Intercept, Science fiction, Traveller, Vector movement on October 10, 2021 by Mr Backman

Belly landers and tail landers

“The ships hung in the sky in much the same way that bricks don’t.”

― Douglas Adams, The Hitchhiker’s Guide to the Galaxy


Most smaller ships in Traveller are of the belly lander variety which like the term indicate land and take-off on their belly, all of this is accomplished by the floater ability of all grav thrust vehicles great and small. When ships get into space however, the depictions of Traveller ships seem to forget about this gravity negating ability but not here in Intercept.

Belly landers should have their belly towards the planet to negate gravity while tail landers should have their tail towards the planet to negate gravity. Because Intercept is a 2D game with ships seen from their top (or bottom if rolled) we decide that belly landers simply can negate gravity to their left or right, the important thing here is that their floater thrust is 90 degrees of their thrust axis while the tail landers have floater and regular thrust co-axial.

Legal floater directions

Movement phase

Let’s go through a movement phase in Intercept to see if and when we can apply floater thrust. We’ll use a belly lander for the example but a tail lander works similarly except what facing it need to have to perform floater thrust. Note that a ship that uses floater is considered thrusting so there is no +2 DM for attacking or being attacked and the ships Signatures will be for a thrusting ship.

Floater drift after gravity

We will use compass directions in this example, we call the up direction north, down is called south, to right is east and left is west. We strongly suggest you use the same conventions when you play Intercept as left and right can be confusing when two players sit opposite each other each with their own map.

The ship is drifting 3 squares up or north and it was facing north in the last turn. Repeat the last move and mark with a thin cross, then apply gravity based on the position of the last turn and mark with a cross. This is the ships Drift and if the ship doesn’t use any thrust, regular or floater, this is where it will end up for the turn.

Now the pilot Pilot should decide if he wants to and is able to  use Floater to negate the gravity pull. Floaters can only negate gravity so it can only move the drift marker back to the light pre-gravity cross. Whether the ship can do it or not depend on the facing from the last turn.

Floater belly lander float directions

A belly-lander facing north can negate gravity to east or west, gravity was pulling to the east so the floater can only thrust back to west making the course a straight line again. So there, the Drift and gravity phase of movement is over, the ship could now turn and thrust (and also aerobrake if adjacent to the planet). The captain decides to neither turn nor thrust here, but that doesn’t mean that it is drifting, as previously stated using floater is considered thrust.

Floater drift after gravity2

 In this turn the ship is still facing north and still has a vector of 3 squares north. The ships last turn was in the north-west gravity arc which will pull the drift one square south-east as shown. Let’s see if the pilot can use his floater again this turn.

Floater belly lander float directions2

No, the gravity pull of south-east cannot be negated by any the floater thrust arrows, the ship should have turned to face north-east in the last turn if it wanted to use floater this turn. We mark the ship with a ring as it is actually drifting this turn. 

Floater drift after gravity3

Using floater thrust is rarely worth the effort but can be used in certain situations, especially for ships such as the subsidized merchants. Subsidized merchants, the lower left part of the topmost image, uses reaction-mass guzzling fusion thrusters for thrust i space, in atmosphere the fusion rocket switches to air breathing with a considerably lower fuel use. Subsidized merchants have Floater grav drives that can only negate gravity but that can be used to reduce fuel consumption from thrust at the cost of longer times to jump. Thrust away from the planet (costing no fuel if the planet has an atmosphere thanks to air breathing) and then turning to keep the side towards the planet (the belly in reality). Then they’ll simply Drift away from the planet gliding gracefully out to the jump point.

Floater, grav and impulse drives

Floater thrusters is the first gravity technology and it appear at Tech Level 10. They can only negate gravity and will work exactly as if it was a rocket for those inside the vehicle. The subsidized merchant is a prime example but there are also some grav vehicles with Floater to negate gravity and some other propulsion means such as propellers, ducted fans or jet engines. Floaters are rated in in how much mass they can negate gravity of, a 10 ton Floater on a ship massing 10 ton or less will negate 100% gravity regardless of gravity strength (this help the referee a tremendously as she no longer need to design vehicles for each strength of gravity).

Grav thrusters appears at Tech level 11 and create thrust proportional to gravity. These are you regular sci-fi grav vehicles. They can reach orbit and beyond but get more and more sluggish as gravity diminish the farther out they gp. Also note that regular grav vehicles rarely have the instrumentation to actually match something in orbit, see this post on reaching orbit in an air/raft. Grav thrusters have have built in Floaters too.

Impulse drives finally appear at TL 12 and above and can create thrust regardless of local gravity. They are the kind of drives most Traveller ships use, except for the Subsidized merchant mentioned above and some Vargr ships which uses the high thrust ratings of fusion drives to make high G corsairs with limited endurance.

