Grav Vehicles, Grav Drives

[Grav_Moped]

(Split off from Reentry and landing)

Some issues:
How does the Grav Drive work (in terms of technobabble and in-universe capability)?
Does it imply artificial gravity?
Does it imply (or can it be repurposed) into some sort of reactionless thruster mechanism (if only at higher TLs)?
Does it (as implemented in the game) break conservation of energy in the same way that Maneuver Drives do (less energy input than output)?

I'm coming at this from CT '81 and Striker, since that's what I have on hand. Inputs from other versions are welcomed.
Seeking canon descriptions of examples, capability, and in-universe explanations.

For RPG purposes, "It's a flying car that can carry X passengers, Y tons of cargo, and go Z kph because it's narratively convenient" is perfectly adequate. I'm just trying to explore the implications of the canon description of their capabilities.

More to follow as time permits.

 

[kilemall]

What drove my questioning of all that is mostly Striker movement, secondarily the ship M-drive version of gravitics.


I am particularly concerned with the intersection of the 1000D limits implying some sort of requirement for a stronger then interstellar gravity field for gravitics to work, the largely unexamined question of how agile a grav vehicle is when the only force down is planetary gravity, whether repulsors must not be A Thing given that it was dropped like a hot potato, how you can have artificial gravity to 3G onboard but not tractor beams.

 

[whulorigan]

From another thread I once posted to:

An interesting possibility might be what are a consequence of some of the current Unified Field Theory candidiates such as Superstrings (in 10 dimensions) and Supergravity (in 11 dimensions): namely that the "graviton" in such models is expressed in three related particle fields:

1. The Graviton: A massless* spin-2 tensor boson
2. The Gravivector (or Graviphoton): A massive* spin-1 vector boson
3. The Graviscalar (or Radion): A massive* spin-0 scalar boson

* - Note that massless particles are stable and travel at light speed, decreasing their respective force-mediation via an inverse square relationship. Massive particles are unstable and decay in short order and also have variable speed, and thus transmit forces only over short range with a strength that falls off statistically/exponentially with distance, with a sharp cut-off range.

​

In particle physics, forces are mediated by virtual bosons (such as the photon for electromagnetism) that give rise to purely attractive forces for even-spin bosons and forces that repel like and attract opposite charges for odd-spin bosons.

In the above models, normally the gravivector and graviscalar effects cancel each other out, leaving only the effect of the graviton field (i.e. always attractive with infinite range dropping off as an inverse square with distance). But if the physics of Traveller has a working Unified Field Theory and knows how to manipulate 11-D graviton fields, then isolating the graviphoton and the graviscalar allows the secondary generation of repulsive and attractive forces, but only over short ranges.

. . . whether repulsors must not be A Thing . . . how you can have artificial gravity to 3G onboard but not tractor beams.

The above can also go to answering your question. If the attractive/repulsive force being generated is a spin-off force derived from gravity (and Traveller materials have suggested that Artificial Gravity is not quite the same thing as normal gravity), then the statistical decay of the massive intermediary bosons would limit the range of the artificial "gravity/anti-gravity" force, since they would create a force out to a given range and subsequently fall off rapidly.

So you could have a tractor beam, but what would be its range? Would it be practical?

I also posted a similar response in this thread:

Interesting uses of Grav-tech (Post #28)

 

[kilemall]

Ok so let's say tractoring is technically possible, but only within x km range that means clamps of a mechanical or electromagnetic nature are more practical (rendering tractors likely more of a research curiosity, security device to stop small craft getaway or limited space industry usage).



Then how does a ship accelerate 3G towards a planet with a 1G at best field, or a gravtank rated at 5Gs per Striker dive down at faster then 1G if there is no attractive function beyond AG in gravitics?


Has to be something more then grav neutralization/grav thrust 'against mass'/short range AG/'attraction'.



If the answer is 'thrust against ethereal spacetime something' then why is 1000D a thing (beyond play mechanics)?


The thruster plate mechanic referenced in the parent thread of rapid cyclical relative compression/release shouldn't require a gravitic field at all.

 

[mike wightman]

How does the Grav Drive work (in terms of technobabble and in-universe capability)?

A null grav module or grav module turns electricity into weight negation and lateral thrust:
"Anti-gravity is the second major breakthrough. The postulated technology
produces both neutralization of weight and lateral thrust.
Each 0.02 m3 of grav generators produces 1 ton of thrust and requires 0.1 megawatts
of power from the power plant. They weigh 2 tons and cost Cr100,OOO per m3."

