R-drives vs. M-drives

[Robbo]

I'm working on a setting with two human cultures, one is an entrenched and established empire that has M-drives and gravitics but lacks fusion power for ships. The other is a group of newcomers with fusion power plants but no gravitics that use R-drives.

The problem is that M-drives in canon are very, very good. Can anyone think of any drawbacks that might give the newcomers (the player characters!) a fighting chance? Is the fact the Empire ships use chemical or fission power enough of a balance? I'm using hyperdrives so Jump fuel is not necessary.

 

[dmccoy1693]

I built a number of ships like that few months ago. I hope I can help.

R-Drives have their own fuel (and thus power supply). So if that side is using missiles, torps and other non-energy weapons, they require almost no power plant. You can get away with whatever power supply supplies them a rating of 1.

R drive ships don't need (and frankly shouldn't have) 2 weeks worth of fuel; 6-12 hours will do most just fine. This leaves more room for weapons and armor

R-Drive ships require heat shields for reentry into a planet. (Grav drives handle gravity wells just fine and thus do not require heat shielding.) To avoid having to lose extra space to heat shielding, have the r-drive ships operate from an orbital station while some non-fighting ships with heat shielding will transport pilots from the surface to the planet.

The R-drive side may have an entirely different design philosophy in ship design. A single large ship (2000+ tons) can carry a large number of short range drones (<20 tons) to handle their ship to ship fighting (think star destroyers and tie fighters). The M-Drive side might desire more independent ships that can travel long distances independent of a mother ship (x-wings)

hope this helps.

 

[Robbo]

Another problem is that the R-drive ships do need a larger power plant for the hyperdrive in the larger ships. So the R-drive fleet might be based on the old battlerider concept, a jump capable ship transports a horde of small fast R-drive fighters with lots of reaction mass. Thanks.

What I was really looking for was some drawbacks I could attach to the M-drive ships. I remember reading somewhere that early grav propulsion lost power when they moved away from a gravity well. I think it was in FF&S1. Any help in this area would be appreciated.

 

[dmccoy1693]

none that I am aware of in mgt. But there is no reason why you can't add them into your home game.

 

[atpollard]

What I was really looking for was some drawbacks I could attach to the M-drive ships. I remember reading somewhere that early grav propulsion lost power when they moved away from a gravity well. I think it was in FF&S1. Any help in this area would be appreciated.

Under the rules as written, the Reaction Drive culture is going to be pretty much trounced by a MD-Fission combination. The Grav drives are going to far out maneuver reaction drives.

One way to 'balance' that disadvantage is to seriously skew the rest of the TLs. For example, Grav culture = TL 8 and Reaction Cutlure = TL 12. That will give the Reaction Drives a serious weapon/armor advantage.

Perhaps the Fission Reactor becomes Fuel Cells or Internal Combustion and needs LOTS more PP Fuel.

Another possibility is to limit MD to under 1000 dTons and Reaction Drives to over 1000 ton ships. Then you get MD SDBs with turrets vs Reaction Battleships with Bays and Spinal mounts. [Of course this strains the rules a little].

 

[Fovean]

I remember reading somewhere that early grav propulsion lost power when they moved away from a gravity well.

I don't have it in front of me but I think T5 has M-Drives losing force as they get farther from a significant source of gravity, becoming useless around 1000 diameters IIRC.

Maybe something simple like -0.3 G of propulsion for every 10 diameters out is applied to M-Drive ships. You'd have to keep track of where the bodies are in the system relative to the ships, but it gives the R-Drive side a 'lure 'em out and pick 'em off' strategy and forces the M-Drive ships to stay near worlds in order to be 100% effective.

This probably wouldn't affect Jump ships either, since general practice is to decelerate to 0G prior to jumping anyway, in order to avoid potential crashes when emerging at the pre-jump velocity.

 

[Robbo]

Maybe the M-drives could be half effective within 10 diameters and become very low low thrust beyond 100 diameters. That way an M-drive ship could still boost like mad for a coup[le of days and build up a good velocity for insystem travel, decelerating at its destination world much like an R-drive.

In a battle the R-drives would have a chance against the M-drives because they would have to stay near a world to have good acceleration. They couldn't keep breaking off and harrying R-drive forces. The R-drive forces would have to win before they ran out of fuel.

