Orbit to Surface with grav equipment

[AnotherDilbert]

So a grav floater and air car can reach orbit at 200-300 kph but cannot leave a ship and get back to the ground?

Of course they can. They can reach (some) orbit and go back to the ground again. For a low powered grav vehicle such as an air/raft it takes hours in either direction.

"Orbit" means both altitude and speed enough to stay there. An air/raft will not just float straight up to orbit, it has to accelerate sideways to get up to enough speed for the orbit. It will move away from the planet, accelerating sideways, moving out in a spiral until it reaches orbit.

If you watch a rocket launch you can see that the rocket isn't just going straight up. Its trajectory curves to the side so it can accelerate sideways up to orbital speed.

I was trying to figure out what of the grav vehicles could be used as a landing craft. Can a grav based APC ferry troops to the surface or do you need a ship with an M-Drive and not a grav unit?

A grav vehicle is basically the equivalent of a small craft close to a planet, within about one diameter, except that they might not be vaccum-proof. A high powered grav vehicle can move to and from orbit just as quickly and easily as any spacecraft.

Air/rafts, grav floats, and grav belts are just limited by their low power. They have barely enough power to stay in the air, leaving very little power for propulsion.

So, a high powered (fast) grav APC should have no problems launching from a spacecraft in orbit, ferry troops to the ground, and go back again.

 

[Reynard]

One Traveller source listed lifters which are used in vehicles and grav belts are only capable of reaching low orbit. Grav belts could be a problem with their limited endurance, 4 hours at TL 12 and 12 hours at TL 15. This is probably why only maneuver equipped ships can make regular trips between ground and orbital facilities.

 

[Condottiere]

LIFTERS

Lifters are anti-gravity modules which effectively negate the force of gravity. Lifters are a hull component; they draw minimal levels of energy from a power source. Lifters operate within 1D of a gravity source; beyond that limit they operate at about 1% efficiency.

Performance. Lifter performance is minimal and related to the nearby gravity source.

A Lifter effectively negates gravity and is able to transform some of its lift into sidewise motion.

Lifters are a backup motion provider primarily used to adjust location on a world surface. Lifters can raise a ship off the ground before engaging maneuver or gravitic drive. They can move a ship (slowly across a world surface). They can be used for very slow ascent to, and descent from, orbit.


1. One diameter of gravity source.

2. Interestingly enough, twelve hundred fifty kilometres is about a tad short of one tenth of Earth's diameter.

 

[phavoc]

Generally speaking, yes.

Terminal velocity of a parachutist is 120mph. You will start slowing down once you hit atmosphere, and you'll slow down even more if you (a) change position and are belly / back towards the surface and (b) if you have more drag - you can get 'winged' suits to parachute in that have flaps between the arms and legs that provide lift and act like flaps to slow you down.

Adding in a grav belt would mean you would not need it to 'fall' from orbit. The de-orbiting process might be a bit tricky, but with a grav belt the normal rules do not apply. It would cancel momentum and your speed. You could basically turn it on and off as needed (or as long as the charge held out).

Without actual specs of what it can do, one might need to use it frequently, or not.

 

[Arkathan]

If a grav vehicle, any grav vehicle, has the continual thrust capability to obtain orbital flight and match speed with an orbiting craft, then it has the capability to decelerate to the same velocity as the mesosphere while maintaining a constant height above the thermosphere/mesosphere interface. Once achieved, the decent is little different from freefall or normal flight for purposes of avoiding a fiery re-entry.
This assumes battery/fuel range sufficient to cover the distance, but if you can do one in a vehicle not reliant on aerodynamics or forward motion for lift, you can do the other.

GDW's Double Adventure 3, Death Station has an air raft going up to the station in vacc suits, so the intention for that capability has been around for forty years.

 

[Condottiere]

I kinda suspect that the motivational system in this case has a linear relationship to the gravitational field it's operating in.

 

[Shirson]

An air raft (Core pg 138) says it can reach orbit, but people must wear vac suits. A grav floater on pg 141 can also reach orbit.
Could a person wearing a grav belt on a vac suit launch themselves from orbit and use the grav belt to fly down onto a planet surface? The entry on page 113 does not say.