So, now you know how ships and vehicles of scifi manage to hang in midair, just like bricks don’t. That is all folks, until next time…

Gas giant skimming

Posted in Intercept, Rules, Traveller, Vector movement with tags , , on September 29, 2021 by Mr Backman

Dangers of skimming they say, bah!

Sure, there are dangers, plenty of ’em too. Flying inside a gas giant’s vast gravity field isn’t dangerous per se, you can orbit them forever with no danger of being sucked in, the problem is that in order to skim hydrogen we must dip into the atmosphere and have it slow us down. Speed is what keep us from falling into the gas giant and we are intentionally slowing down while being as close to its crushing atmosphere as possible.

So, we dip in as deep as we dare and let the atmosphere slow us while our scoops gulp in as much gas as possible. Then we thrust away building up speed again to avoid the huge planets cold embrace. Some make one high speed approach and hard braking, others make a series of gentler dips instead, spreading out the danger. System Defense Boats and other daredevils sometimes even slow their ships down to zero deep within the atmosphere, hovering using their grav floaters and slowly scooping gas. No matter how you go about it the risks are real and minor damage to the hull is common.

As if the skimming itself wasn’t hard enough, there is also the real risk of pirate scum picking on us when we are the most vulnerable, they are rare but they do exist.

So there should be no surprise when I tell you that we do not skim that often. The time to enter and exit the huge gravity wells of gas giants offset the cost of getting fuel for free, time is money you see, especially as you must also pay for any hull damage inflicted by the skim.

So there you go, the dangers of skimming are real but not what you think!

Gas giant maps

Gas giants are, as is evident by their very moniker, huge. They are so large so they need special maps when used where the planet take up a large chunk of the map and their gravity field cover the rest of the map. Map sheets for small and large gas giants are available in the InterceptBundle here.

There is the possibility to use the Large scale rules on pages 34-35 and use the large planet maps for large gas giants and small planet maps for the small gas giants. This won’t be covered here though, just make sure you take the scale changes into account as outlined by the rules there.


Let us say we ha a streamlined ship with 1G of thrust, a Size of 8 (100 dTon) and a Pilot skill of 2, the computer is a model 1 giving us just 1D6 dice pool. The Pilot defaults table will give us 3(Miss) for 3 steps of turning every turn and the task result will be treated as a Miss. We will use the Pilot default on our approach flight but for the aerobrake turn we’ll dare an actual Pilot task roll trying to get a better result as the damage roll depends heavily on our Pilot task result.

The ship will fly towards the gas giant ass first, brake-thrusting to keep the speed from becoming too large from gravity – maximum safe speed/drag is 4/2 for airframes and 2/1 for streamlined. Keep those numbers in mind as you approach the gas giant. Try to hit the atmosphere edge on rather than head on when entering the voluntary aerobrake squares (the light grey area bordering the planet), also make sure that your ship face in the direction it will be going to head in the next turn.

The ship in the picture is thrusting to negate gravity (each x show where it would be if it drifted that turn). In the last turn it drifted and let gravity pull it into the voluntary aerobrake zone and speed it up to a speed of 4 (4.5rounded down). The circle around the ship show that it is drifting

The next turn we’ll go through aerobrake skimming step-by-step so please pay attention. This is a good moment to tell your crew over the intercom to buckle up, things can get bumpy.

Aerobrake skimming

Aerobrake steps

We’ll follow the steps from page 25 of the rulebook in some detail here, don’t worry though as it is much easier in practice. Print out a mapsheet and doodle away.

Decide drag and determine Pilot task result

Decide drag, then roll Pilot or use Pilot default to determine the level of success and the amount of turning available this crucial turn. Rolling the Pilot task will give you a better result on the average but with the Pilot default table you know what you get. We are moving at a speed of 4 and have decided to use 2 drag to stay in the 5-6 column of the Aerobrake DAM table, more on that later.

After deciding drag we will determine our Pilot task result. Pilot default has given us 3(Miss) for all of the turns of our approach but we will try to roll the Pilot task for the aerobrake hoping to get at least a Fair result, aerobrake damage depend heavily on the Pilot task result and a Miss is just not good enough we think. Rolling may give us worse than Miss of course but we bet that fortune smiles upon us. The ship is Size 8 and our Pilot skill is 2 with 1D6 dice pool so we roll 3D6 and pick the two highest and add 2.

Pilot task is rolled against the ships Size of 8 so we roll 1, 3 and 3 and use the two 3s for a die roll of 6 with 2 added, 8 – we just barely managed to roll a Fair result! This will give us 4 steps of turning but more importantly our damage roll will be based on a Fair result instead of a Miss, more on that later.

Drift and gravity (including Floater)

Gravity will take us back into the voluntary aerobrake zone again and our speed is still 4. Let’s zoon in a bit so we can see clearer.