Does it imply artificial gravity?

Nope, ship like artificial gravity plates and acceleration compensation remains a complete mystery since it is not in the Striker design sequence and are just handwaved away by High Guard.

Does it imply (or can it be repurposed) into some sort of reactionless thruster mechanism (if only at higher TLs)?

If you want to it can, but my take is no. The grav module and the ship maneuver drive are different.

Does it (as implemented in the game) break conservation of energy in the same way that Maneuver Drives do (less energy input than output)?

If it breaks energy conservation laws then you have to hand wave dark energy physics

I'm coming at this from CT '81 and Striker, since that's what I have on hand. Inputs from other versions are welcomed.
Seeking canon descriptions of examples, capability, and in-universe explanations.

Stick to one edition - as you move from edition to edition the handwaves change. That said MT and later had artificial gravity and acceleration compensation as separate systems, MT differentiated betwenn grav modules and the maneuver drive, TNE changed grav modules into contragrav that only negate weight and produce no thrust.

For RPG purposes, "It's a flying car that can carry X passengers, Y tons of cargo, and go Z kph because it's narratively convenient" is perfectly adequate. I'm just trying to explore the implications of the canon description of their capabilities.

Things were much simpler in LBB:3 days

More to follow as time permits.

Looking forward to this discussion.

 

[mike wightman]

I would be very hesitant to use any form of quantum gravity handwave. There is no experimental evidence for quantum gravity - the best latest contender loop quantum gravity has failed its only experiment to date - physicists are finally starting to think the unthinkable
, you can not reconcile quantum mechanics and general relativity.

What is needed is new physics beyond out understanding, which is certainly possible in a science fiction universe. Don't try and handwave gravitons and spin integers etc as they are likely going to be replaced by the new physics, in much the same way quantum mechanics replaced classical.

 

[mike wightman]

At TL8 there is a breakthrough that allows grav modules to be built.

Where it gets interesting is the disconnect between CT LBB2 drives and HG (just for a change).
Under LBB2 TL9 is the lowest TL of the letter drives - up to 6g performance.
Under LBB5 TL7 is the lowest TL of the maneuver drive but it is limited to 2g, at TL8 up to 5g is now possible, while at TL9 the full 6g performance opens up. And then there is this bit:

Tech level requirements for maneuver drives are imposed to cover the grav plates integral to most ship decks, and which allow high-G maneuvers while interior G-fields remain normal.

Does this imply grav plates at TL7?

 

[Adam Dray]

Grand Unification might happen via Kohl Furey's work on octonions, or in a more whimsical universe, we could let the crazies like quantumantigravity.wordpress.com (HTML) be correct: the right balance of spin, magnets, and electricity creates gravity fields.

 

[Grav_Moped]

At TL8 there is a breakthrough that allows grav modules to be built.

Where it gets interesting is the disconnect between CT LBB2 drives and HG (just for a change).
Under LBB2 TL9 is the lowest TL of the letter drives - up to 6g performance.
Under LBB5 TL7 is the lowest TL of the maneuver drive but it is limited to 2g, at TL8 up to 5g is now possible, while at TL9 the full 6g performance opens up. And then there is this bit:


Does this imply grav plates at TL7?

It might suggest that TL7 maneuver drives aren't gravitic at all, and the 2G limit is what spaceship crews can endure over the duration of a HG battle. Doesn't explain fuel consumption though.

 

[Grav_Moped]

...
I also posted a similar response in this thread:

Interesting uses of Grav-tech (Post #28)

Thanks for bringing that one up again!

 

[whartung]

Does it imply (or can it be repurposed) into some sort of reactionless thruster mechanism (if only at higher TLs)?

I say yes, because I subscribe to the "Johhny Quest" world view of gravitic drives in Traveller.

As you said, the Air Raft is an "air car" that conveniently, and comfortably lets folks travel from point A to point B, just like in Star Wars. That means that whatever exhaust there may be is essentially harmless to folks nearby. That landing the car does not turn your driveway into molten glass. That as the car lifts away, your wife holding your child can step forward in to the air draft of the life, and wave bye bye with a smile and no radiation burns.

We also have "grav belts". I don't visualize grav belts as some kind of EVA rig with arms and a huge pack.

No. It's a BELT, not a PACK. You click it on, press a button, and it makes light, friendly, burbling electronic noises as it lifts you over the trees and lets you move forward, rifle in hand.

So, to me, that's what grav technology is like.

