Grav drive question

[Blue Ghost]

So, I don't recall in the basic rules how the grav drive works. By that I mean how many G's does it negate? I'm thinking an air raft, if it can reach orbit and beyond from a world with 1g, can exert 1g, and then exert another 1g to accelerate upwards against gravity.

If you're skimming fuel, as per a previous discussion, I understand that it's a parabolic ride with your drives full wide open to help A) facilitate copious fuel skimmed from said gas giant, and B) since the pull of gravity is two or three times or more from a gas giant, you therefore need to keep the ship's engines in zone-5 to keep your acceleration constant.

Now, if you had a grav drive underneath your ship, could you not just flip it on like you were coming into land on a normal world, and then simply fly or even glide through the gas giant's atmosphere without having to worry about it pulling you down to certain death?

 

[Timerover51]

So, I don't recall in the basic rules how the grav drive works. By that I mean how many G's does it negate? I'm thinking an air raft, if it can reach orbit and beyond from a world with 1g, can exert 1g, and then exert another 1g to accelerate upwards against gravity.

If you're skimming fuel, as per a previous discussion, I understand that it's a parabolic ride with your drives full wide open to help A) facilitate copious fuel skimmed from said gas giant, and B) since the pull of gravity is two or three times or more from a gas giant, you therefore need to keep the ship's engines in zone-5 to keep your acceleration constant.

Now, if you had a grav drive underneath your ship, could you not just flip it on like you were coming into land on a normal world, and then simply fly or even glide through the gas giant's atmosphere without having to worry about it pulling you down to certain death?

I go with the gravitational lifters can lift a fully loaded ship against a gravitational pull of about 1.5 G. That is why I view having a specified mass load per Traveller dTon as am important factor in ship design. I am not a fan of allowing an air raft to achieve orbit unless on a small world with little to no atmosphere simply because of the power requirements. Somewhere in my files I have the figure that to raise one pound of mass to orbital velocity for Earth takes 4 kilowatts of power. I can see a ship generating that much power, but not an air raft.

 

[mike wightman]

There is a difference between the grav modules used on air/rafts and the like and maneuver drives.
In original CT air/rafts and the like were propelled by null grav modules:

A light anti-gravity vehicle which uses nullgrav modules to counteract gravity for lift and propulsion.

note that the air/raft has enough power for ten weeks continuous operation before recharging from a fusion plant - more than enough energy to lift it to orbital altitudes - the actual achievement of an orbit is a bit trickier since it would have to use the planet to accelerate to orbital velocity but it is physically possible if you accept the setting physics.

Later the null bit was dropped and the name became grav modules, eg Striker which also states that:

Anti-gravity is the second major breakthrough. The postulated technology
produces both neutralization of weight and lateral thrust.

The nature of the maneuver drive it a lot more difficult to pin down since it has gone through so many iterations and paradigm changes.

Current Traveller editions have the m-drive being some sort of advanced physics gravitational spacetime fundamental particle quantum field interacting handwavium.

 

[Brandon C]

Somewhere in my files I have the figure that to raise one pound of mass to orbital velocity for Earth takes 4 kilowatts of power. I can see a ship generating that much power, but not an air raft.

The Dodge Demon must be one helluva starship. It's 840 hp engine converts to over 625 kw. Over half a megawatt of brute force.

Remember that an air/raft doesn't _weigh_ 4 tons, it displaces 4 tons of liquid hydrogen in volume.

 

[mike wightman]

So, I don't recall in the basic rules how the grav drive works. By that I mean how many G's does it negate? I'm thinking an air raft, if it can reach orbit and beyond from a world with 1g, can exert 1g, and then exert another 1g to accelerate upwards against gravity.