As part of the game mechanics - yes.
Within real orbital mechanics - mostly no.

Orbit entry is not just about reaching some altitude, but, much more importantly, gaining a very high horizontal velocity.
To descend from orbit, it is necessary to reduce the horizontal velocity, in the case of atmospheric braking - partially, in the case of a non-atmospheric body - completely. In the real world, orbital altitude change maneuvers are accomplished precisely by manipulating horizontal velocity, not vertical velocity (to return to Earth, vehicles brake their horizontal velocity, not attempt to accelerate toward Earth).
The antigravity "engines" of the described vehicles are repelled from the planetary body instead of being attracted to it. That is, the thrust vector is always directed away from or toward the planet's center of mass. Getting to some height above the planet is quite easy. But to increase the horizontal velocity is much more difficult. It is possible, in principle, but will take a lot of time for extremely inefficient maneuvers (including flying to the edge of the planet's sphere of influence and using the gravity of other bodies, like a star or moon(s), to accelerate not vertically relative to the planet).
If the grav engine can change the vector of interaction - then the problem is solved very easily. Within the game, however, just don't bother with it. The rules simplify a lot of things for ease of play, especially in terms of spaceflights and fair orbital mechanics.If the rules say that a raft can get to and from orbit, then so can a person with a gravity belt. If it doesn't mess up the current game situation and fits into the logic of the setting

 

[Condottiere]

My original theory, and current one (expanded, anyway), is that forward momentum with gravitational motors is achieved by falling in the desired direction.

 

[heron61]

One Traveller source listed lifters which are used in vehicles and grav belts are only capable of reaching low orbit. Grav belts could be a problem with their limited endurance, 4 hours at TL 12 and 12 hours at TL 15. This is probably why only maneuver equipped ships can make regular trips between ground and orbital facilities.

It seems possible that 4 hours would be too short, but I doubt 12 hours would be. One issue is speed - one bit I liked in MegaTraveller was a note I remember reading that converted between speed and acceleration. Various grav vehicles have a listed top speed and this makes sense for atmospheric operation, but makes no sense for operating in vacuum or near vacuum. There's absolutely no reason that an air raft or grave belt would be limited to speeds of a few hundred kph in vacuum or near vacuum, especially since we know from the Vehicles book that streamlining a grav vehicle makes it go faster. So, in vacuum it makes sense that what all grav vehicles have instead is maximum acceleration rather than maximum velocity, and even if a grav belt or air/raft has a maximum acceleration in vacuum of only 0.1 G, it can still reach Earth escape velocity in about 3 hours.

As a result, I see no issue with reaching or leaving low orbit with an air/raft or grav belt, but it is also likely that using a ship is faster, and I could see populous worlds having regulations against using non-spacecraft to enter or leave orbit, since if lots of people are doing this it could get crowded and potentially dangerous.

 

[Condottiere]

The rate of change of velocity per unit of time.

So it would depend likely on thrust to weight ratio, less local gravity and atmospheric friction.

 

[PsiTraveller]

Does weight actually matter anymore with grav equipment? Is all weight considered zero, or 1 gram when contra grav is in operation? Page 130 of the Core book has Anti-grav ignoring atmosphere and Size of a planet. So that seems to go against the local gravity and atmospheric friction issues.

 

[Condottiere]

It wouldn't if we used lifters on spacecraft.

I can't really comment on vehicular design, since I'm currently trying to study it.

 

[Arkathan]

Does weight actually matter anymore with grav equipment? Is all weight considered zero, or 1 gram when contra grav is in operation? Page 130 of the Core book has Anti-grav ignoring atmosphere and Size of a planet. So that seems to go against the local gravity and atmospheric friction issues.

Mass matters. Weight is dependent on local gravity.

 

[PsiTraveller]

I'm not sure if mass actually matters. Spaceship functionality is measured in displacement tons, not mass. A ship carrying 100 dtons of lead and a ship carrying 100 dtons of feathers have the same M-Drive and J-Drive stats. So mass is not a factor in performance. Contra grav may work the same way and just move anything within its operational field, regardless of weight and mass.