I'm going to give it a try and set the R-drive humans at TL 12. The M-Drive humans will have TL 10. I think that will be enough to give the rocket jocks a chance.

Of course gravitics are also necessary for laser weapons. So the rocket jocks will have to make due with missiles and sandcasters for turret weapons.

 

[Fovean]

Of course gravitics are also necessary for laser weapons.

How so? I'm no physicist, but I've always thought of lasers as massively focused beams of light... not sure why gravitics would be necessary.

Not doubting you, just curious...

 

[Robbo]

Uhm. Don't the lasers use gravitics to focus their beams over great distances? I remember reading that in FF&S. Is that not canon anymore?

 

[BP]

FF&S is not part of MGT Cannon. As to this 'explanation', not sure if it appears anywhere in MGT...

 

[atpollard]

Of course gravitics are also necessary for laser weapons.

How so? I'm no physicist, but I've always thought of lasers as massively focused beams of light... not sure why gravitics would be necessary.

Not doubting you, just curious...

Ultimately it is very hard to create a perfect column of light. Real lasers tend to be very narrow cones, so the light spreads over great distances (like starship combat) and damage should decrease with the square of the range (ie. 2x the range = 1/4 damage; 3x the range = 1/9 damage).

Some 'magic' technology overcomes this. Previous versions of Traveller tended to use 'gravitics' as the magic wand to overcome all of these annoying real world issues.

I think that Mongoose Traveller just waives its arms and says 'this is how it is' without trying to explain the unexplainable.

 

[Rikki Tikki Traveller]

Another thing to think about is that the Reaction drive described in HG is really a chemical drive.

If you want to postulate a Plasma Drive or Fusion Drive (Per TNE), it would be much more fuel efficient while still being a R-Drive. I use a Fusion Drive IMTU that is twice the size of an M-Drive and uses 1% of the ships volume per G-Day; 3G max sustained acceleration, but up to 10G for Combat Maneuvering.

Another way to limit the effectiveness of the M-Drive is the natural 6G limit from the rules. Allow R-Drives to go to higher accelerations (perhaps up to 10G like I did above), that gives them a maneuvering advantage for short periods of time. Humans cannot stand much more than about 2G for long periods of time (hours), but given acceleration couches and perhaps advanced drugs etc, people could sustain 10G for 1 turn(?).

Just some thoughts.

 

[Robbo]

Another thing to think about is that the Reaction drive described in HG is really a chemical drive.

That is one HECK of a chemical drive. For 50% fuel it gives almost a day's acceleration at one gee. You could go to the moon and do a few laps around it then go back to Earth no problem.

I like the fusion rocket. It could be an analogue to an MD with a fission reactor.

I realized that if the MD culture uses reactors and chemical power then they are tied to refueling bases. The RD culture can still refuel from gas giants. Another perk for the underdogs.

 

[dmccoy1693]

Humans cannot stand much more than about 2G for long periods of time (hours),

Oh Pish Posh. So our heads will cave in and our chests will crumple. So what. Big deal. Its just so much fun going 12G though. :wink:

Seriously though, if humanity has achieved some way of going that fast, they would have made some way of compensating for it. Inertia Dampers maybe bogus science fiction, but there has to be some way out there that we haven't even thought of yet that can allow us to travel at high rates of speed without dying.

And I do like the idea of limiting a grav drive to 6G while an R-drive can go much faster.

 

[Robbo]

Acceleration couches and g-tanks can compensate for a lot of acceleration short term (like a dogfight). Joe Haldeman's Forever War had crew immersed in liquid fluorocarbons to withstand 10's of gees. They needed some surgical mods too.

I'd probably introduce new augments to deal with acceleration problems. Neural interfacing would be a big advantage over working a 'primitive' joystick. Also Expert of Intellect programs could let your ship take over for a few seconds if you black out.

I think I'm going with the following technology:
The Empire uses MDs that work by pushing or pulling on planets. They can go up to 6 gees. But beyond ten planetary diameters they drop to half their maximum acceleration. Beyond a hundred diameters acceleration drops to a quarter. The Empire uses chemical PPs (fuel cells) and more rarely fission plants. Fissionables are in short supply and reserved for military ships.