Similarly, as I mentioned elsewhere, grav tech allows your Free Trader to land softly on a flat space, kicking up some dust, with no more than bugs on the windscreen rather than seared, blistered and cracked paint and panels, too hot to touch, too hot to be near, and a fire hazard if you land in a field.

T-Shirt Traveller. Ship lands, door immediately slides open, and out you pop in boots and cargo shorts, and a tank top, gauss rifle in hand.

 

[Grav_Moped]

Star Truck!

I'd think at TL8 (and only TL8) you'd see grav surface-to-orbit vehicles built to accommodate single standard shipping containers. Basically, antigravity space semitrucks.

They'd carry 5 Td cargo (note: much more than 5000 kg...), and cost 3x what an air/raft does. Crew compartment (2 seats) is pressurized with 72 man-hours life support. One driver needed, with grav vehicle skill (and possibly small craft skill as well if your TU doesn't include sophisticated autopilots). The second seat is for a relief driver. Performance is similar to an air/raft, but airspeed can be a bit higher at low altitudes.

On worlds with sufficient atmosphere, they'd be built for aero-braked reentry and look like miniature space shuttles. Otherwise, they'd look like scaled-down Cutters.

There might be antigravity analogs of the standard small craft as well, but with chemical rockets in addition to the contragrav system (this is specifically at TL8 only). I don't think pure antigravity vehicles much larger than 100Td are common at TL8 -- by the time you've scaled things up that far, you're approaching TL 9 anyhow (at least in the LBB "better is bigger" tech ladder).

 

[coliver988]

I say yes, because I subscribe to the "Johhny Quest" world view of gravitic drives in Traveller.

As you said, the Air Raft is an "air car" that conveniently, and comfortably lets folks travel from point A to point B, just like in Star Wars. That means that whatever exhaust there may be is essentially harmless to folks nearby. That landing the car does not turn your driveway into molten glass. That as the car lifts away, your wife holding your child can step forward in to the air draft of the life, and wave bye bye with a smile and no radiation burns.

We also have "grav belts". I don't visualize grav belts as some kind of EVA rig with arms and a huge pack.

No. It's a BELT, not a PACK. You click it on, press a button, and it makes light, friendly, burbling electronic noises as it lifts you over the trees and lets you move forward, rifle in hand.

So, to me, that's what grav technology is like.

Similarly, as I mentioned elsewhere, grav tech allows your Free Trader to land softly on a flat space, kicking up some dust, with no more than bugs on the windscreen rather than seared, blistered and cracked paint and panels, too hot to touch, too hot to be near, and a fire hazard if you land in a field.

T-Shirt Traveller. Ship lands, door immediately slides open, and out you pop in boots and cargo shorts, and a tank top, gauss rifle in hand.

Hey, you are playing in my game! That's also how I play it - I want my flying cars and relatively quiet grav ships.

After watching the last Star Trek movie and it's "what could we do with gravity control" starbase, as well as the GURPS cutter module having the engineering module have its gravity length-wise vs belly-wise and mentioning the fun to be had when your gravity field shifts 90 degrees, I sometimes throw that out to the players. There was one time 2 ships were docked at 90 degrees from each other. Oddly only the Aslan failed the dex roll and did NOT fall on her feet.

 

[Grav_Moped]

I say yes, because I subscribe to the "Johhny Quest" world view of gravitic drives in Traveller.
...
T-Shirt Traveller. Ship lands, door immediately slides open, and out you pop in boots and cargo shorts, and a tank top, gauss rifle in hand.

Hey, you are playing in my game! That's also how I play it - I want my flying cars and relatively quiet grav ships....

I like the idea of the hard edges of physics sticking out (except where handwaved). The grav-lifters are quiet and don't have significant outside effects. At lower tech levels, the primary lateral propulsion system might not be quite so quiet (turbojets, rockets, etc.)*...

Land your ship "hot" (not decelerating to sub-orbital velocity before entering the upper reaches of the atmosphere, or to subsonic velocity before entering the denser lower altitudes) and the hull's going to get a bit toasty. Standard procedures would be to avoid it, but there might be a narrative reason for doing so anyhow. Make that sort of thing stand out!


*Oh, now there's a question: How sharply can an air/raft turn? How quickly can it stop? In a vacuum, trace, or even thin atmosphere, you're limited by your lateral propulsion system (at low TL, rockets, probably) and the 1/10G that you can get from the lifters. In a more standard atmosphere, slowing down is fairly easy -- just go nose-up and the craft becomes its own airbrake. Turning? You can change direction the same way, but the lift/drag ratio is going to be appalling...