Which leads to my next question, the cargo capacity is in tons. Is that a measure of mass or dtons? Shipping is listed in tons, and means dtons. While weapons are measured in tons instead of kilograms (pg 136). So is cargo in tons or dtons? It makes a difference.

 

[Arkathan]

I'm not sure if mass actually matters. Spaceship functionality is measured in displacement tons, not mass. A ship carrying 100 dtons of lead and a ship carrying 100 dtons of feathers have the same M-Drive and J-Drive stats. So mass is not a factor in performance. Contra grav may work the same way and just move anything within its operational field, regardless of weight and mass.

Which leads to my next question, the cargo capacity is in tons. Is that a measure of mass or dtons? Shipping is listed in tons, and means dtons. While weapons are measured in tons instead of kilograms (pg 136). So is cargo in tons or dtons? It makes a difference.

I was comparing the comment on weight to the correct term, mass. Weight changes in relation to the strength of the gravitational field. Mass is constant.
Ships are measured by volume. This is a concession to TTRPG mechanics where knowing how many 5x5 squares are blocking fire matters more than specific cargo density.
The original post is not concerned with ships or small craft.
In older rules, grav units had a mass based thrust. That is, you divided the robot/vehicle's mass (including cargo) by the thrust rating of the grav units installed. If the result exceeded 1, the unit could lift off. Subtracting 1 gave you the thrust available for maneuvering. The new vehicle construction rules assume that sufficient grav thrust is provided for a vehicle of the given size. This does not aid you in your question about grav belts.

Realistically mass matters.

 

[Condottiere]

Depends on how gravitational based motors are meant to interact with our physical universe.

Because principally there would be two variants, thrust based and field effect.

Or maybe you'd have both simultaneously, which is basically implied with inertial compensation.

 

[Condottiere]

Oh, as regards to vehicular spaces in relationship to spacecraft volume:

1. Usually, shipping volume is one default space to half a tonne.

2. This includes chassis, fuel tank, and motivation, which isn't mentioned.

3. Somewhere between thirty to sixty tonnes, it expands to four spaces to a tonne, as an economy of scale.

4. Considering that this is about how spacecraft systems are converted, seems correct (in terms of conversion).

5. Exception seems to be unpowered boat.

 

[ochd]

I'm not sure if mass actually matters. Spaceship functionality is measured in displacement tons, not mass. A ship carrying 100 dtons of lead and a ship carrying 100 dtons of feathers have the same M-Drive and J-Drive stats. So mass is not a factor in performance. Contra grav may work the same way and just move anything within its operational field, regardless of weight and mass.

Which leads to my next question, the cargo capacity is in tons. Is that a measure of mass or dtons? Shipping is listed in tons, and means dtons. While weapons are measured in tons instead of kilograms (pg 136). So is cargo in tons or dtons? It makes a difference.

Not sure if this is relevant, but I have long wondered about this sentence in Pirates of Drinax (book 1, p. 173), referring to a far trader that has just emerged from jump: 'From the way she [sic] turns and accelerates, the Travellers can tell that the Trader has a full cargo bay.' This would suggest that the mass of cargo matters.

But from the very little I understand of how M-drives, etc. work, it never made sense to me how a full cargo bay would affect the way a ship turns and accelerates in space.

Dan.

 

[cunningrat]

Not sure if this is relevant, but I have long wondered about this sentence in Pirates of Drinax (book 1, p. 173), referring to a far trader that has just emerged from jump: 'From the way she [sic] turns and accelerates, the Travellers can tell that the Trader has a full cargo bay.' This would suggest that the mass of cargo matters.

That one might not be about the mass. Inertia is still a thing, and cargo tiedowns fail occasionally. I could totally see a Trader pilot making slow and careful turns in order not to shake cargo containers loose inside the hold. Not enough to affect game mechanics, but enough for another pilot to notice.

 

[ochd]

Ah, okay. I thought a ship's internal grav plates counteracted inertia?