The Newcomers never developed MDs and use the RD from High Guard. They are working on a fusion rocket that uses 1% mass for one g-day, but that's in the future when the campaign starts. Their RDs can go faster than 6 g's.

The Empire uses artificial gravity on their ships and stations. They use inertial compensators to offset the effects of acceleration. The compensators work perfectly for steady acceleration. Sharp movements will 'overload' them so they are far from perfect in combat and crews still use acceleration couches and strap in. Newcomers use acceleration couches, exoskeletons and augments to combat acceleration.

Ships use Hyperdrives. MDs cover a number of parsecs each day equal to their g rating. RDs work with hyper but use thrust points to cover the parsecs. they are typically 2-3 times slower than MDs to conserve fuel.

The Empire is TL 10. The Newcomers TL 12 with some early TL 13 armaments.

 

[Robbo]

Right! Megatraveller. Thanks. I knew I read that somewhere. That would fit in nicely with putting the Empire around TL 10. Of course I'm probably going with no true reactionless drives. The Empire ships work by pushing or pulling on various planets.

 

[BP]

...Grav drives handle gravity wells just fine and thus do not require heat shielding.

I've seen this posted quite a bit - apparently it became 'canon' knowledge at some point in the past.

Someone got caught up in simple mechanics and showing off some simple physics 'knowledge'...

Unfortunately, this notion is not entirely true - and definitely not 'realistic'. Especially if landing from 'orbit'. Or around higher G planets with lower G M-Drives. Even earth mass sized worlds would present problems for 1G limited ships.

Orbits require certain velocities - drastically different from the linear velocity at ground and from the gaseous envolepe around a planet (which varies by distance from the equator (latitude), but is infintesimal compared to orbital speeds) . From orbital velocity, one would have to use the M-Drive to decelerate while also repelling the gravitational attraction of the planet. Thus, 1G might not cut it (causing a more ballistic, high temp/plasma generating trajectory into atmo at high relative velocities). Or doubly long overall reentry time. But, there is still another major issue.

Non-orbital altitude - i.e., using G-Drive to constantly oppose gravity - avoids most of this (though the linear velocity of atmo relative to the ship can still be large enough to cause issues if not addressed). But, again, there is one major, unaccounted for issue.

The major, overlooked issue? Simple. Running into stuff that is not gravitic (not that the other kind wouldn't still hurt)! Stuff that is at orbital speeds! Effective heat shielding would be a lot cheaper than kinetic energy shielding.

Besides sophont-made orbital garbage, there will generally, always be natural objects grabbed by the planet's gravity - such as micrometeorites.

So, IMTU, heat shielding has always been an integral part of spacecraft - and atmo approaches generally involve orbital speed transfers and/or polar insertions to avoid the reality of the danger posed by the inevitable space junk in a high-tech society.

FYI: The U.S. Space Surveillance Network currently tracks over 13,000 objects over 10 cm in diameter. A ten-centimeter long piece of space trash can cause as much damage as twenty-five sticks of dynamite.

There are an estimated 600,000+ objects 1 cm to 50 cm in diameter. And 10s of millions of other, smaller objects (like paint flakes) - that can fatigue a spacecraft's hull and damage more sensative parts.

In the real world, impacts have caused problems already. And this is without remaining 'relatively stopped' as compared to normal orbital velocities. This would make a huge difference, especially given high traffic and large vessels in Traveller, even when one accounts for the fact that near-planet space is a tremendously huge volume.

Clunk - steps off platform...

 

[AndrewW]

FYI: The U.S. Space Surveillance Network currently tracks over 13,000 objects over 10 cm in diameter. A ten-centimeter long piece of space trash can cause as much damage as twenty-five sticks of dynamite.

Yup, they have a lot of kinetic energy built up from orbital velocity. At times they have the astronauts / cosmonauts move into a safe area on the ISS due to this danger.

 

[BP]

At times they even move the ISS as well!

 

[Robbo]

I was wondering why R-drives needed a hest shield since they were at least as powerful as MDs. While we're on the subject of acceleration how much thrust do you need to skim a gas giant? I'm guessing 2 g's is bare minimum and 3 g's is comfortable.You need to neutralize 1-2 g's, fight cyclone winds and move fast enough to take in the gas. Talk about needing heat shields!