This implies some sort of vernier rockets for emergency braking or collision avoidance that are probably automatically locked out unless needed. At higher TLs, that's probably addressed with reactionless thrusters of some sort. It also suggests the existence of anti-gravity-assisted aircraft at TL 8 and perhaps beyond.

 

[McPerth]

My take on ship's drives

I'm not an expert on physics, so take it with a big grain of salt (or a whole meteorite of it, I let you decide its size).

First of all, I must tell that I don't know if 1979 HG even reached Barcelona, and my first sight on HG was the 1980 version.

One of the first things that caught my attention on Traveller drives is the fact acceleration is the same regardless the ship's mass. This is quite clear in MT, where mass is separated from tonnage, but it's also hinted in CT, as ships with empty holds have the same acceleration than ships with full ones, and hte density of the cargo does not affect performance.

IMHO, this ruled out the possibility of any drive produceng tons of thrust (as reaction ones or, in MT -as I didn't have access to Striker- the grav vehicles plates), as their acceleration would be mass dependant.

OTOH, if gravitic tech was controled enough, I though a ship could use its onw mass generated gravity on those gravitic thrusters, so, as its own mass increases, so does the gravity generated by it, to a point the game assumes they cancell their effects. If so, the volume (dtons) dependency of them was for the volume they can take the mass from.

So, as I understand it, there are two kinds of gravity tech plates: in one, they just generate tons o thrust by "pushing" against a large graity mass, while the other uses the own mass generated gravity to work.

The fist ones are cheaper and use less power, and so are used for vehicles and small crafts, but depend on the large gravity well to push against. The second class does not depend on this graity well, as it uses the craft own gravity, but are more expensive and use lots of power (and in MT cannot be miniaturized to be used in crafts that less tan 20 dtons), so they are used by larger boats and ships.

Of course, that's just my take on it, and YMMV...

 

[Grav_Moped]

Moved from a thread where it was off-topic

Agree completely, except that in atmosphere the thrust giving the 0.1 g is used to maintain the 100 km/h speed. So in atmo, we are limited to 100 km/h horizontal speed with no extra acceleration.

Exactly. The ascent profile I proposed was essentially vertical (aside from wind drift, which may be significant but could likely be accounted for in advance) until clear of atmospheric effects. Once out of the atmosphere, that limit doesn't apply. [Sentence deleted]

Agreed, but any result we calculate has to be reasonably close, despite that number being presumably picked at random.

I may disagree with your definition of "reasonably close" but fair enough.

Agreed, unfortunately.

But it is difficult to ascend and descend vertically in a rotating system. Ovals and spirals are the natural shapes of movement in rotating system in a gravity field. As you ascend you have to increase speed to stay over the same spot on the ground as the angular speed is constant but the radius is increasing. But in atmo we have a top speed of ~100 km/h so we can't increase speed, hence the planet will rotate away from us.

So float "straight" up and take the drift, then go fast when you're above the atmosphere. 1 hour and 15 minutes due to aerodynamic drag below 30km gets you to the 100km Karman line (effectively out of the atmosphere).* That's not much time for wind to push the vehicle sideways, and you retain the surface rotational velocity of your point of departure. This will yield a westward displacement at altitude, but the effect at 100km is under 10m/sec at the equator.

As far as getting to orbit,the first half of the ascent to orbital altitude (not velocity) takes 2.25 hours : 1 hour to get to 30km altitude, 1.25 hours to get to 1000km altitude. The second half of the ascent has the 0.1G that was used for ascent thrust redirected laterally to achieve orbital velocity.** After another 1.25 hours of this, the craft is at the 2000km orbital altitude but at only half orbital velocity (about 4km/sec). At this point the antigravity is dialed back to about 0.5G and progressively reduced to zero over the next hour of continued lateral acceleration. 55 minutes later it will have achieved 7.8km/s orbital velocity at 2000km altitude.

Time to establish Low Earth Orbit (2000km altitude, 7.8km/sec): 5.66 hours [Edit: 4.66 hours was incorrect.]
Time to 2000km altitude: 3.5 hours [Edit: 2.5 hours was incorrect.]
Time spent accelerating laterally at 0.1G: 2.16 hours, starting 2.25 hours after takeoff.

This is for a nonspecific orbit. Achieving a specific orbit (a particular orbital inclination, or rendezvous with an object in a specific orbit) may take substantially longer -- which may explain the nominal 8-hour trip time.
Descent takes similar time, especially if aiming for a specific destination from an arbitrary point in the orbit. Aerobraking can significantly reduce this if the craft has the capability to do so.†

This excludes the contribution of the surface rotational velocity at the point of departure (460m/sec at the equator, 390m/sec at 45N or 45S)



* Due to the low vertical acceleration and the exponential decrease in atmospheric density with altitude, the Air/Raft never reaches its drag-limited airspeed even considering it's a 3m*6m flat plate when going straight up.

** Antigravity is turned down to 0.9G (effectively a 0.1G downward gravitational effect) in order to neutralize the vertical vector by the time orbital altitude is reached. I'm assuming that this "extra" 0.1G can't be used laterally, just the normal 0.1G that was used for vertical acceleration until this point.

† Air/Rafts carried on commercial starships might be equipped with a re-entry kit (single-use spray-on ablative foam, and a stabilizing drogue), while military or Scouts' Air/Rafts likely incorporate re-entry shielding as standard equipment. These enable 3G deceleration within the mesosphere (<100km altitude in standard atmosphere). General-purpose civilian Air/Rafts aren't capable of this degree of aerobraking and need to slow to a safe speed using their grav drives before re-entering the mesosphere.

 

[AnotherDilbert]

So float "straight" up and take the drift, then go fast when you're above the atmosphere. 12 minutes at 0.1G gets you to the 100km Karman line (effectively out of the atmosphere).*

The air/raft is still limited by air resistance in atmosphere. If it is limited to 100 km/h forward, it is reasonably limited to far less upward since it presents a far larger area to air resistance in that direction.

I believe it would take hours to leave the atmosphere, but I can't quantify it since I don't know how the craft is limited at different heights. But we can't just ignore air resistance in atmosphere.


Note that grav vehicles are described as air and land transport but not space transport which is the realm of spacecraft. MT and T5 notes that grav drives only work close to planets, e.g.:

Because of its nature, artificial gravity is not a very efficient means of locomotion in deep space where there are no strong gravity wells to push against.

We don't know the specifics, but reasonably the effectiveness of grav drives decrease as the distance to the world increases.

LBB3, p23:

An air/raft can reach orbit in several hours (number of hours equal to planetary size digit in the UPP); passengers must wear vacc suits and interplanetary travel in an air/raft is not possible.

Even if grav vehicles can reach orbit (eventually) they are not spacecraft and not intended for space.


(D = at²/2. Even if we accelerate at 0.1 g ≈ 1 m/s² travelling 100 km it would take 100000 m = 1t²/2 ⇒ t = √(2×100000) ≈ 447 s ≈ 7.5 minutes.)

 

[Grav_Moped]

Missed my drag-limited airspeed calculations earlier in this thread! Revised:

Assuming that the drag-limited airspeed in vertical ascent of an Air/Raft is 1/4 of its drag-limited airspeed in horizontal travel (that is, 30kph), aerodynamic drag is a constraint until 30km altitude is reached, and this will take an hour. Above that point, the air thins out faster than you gain velocity.

For a 100km trip, leaving the atmosphere isn't worth the detour to 100km altitude to get to soft vacuum (1/2,200,000 of sea level air density). There's no time advantage for trips less than 300km, and counting the time to suit-up for the trip and repack the vacc suit on arrival, it's more like 350km.

The only limit is that average top speed above the atmosphere is limited to 12,500km/hr since at that rate the peak velocity exceeds escape velocity and you can't get back down again... oops.

For time-to-orbit, the drag below 30km issue adds one hour to the figures I gave.
I might do all of the math again later, but it's basically the same but with an ascent distance of 1970km and starting with an 8.3m/sec upward velocity.

 

[AnotherDilbert]

If I'm understanding the last line of your post correctly, trip time would be 15 minutes up*, 7.5 minutes sideways, then 15 minutes down.

My point is that in atmosphere we are limited by aerodynamics and can't use constant acceleration, but constant top speed.

At sea level an air/raft might be limited to something like 10 km/h vertically, much less than the 100 km/h horizontally limit (because larger area). Even if that limit increases at altitude it would take hours to leave the atmosphere.

 

[AnotherDilbert]

Assuming that the drag-limited airspeed in vertical ascent of an Air/Raft is 1/4 of its drag-limited airspeed in horizontal travel (that is, 30kph), aerodynamic drag is a constraint until 30km altitude is reached, and this will take an hour. Above that point, the air thins out faster than you gain velocity.

This is based on a few assumptions. Unfortunately the conclusion is incompatible with the stated performance ("An air/raft can reach orbit in several hours (number of hours equal to planetary size digit in the UPP...)".