Archive for December, 2010

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).

Playthrough session

Posted in Intercept on December 12, 2010 by Mr Backman

Me and Cicci played out a battle between a TL 14 Scout on one side (with a single large laser in its turret) versus a TL 12 Subsidized merchant and it’s Launch. In our Traveller game she usually flies a Scout while my kids have a Subsidized merchant and a Launch, this was a test to see if the low tech Sub and launch could actually beat the Scout.

Both sides started secretly at the top or bottom box rows of the map (yes, they could have started on the same row, even the same square). The Sub was to touch any square adjacent to the planet and then exit out the same map edge it came from, the Scout was to stop it from doing that. We used the new 3.1 sensor rules which was much easier than the old  ones to use. The Scout spotted the Launch early on but didn’t see the Sub until it was close to the planet (I was drifting the Sub most of the time to save on Visual/IR signatures and also to save on fuel). The Scout shot out the Subs’ powerplant and the Launch killed the Scout’s thruster. The Sub could limp back out as its fusion drive requires no power, leaving the crippled Scout in orbit around the planet. Marginal victory for the Sub and Launch.

The game (as always) made for some changes in the rules and components. The main change was that the hybrid hitlocation system could somewhat unfair, a higher degrees of success should always result in equal or better options but that was not the case for the poor Scout player. She attacked the Sub from its RearLeft arc and as she got a Good hit the rules forced her to hit the Powerplant. Had the hit been fair she would have rolled randomly and had the hit been VGood she would have picked the location but a Good hit forces a certain hitlocation, in this case pretty much the worst location as neither weapons nor thrust relies on power in a Sub. I rewrote the hitlocation rules as follows:

Regular hitlocation

  • Fair hit Roll hitlocation
  • Good hit Attacker choose to roll or use facing
  • VGood hit Attacker picks hitlocation

Deterministic hitlocation

  • Fair hit Hitlocation based on attack facing
  • Good hit Hitlocation based on attack facing
  • VGood Attacker picks hitlocation

The big mapboard should have sensor boxes on the to make it easier to go between the boards. Not an important change for sure but still needed I think. I also added the option to make missiles with longer endurance at the expanse of thrust, this option can be used together with the coldstart option. Missile versions:

  • Regular Performance and as stated, price as stated.
  • Cold start Subtract 2G from thrust and double price.
  • High endurance Subtract 1G from thrust, double the endurance but keep price. Reduce PEN and DAM by -2.
  • Cold start high endurance Subtract 3G from thrust, double the endurance and double the price. Reduce PEN and DAM by -2.

Aerobraking never entered into the fight but I kind of liked the older version better, the one before the 3.1 update. I think I’ll go back to the more accurate and more complicated version, mainly because the chance of crashing into the planet goes up. If one cannot see through the planet one shouldn’t easily fly ‘through’ it either.

The final change was to add back in the very first integration rules where you get a cumulative +1 in Scan for each subsequent scan with the same ship, sensor and scan area. This integration bonus can never go higher than +3 and must be from the same ship and sensor type and it must be continuous from turn to turn. Actually, this rule highly overstate how well integration time behave in reality, realistic rules would give you +1 after 10 turns of integrating, +2 after 100 (yes, that’s more than a full day), one could rationalise this by assuming that the scan is actually concentrated on the parts of the scan area with better signal to noise than the average. The real reason for doing it this way is to give the playef a way to boost signal a bit, if you have the time and the target is stupid enough to linger.

After playing we looked at each other’s plotting traces and Cicci showed how sneaky/munchkin she can be. She deployed at the narrow, rightmost square-column knowing I would never waste scan area there and also helping her avoiding the sun direction. Sneaky! maybe I should shave off any partial boxes from the small plotting map? All the changes except adding boxes to the large map have been added to the Intercept bundle (including a fix for the sneaky trick Cicci discovered).

Favourite unit for space combat calculations: Rick, one Rick equals about 3 km/s and it’s the speed where matter has the same kinetic energy as TNT.

Sensor rules 3.1

Posted in Intercept on December 2, 2010 by Mr Backman

Ah, finally something new at this website! My excuse; well, mostly work actually but I have also been fiddling with some En garde inspired rules for Traveller which I will post here whenever I have enough stuff written.

I have playetested Intercept with my girlfriend mostly but also with some of my colleagues at work and certain errors cropped up regularly enough to warrant investigation. Intercept is a double-blind system without a referee so it requires a great deal of trust between the players. If a player does his scanning wrong it is hard to detect the error until after the session when the players typically look at each other’s plotted maps. The errors were of three kinds:

  • Coordinate errors People weren’t as used to cartesian coordinate systems as I thought they’d be. If one could merely point at the box or boxes scanned on the common map a lot of errors would go away.
  • Planet LOS errors The planet Line-Of-Sight rules were hard to do right as one had to match the square scan against the jagged blocked arc.
  • Aerobrake errors We had only a few of those because aerobraking is so rare but whenever a player tried such a stunt the chances were high that he would do it wrong.

Coordinate improvements

The plotting map is now divided into 5 square x 5 square areas called boxes. These boxes are easy to see on the map and if player A says he scans in the x = +5, y = +15 box (or simply points at the box on the common plotting map) the other player can easily check if his ship is inside or outside of the box. No more counting on scan radii to see if one is inside or not (or using the my custom-made plexiglass scan guides). Scanning individual squares (either 1 x 1 or 3 x 3 squares) require a bit more scrutiny but the 3 x 3 square scans can only be done in the same or adjacent boxes of the scanning ship and the 1 x 1 square scans can only be performed inside the scanning ship’s box itself. These small scans are rarely used and are dead giveaways to the scanner’s own position and as they are so rare the players can agree beforehand to not allow them if you both feel they add too much complexity.

Planet LOS changes

Planet LOS is much simplified thanks to the addition of map boxes. Check what arc of the planet the scanning ship is in and the boxes along the opposite centerline are illegal for scans. If a scan includes the central box containing the planet the scanner must tell his opponent what arc is blocked, the arc opposite of the scanner’s arc that is. Yes, scanning the central box gives the opponent more information that before but that is a small price to pay for a much simpler LOS system with far less errors. If you are scanning the tiny 1 x 1 squares or 3 x 3 squares you still have to abide the old rules which simply states that the scans cannot include squares in the opposite arc from the scanner. As 1 x 1 and 3 x 3 square scans are so rare one could agree on not allowing such scans if you both feel they add too much complexity.

Aerobrake changes

Aerobrake manuevers are typically done before being spotted as it allows you to slow down the ship while still being stealthy (no large Visual, IR, Neutrino or Mass signatures from the drive) and must therefore be easy to do without asking your opponent how it works. The new aerobrake rules uses requires the ship’s Present to be on the planet for aearobrake to take place, if the ship merely crosses the planet square it has flown above or below it. A ship can actually maintain a steady polar orbit by jo-jo-ing back and forth across the planet as long as its Present never end up on the planet.

The singularity; Rapture for nerds – Ken MacLeod