Turn and thrust (or Initial Split-movement)

Typically you’ll only turn here, to face in your drift direction but nothing stops you from thrusting too, you may even do the first half of Split-movement here, go crazy but don’t forget that if you hit the planet you are dead. Our facing and vector are actually perfect so we’ll leave the ship as is here.

Pop-in and Forced facing

Now the ship pop in its surface fixtures which means that Visual, IR and Radar cannot Scan later in the turn, you do get to keep your Tracked targets and any launched missiles. We’ll pop in and hope that no coward pirate sneak up on us, for new targets we’ll be completely blind this turn.

Our ship is facing in the direction of our vector so there will be no Forced facing. Try to avoid this forced facing adjustment as it will increase the risk of damage quite a lot (your ships hull will be treated as one degree worse if adjusted here.

Aerobrake (first drag and then maneuver)

We decided on a drag of 2 two so let’s do them. We will simply move the drift two squares, one at a time. In our case there are no choices but sometimes there will be two options (zag-zig or zig-zag) and the Pilot can decide which one that is preferable as long as each step moves the drift closer (and thus reducing speed).

The ship has no wings and we didn’t adjust facing so there is no Maneuver to do. If we had some maneuver to use it would still be limited by the drag used (in this case 2), so maneuver could move the drift after aerobrake up to 2 in any direction. We have zero maneuver but if not we could maneuver to any square that wasn’t greyed out in the picture. Note that brake Gs are determined by counting the squares from our position before the aerobrake to the position after drag and maneuver.

Adjust facing

Our ship is still facing in the direction of our vector so no final facing adjustment is needed, this is mostly happening when a ship also uses Maneuver. This final adjustment of facing does not affect damage, it simply turns the ship to face the direction of travel.

Roll aerobrake damage

Aerobrake damage

We came in with a speed of 4 and our aerobrake took us 2 squares from that so our brake is 2. We’ll use the 5-6 column as 4 + 2 = 6. Our ship was Streamlined so our aerobrake DAM is 3. Now it’s time to roll the actual damage.

Aerobrake damage roll

Our Pilot task roll was Fair from the Decide drag and Pilot task step and the table tell us a Fair result will be rolled using 2D6 and picking the lowest and that the location of any damage is Hull.

We roll a 2 and a 5 and use the lowest one so 2 it is. Add 2 to the 3 we got from the aerobrake DAM table, 5 on the damage table is (Scratch), one point shy of Light damage. Some scorched paint and scratches is all we got from the aerobrake, let’s get out of here!

Skimming fuel

  • Skimming will net speed x brake x 5% of its hull per 15 min
  • Hovering will net 1.25% of its hull volume per 15 min

The ship did an aerobrake of  speed 4 and drag 2 which give us 4 x 2 x 5% = 40% of the ships hull volume skimmed. This is probably far more than the actual tankage we got so we leave the gas giant with full tanks and some scratches on the hull for our efforts.

Hover skimming

Hover skimming using Floater will be dealt with in a future post, stay tuned!

Large scale maps

Skimming can also be done using the 100 000 km per square, 1 hour per turn scale. In this scale large gas giants use the large planet maps and small gas giants use small planet maps. This will also be dealt with in a future post so stay tuned for that too!

So, to finish off my diatribe about the dangers of skimming by saying that the subs, the subsidized merchant crews are the bravest and here’s why:
A fully loaded sub has enough fuel for 4 hours of continuous 1G thrust! You normally use less than half of that to get to the jump point, which means that you have a bit more than half to maneuver towards the gas giant. What is even worse is that when fully loaded a sub’s Floater is only capable of negating about 60% of gravity, yeah, that is why they have those wings; to assist in takeoff when the Floater alone cannot do the job, and these guys sure need to turn every dime as their sponsors take half of what they earn.

I dare you to find a single subs skipper that has ever skimmed a gas giant fully loaded!

Vector movement game units

Posted in Intercept, Other vector movemet systems, Rules, Science, Traveller, Vector movement on July 16, 2019 by Anders Backman

When you create your own vector movement system, as I am sure everyone does, you need to determine what map scale, turn length and acceleration units are. There is an obvious formula from school that most seem to use and but will argue for why this is wrong and why one should instead use another formula, also from school.

Plotting example


Typically one decide on the map scale first (how large will the hexes, squares, inches or centimeters be?). Deciding on scale is mainly about what you want in your maps, do you want planets to be one unit or less in scale? Do want to show Earth and the moon on the same map? Do you want to fit the inner solar system on your map, like Triplanetary? And so on.

Some examples:

  • Intercept 10 000 km per square, 15 minute turns, 1G per square.
  • Intercept large scale 100 000 km per square, 60 minute turns, 1G per square.
  • Traveller LBB 1000 km per inch, 5 minute turns, 1G per inch.
  • Mayday by GDW 300 000 km per hex, 100 minute turns, 1G per hex.
  • Triplanetary ~10 million km per hex, 1 day per turn, ? G per hex.