The basic rules of CT have the grav modules negate local gravity with enough thrust left over to produce a thrust that can accelerate the air/raft to 120KMperH. Striker later provided rules for designing grav vehicles - the gravitic thrust had to negate local gravity for the vehicle to float, with left over thrust providing acceleration.
Design a grav vehicle with a power to mass ration that grants a 1.5g rating. If local gravity is 0.8g then you have 0.7g to accelerate with - top speed will depend on streamlining, air resistance etc (there is a table)

If you're skimming fuel, as per a previous discussion, I understand that it's a parabolic ride with your drives full wide open to help A) facilitate copious fuel skimmed from said gas giant, and B) since the pull of gravity is two or three times or more from a gas giant, you therefore need to keep the ship's engines in zone-5 to keep your acceleration constant.

The magnitude of the gas giant's gravity has little effect on a Traveller ship approaching from elsewhere in the system or from a jump point. You make sure your vector allows you to enter orbit around the gas giant.
You change your orbit so it dips into the gas giant atmosphere, you only need to use your engines to overcome the atmospheric drag from entering the atmosphere and scooping the fuel.

Now, if you had a grav drive underneath your ship, could you not just flip it on like you were coming into land on a normal world, and then simply fly or even glide through the gas giant's atmosphere without having to worry about it pulling you down to certain death?

If you have a 3g maneuver drive then you do not have to worry about the gas giant's gravity unless it is much larger than Jupiter (see A:12 SotA). It was not clear if the original m-drive included grav modules or not.

 

[mike wightman]

The Dodge Demon must be one helluva starship. It's 840 hp engine converts to over 625 kw. Over half a megawatt of brute force.

Remember that an air/raft doesn't _weigh_ 4 tons, it displaces 4 tons of liquid hydrogen in volume.

Back in CT days the mass of vehicles etc is their metric mass, not their displacement tonnage. See the trade section in LBB2 for clarification.

 

[Brandon C]

Back in CT days the mass of vehicles etc is their metric mass, not their displacement tonnage. See the trade section in LBB2 for clarification.

Really? I thought CT used volume, not weight.

I never bothered with trade in any campaign, honestly. It is the least read of and section of and edition of Traveller I have.

 

[aramis]

Really? I thought CT used volume, not weight.

I never bothered with trade in any campaign, honestly. It is the least read of and section of and edition of Traveller I have.

Ship Design was always volume - per Marc and the deckplanning rules.
Trade goods were mass - per the examples - but the mass/volume conflation was common, and by the time book 7 was released, the mass tonne no longer was relevant.

as for ContraGrav and Anti-Grav -
In CT, AG is pure thrust, and it can be used for up or lateral, with full effect.
In MT, the core uses it like striker; Official 3rd arty supplements note that off-design-axis is reduced - 100% at ±15°, 10% at >165°, 25% at 90°.
In TNE, Contragrav is weight nullification - 98%. Gravitic thrust is an optional rule.
In T4, CT style AG thrust returns, alongside TNE style contragrav.

 

[Blue Ghost]

Ah Striker. I never did purchase Striker.

Great replies. Thanks everyone. Really insightful answers.

 

[Brandon C]

Ship Design was always volume - per Marc and the deckplanning rules.
Trade goods were mass - per the examples - but the mass/volume conflation was common, and by the time book 7 was released, the mass tonne no longer was relevant.

And since the air/raft appeared as part of the ship design process and trade system, the four tons got taken by some (many? most?) to mean volume and mass. Gotcha.

as for ContraGrav and Anti-Grav -

Lift and thrust were separate systems in GT. Both first appeared at the same TL though, so if a ship or vehicle had one it usually had the other.

 

[Timerover51]

Ship Design was always volume - per Marc and the deckplanning rules.
Trade goods were mass - per the examples - but the mass/volume conflation was common, and by the time book 7 was released, the mass tonne no longer was relevant.

as for ContraGrav and Anti-Grav -
In CT, AG is pure thrust, and it can be used for up or lateral, with full effect.
In MT, the core uses it like striker; Official 3rd arty supplements note that off-design-axis is reduced - 100% at ±15°, 10% at >165°, 25% at 90°.
In TNE, Contragrav is weight nullification - 98%. Gravitic thrust is an optional rule.
In T4, CT style AG thrust returns, alongside TNE style contragrav.

I go with contra-gravity lifters separate from thrust drives, for which I use the Dean Drive a.k.a. Abbott Drive, with the power plant capable of handling both. Using the 1977 Edition of the LBB, the power plant has to match the maneuver drive, not the higher of the Jump or Maneuver Drives. I also use boost then cruise to jump point, charging the Jump Capacitors on the way if using a Jump Drive or the equivalent for a Hyperdrive.

 

[jec10]

I like the contra-grav (negation of mass but no lateral thrust) solution because somehow for me it just seems less hand-wavy.

And then you get to add all sorts of fun thrust agencies to make your craft actually move about laterally. Could be a prop, could be a ducted fan, could be a turbofan, could be a rocket.

 

[mike wightman]

Trouble with changing grav modules (gravitic buoyancy and thrust) to contra-grav lifters (gravitic buoyancy only) is you can kiss goodbye to the gravbelt.

Or rather the grav belt must become a much more bulky affair requiring the contragrav modules and some sort of thrust agent - rotors, mini-jet engines or rockets - as well as an energy supply - fuel, batteries etc.

 

[whartung]

So, I don't recall in the basic rules how the grav drive works. By that I mean how many G's does it negate? I'm thinking an air raft, if it can reach orbit and beyond from a world with 1g, can exert 1g, and then exert another 1g to accelerate upwards against gravity.

The way the air raft works is, you and Hadji jump in to it as Race gives cover fire, meanwhile Bandit barks wildly and, fortuitously, as Race has to duck and cover, kicks the "go lever", and away you go.

 

[jec10]

Trouble with changing grav modules (gravitic buoyancy and thrust) to contra-grav lifters (gravitic buoyancy only) is you can kiss goodbye to the gravbelt.

Or rather the grav belt must become a much more bulky affair requiring the contragrav modules and some sort of thrust agent - rotors, mini-jet engines or rockets - as well as an energy supply - fuel, batteries etc.

For me at least, a grav belt with small ducted fans was not a big concession.

And I loved the colour of the TNE two-person "broom-stick".

 

[infojunky]

I like the contra-grav (negation of mass but no lateral thrust) solution because somehow for me it just seems less hand-wavy.

I go that route kinda in my head as well, Though I have been abusing Grav Displacement Drives for Landspeeder goodness, which in turn makes all the Tank-like Grav Tanks make much more sense.

And then you get to add all sorts of fun thrust agencies to make your craft actually move about laterally. Could be a prop, could be a ducted fan, could be a turbofan, could be a rocket.

Can you say multifuel Plasma Rockets... Was watching Elysium the other day, the Vtols there in are the perfect examples of Contragrav with Heplar thrust...

 

[Straybow]

I go with the gravitational lifters can lift a fully loaded ship against a gravitational pull of about 1.5 G. That is why I view having a specified mass load per Traveller dTon as am important factor in ship design. I am not a fan of allowing an air raft to achieve orbit unless on a small world with little to no atmosphere simply because of the power requirements. Somewhere in my files I have the figure that to raise one pound of mass to orbital velocity for Earth takes 4 kilowatts of power. I can see a ship generating that much power, but not an air raft.

Wattage depends on how fast you want to reach orbit. It is joules/kg for energy.

Orbital altitude (160 km for Earth) = m·a·d = 1.6 Mj/kg
Orbital velocity 7800 m/s = ½m·v² = 30.4 Mj/kg

If you're simply lifting up, the energy requirement is low. But you'll only be going 400-500 m/s and have to keep power on the lifters to stay up. Once above the atmosphere it easier to accelerate slowly to reach ballistic orbital velocity. All told, you'll need 32 Mj/kg. If my minivan's 200kW engine could power a grav unit, the 2 ton vehicle would reach altitude in about 5 hours. It would take about 4 days to accelerate to 7800 m/s if all the power could then be dedicated to the task. Since a certain percentage would be required for lift while accelerating, it might take a couple weeks. Lack of adequate zero-gee bathroom facilities would force me to cancel the experiment.