Escaping gravity in Traveller

[Michael Taylor]

Just a simple question for the geniuses on Coti:

Traveller's minimum size man drive is 1g (9.8m/sec2 acceleration). So if you have a free trader for instance, and the escape velocity of a size 10 planet (or a gas giant) is more than 9.8m/sec2, how does a free trader break free of that planet's gravity well?

So what are the escape velocities of planets larger than Earth? I'm aware the escape velocity of Earth is 11.2 km/sec (ignoring air resistance) but I guess we are talking about acceleration with maneuver drives, not constant velocity.


Thanks in advance for any answers.

 

[flykiller]

... the geniuses on Coti:

hmph.

how does a free trader break free of that planet's gravity well?

1) the ship lands and takes off like an airplane. this seems to be the classic ct approach.

2) maneuver drives can be overloaded for short periods of time. this allows vtol. many game referees take this approach.

3) ignore the whole problem. lots of people take this approach, and just play the game.

 

[Malenfant]

Take a look at this:
http://en.wikipedia.org/wiki/Escape_velocity


This has bugged me actually... all you really need is to pick up enough speed to get higher than the escape velocity. You'll note that the escape speed is defined as such assuming an object with no acceleration. But I'm not entirely sure where acceleration comes into this - i.e. if you have a 1g drive and you launch from the surface of a planet with a surface gravity of 2g on the ground, would you be even able to get off the ground? I'd assume you couldn't.

But if you were already in space, you could possibly slingshot past the planet and pick up enough speed to escape its gravity with a 1g drive, if you set things up right. Maybe.

I dunno, spacecraft orbital dynamics ain't my thing...

 

[BrennanHawkwood]

IIRC, if the ship is sufficiently streamlined or has enough contragrav to remain airborne (i.e. enough lift to effectively 'cancel out' its own weight) it can still get out of the well, it just takes longer.

I'll admit, I don't particularly worry about it IMTU. My PCs have a ship with 4g acceleration and aren't that much into the physics of it anyways.

[edited for clarity]

 

[mike wightman]

It's the difference between velocity and acceleration.
A 1G drive can reach near light speed if it's left on long enough.
The clue in CT is that streamlining is required to land on planets with atmospheres - that streamlining provides lift.
A 1G drive will eventually accelerate a ship to escape velocity providing the hull provides the lift to allow it to take off in the first place.

Another way to think off it is raising the gravitational potential energy of the ship, but I'll have to go and check my potential well notes before going anyfurther

 

[BrennanHawkwood]

Originally posted by Malenfant:

This has bugged me actually... all you really need is to pick up enough speed to get higher than the escape velocity. You'll note that the escape speed is defined as such assuming an object with no acceleration. But I'm not entirely sure where acceleration comes into this - i.e. if you have a 1g drive and you launch from the surface of a planet with a surface gravity of 2g on the ground, would you be even able to get off the ground? I'd assume you couldn't.

I'd agree with you...Unless contragrav is used to cancel a portion of the gravitation pull or the craft is aerodynamic enough to take off like an airplane.

 

[Malenfant]

Originally posted by Sigg Oddra:
It's the difference between velocity and acceleration.
A 1G drive can reach near light speed if it's left on long enough.

Yes, but can a 1G drive even get off the ground if the local g-field is higher? I presume that it has to, but I'm a bit crap at mechanics...

Even if you have contragrav, doesn't that just cancel out weight, not mass?

 

[mike wightman]

Very few light aircraft engines can generate 1G of thrust...

think of it this way. A car starts to accelerate from stationary, does it have 1 G of thrust? No.
It gets faster and faster as it accelerates to build up speed.

Now put wings on it, the wings provide lift. So once a certain critical velocity is achieved the vehicle leaves the ground.

It can now accelerate to a faster speed because it is no longer suffering friction between its wheels and the ground. Only air resistance is a problem.

As the aircraft climbs higher, at much less than 1G of thrust, the force of air resistance gets less so the plane can go faster.

With a Traveller 1G free trader it climbs slowly to a high altitude where lift from the atmosphere becomes negligable but the forward velocity is now quite large, the ship is also at an altitude where the force of gravity is less than 1G.

 

[TheEngineer]

Hi !

That old question again.
Perhaps we should create a Traveller FAQ ?

Guess its just a rule thing.
Taking MT it is stated, that thrusters ignore "local" gravity, when calculation cruising speeds. Also the given travel time charts "to reach orbit.." do not give any hint, that gravity plays a role.
This is why gas giant skimming is such an easy thing (regarding gravity) even for a humble 1g merchant.
Thats Traveller

So IMHO and in my gaming practise, thrusters cancel out weight and somehow ignore mass, too.
(Yes, thats the fictional part of SF)
Really strange things, but you never know what stuff is developed by these engineers next....

@Sigg:
I always expected streamlining to provide some way to get along during athmoshpere crossing without being torn into parts or melting away.
To provide lift by aerodynamics is perhaps a step further and maybe not an attribute of the "streamlined" designs we know in Traveller.
Anyway, even without aerodynamical lift the increasing velocity vector will drag you away from the surface sooner or later

Regards,

TheEngineer

 

[Malenfant]

Originally posted by Sigg Oddra:
Now put wings on it, the wings provide lift. So once a certain critical velocity is achieved the vehicle leaves the ground.

*snip*

With a Traveller 1G free trader it climbs slowly to a high altitude where lift from the atmosphere becomes negligable but the forward velocity is now quite large, the ship is also at an altitude where the force of gravity is less than 1G.

Yes, but an aeroplane isn't flying because of its acceleration, it's flying because it's going fast enough to generate enough lift to overcome gravity (and other aerodynamic reasons).

I was thinking more in terms of a rocket, pointing straight up. Does that need an acceleration of more than 1g in order to leave the ground? I presume it does, but I'm feeling particularly dense today

 

[mike wightman]

Yes, a rocket going straight up requires more than 1G of thrust (on Earth) or all it'll do is hover then fall over

Contragravity or grav lift modules like in an air/raft (called null grav modules in CT) produce the buoyancy (lift) and a small amount of propulsion. Attach a rocket to an air/raft and watch it go

 

[Anthony]

A rocket needs a thrust of more than local surface gravity, at least briefly, to leave the ground.

 

[Michael Taylor]

This lead sme to a simpler question: is the scale of vehicle EP to starship EP 1000:1? ie: 1000 vehicle EP = 1 starship EP. I would assume so, lookimg at the size of a vehicle fission powerplant compared to a starship fission powerplant.

 

[far-trader]

I think I was getting something like 500 to 1 by comparing fusion power plants iirc.

 

[Michael Taylor]

If you compare fission vehicle PP with a starship fission PP the ratio is 1000:1. Why?
A vehicle fission PP has 2.14 starship tons of mass and produces 1000 vehicle EP. A starship fission PP unit produces 1 starship EP and is 2 starship tons in displacement.

So that means the ratio is 1000 to 1?

 

[far-trader]

Originally posted by Michael Taylor:
If you compare fission vehicle PP with a starship fission PP the ratio is 1000:1. Why?
A vehicle fission PP has 2.14 starship tons of mass and produces 1000 vehicle EP. A starship fission PP unit produces 1 starship EP and is 2 starship tons in displacement.

So that means the ratio is 1000 to 1?

Ah, you're using the vehicle TA's conversion of vl to tons, rather than the one in The Traveller's Handbook. If you used the latter you'd get half that, or about 500 to 1.

edit - Since this is your topic Michael and you brought it up I'll leave my rant here as I first composed this reply (except for this edit). If you'd rather I move it somewhere else just drop me a note and accept my apologies in advance.

<rant on>

So which is the "official" vl to ton conversion?

And why weren't the two design systems better integrated?

A little internal consitancy would have been nice. As it is when comparing the fusion tech of the two the vehicle types use about half the fuel and cost 10% that of a plant of the same size. But does that even mean that solving for EP ratio is valid? They are obviously not equal and yet have exactly the same name, brilliant.

<rant off>

 

[far-trader]

Originally posted by Michael Taylor:
Just a simple question...

As you see everybody, in Traveller(tm) there are no simple questions and no easy answers.

 

[Michael Taylor]

I'm happy for you to Rant Dan. Where is the conversion in the THB?

 

[Michael Taylor]

As for getting off the ground, a look at the power to weight ratio of some launch vehicles:

Saturn 1b: 1.26
Saturn V: 1.19
Soyuz SL4: 1.32

It appaears all are greater than 1:1 whcih means a 1-G maneuver drive would get you into space very slowly.

 

[Malenfant]

How do those power to weight ratios convert to anything meaningful in Trav? (eg what's the power to weight ratio of a Suleiman, or a Far Trader)?