Intercept let you play using two different scales and switch back and forth as you like, there’s even a smaller scale in the works if I can iron out the problems with planets taking up large parts of the map, that scale will be 1 000 km, 4 minute turns and 1G per square as usual.

We will later calculate what acceleration Triplanetary is likely using based on the distance and time scales and formulas learned.


High school math teaches us two formulas for determining distance traveled, one for when applying constant acceleration from a standstill and another when having constant speed.

The two formulas are:

Formula one and two

Notice how formula 2b is the same as formula 1 but without the 1/2 multiplier. Distances traveled become twice as far in formula 2 so one of them must be wrong, right?

Not so fast! The formula from high school actually looks like this:

Formuila one with prior speed

Formula (3) also take the velocity from the previous turn into account (the v0 term). As v equals a multiplied by t we get our beloved formula (2b) as the first term, or something similar at least.Why is the first term twice as big as the second term? Well, the the first term assumes the speed is constant through the time segment t while the second term treat is as increasing, the distance traveled can be seen as areas in graphs with speed plotted versus time, like this:

Formula and area

If we use formula (3) to determine total distance traveled while t keeping t as the turn length and v as v (n-1) where n is the number of turns we’ll see that as the number of turns increase the grey area will more and more resemble a rectangle (the triangle of the last turn will contribute less and less of a fraction of distance traveled).

Formula graph

The grey area is the distance traveled. If we call one rectangle as 1.0 and one triangle as 0.5 we get the following distances:

  • Turn 1: 1 triangle plus 0 rectangles = 0.5
  • Turn 2: 2 triangles plus 1 rectangle = 2.0
  • Turn 3: 3 triangles plus 3 rectangles = 4.5
  • Turn 4: 4 triangles plus 6 rectangles = 8.0

and so forth…

You see that as the number of turns increases the number of rectangles increases faster than the number of triangles. So, as the number of turns increase the the number of rectangles will outstrip the number of triangles.

In the vector movement systems of Triplanetary, Mayday, Traveller, Intercept etc we use a vector that both represent velocity and acceleration however. So we either decide that one unit length should be correct for acceleration from a standstill but wrong for drifting or accelerating with a prior velocity (1) or we decide that one unit length should be correct for drifting and approach correct when handling prior velocity (2).

Too much theoretical bullshit you say? OK, let’s do a practical example.

Let’s travel from a standstill to the moon as see which of formula (1) or (2) most closely fit (3). We ignore braking at the moon just go to the moon as fast as possible. The average distance between the earth and the moon is 380 000 km so let’s go with that.

Units for formula (1)

  • A = 10 m/s^2
  • T = 1000 s
  • S = 5000 km

Distance earth – moon will be 76 squares.

Traveling 76 squares with 1 unit acceleration will be

1+2+3+4+5+6+7+8+9+10+11+12 = 78 units after 12 turns (12 000 s) = 3 h 20 m
(We overshot the moon by 2 squares but this is the closest we could get)

Units for formula (2)

  • A = 10 m/s^2
  • T = 1000 s
  • S = 5000 km

Distance earth – moon will be 38 squares.

1+2+3+4+5+6+7+8+9+10 = 46 units after 10 turns (10 000 s) = 2 h 45 m
(We overshot the moon by 8 squares but this is the closest we could get)

The correct value using formula (3) and setting v0 to zero is (8 718 s) = 2 h 25 m


Sorry about the long winded explanation but for some reason most vector systems get this wrong. Doesn’t matter when you play of course but say you want to travel from earth to the moon using actual mapboard movement you’d find that the travel time would not match the calculated value.

Apollo 11 50 years anniversary July 16 1969

Yes, 50 years ago today a couple of Americans started their travel from earth to the moon , certainly not under constant 1 G acceleration and they made damned sure their velocity was as close to zero as possible before they hit the moon. Apollo 11 did the trip in about half a week.



Traveller day

Posted in Boardgames, Computer games, Elite Dangerous, Films and TV, Other vector movemet systems, Traveller, Vector movement on May 1, 2019 by Anders Backman


Today is Traveller day.
Just read what’s written on the box and you’ll get it.

Traveller was the first science fiction rpg ever published, it came out the same year as Star Wars but before the movie was released, a happy coincidence. Traveller was a strong influence for the Elite computers games as well as the Firefly and Expanse TV series, Traveller has also influenced Intercept which of course is eminently usable as a space combat system for the Traveller rpg.

Oh, and Traveller used vector movement for its starships instead of some lame-ass Trek thing…

Marc Miller, the creator of the game just started a Kickstarter for his new edition of Traveller today: