A lot of worlds over 1g gravity. How do 1G thrust ships take off?

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Moppy
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A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Moppy » Sat Jan 26, 2019 10:12 pm

The m-drive is gravitic? Is the thrust relative to the local gravity?

Anti-grav neutralises local gravity then the ship does 1G on top of that?
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Old School » Sat Jan 26, 2019 10:36 pm

G rating on a drive represents the g-forces (in the conventional sense with 1G equalling the gravitational pull of Earth) of acceleration that the drive can generate. 1G is about 9.5 meters per second of acceleration. Not affected by the gravitational pull of any planet that may be nearby. Note that a ship with a 1G drive can generate 1G acceleration when taking off, but also when millions of miles from any planetary body. So the gravitational pull of any neaeby body doesn’t factor in. Why? Who cares. Its a scence fiction game with make believe gravitational drives.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Moppy » Sat Jan 26, 2019 10:50 pm

Can you clarify this part of your response?
Old School wrote:
Sat Jan 26, 2019 10:36 pm
Not affected by the gravitational pull of any planet that may be nearby. ... So the gravitational pull of any neaeby body doesn’t factor in.
So, the planet's gravity has no effect on the ship? Therefore it doens't matter how strong the gravity is, the ship can still take off?
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby phavoc » Sun Jan 27, 2019 12:09 am

Ships have antigravity for lift, so that negates the planetary gravity.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby AnotherDilbert » Sun Jan 27, 2019 8:51 am

Moppy wrote: The m-drive is gravitic?
Yes. (Whatever that means.)

Moppy wrote: Is the thrust relative to the local gravity?
No, it is added to gravity, or subtracted as the case may be.

Moppy wrote: Anti-grav neutralises local gravity then the ship does 1G on top of that?
Ships don't necessarily have A/G drives in addition to M-drives.

That ships have anti-grav lifters is often assumed, but not specified in CT, MT, or MgT. TNE ships do have contragrav, a different system with similar effects.

That ships are affected by the full force of gravity is specified in CT (e.g. LBB2, p29) and MgT1 (HG, p86).



So, how does ships take-off from hi-grav planets? Either they have wings and can fly like airplanes (cf. Subsidised Merchant) or they simply don't land (and use a highport instead).

This makes a M-2 drive very useful.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Moppy » Sun Jan 27, 2019 12:10 pm

AnotherDilbert wrote:
Sun Jan 27, 2019 8:51 am


So, how does ships take-off from hi-grav planets? Either they have wings and can fly like airplanes (cf. Subsidised Merchant) or they simply don't land (and use a highport instead).

This makes a M-2 drive very useful.
Wings take-off with thrust less than gravity needs runway. Possible but i never heard of it within the setting for regular spacecraft. Assuming no counter-gravity.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Sigtrygg » Sun Jan 27, 2019 1:23 pm

They don't have to have wings - a streamlined lifting body with TL9+ computer controlled control surfaces will suffice.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Old School » Sun Jan 27, 2019 2:38 pm

That ships are affected by the full force of gravity is specified in CT (e.g. LBB2, p29) and MgT1 (HG, p86).
You’re taking the combat movement rules, which are entirely different from those that govern normal ship movement, and applying them to whether or not a ship can take off. So by that logic, a ship with a 1G M drive cant take off from an earth or larger world without aerodynamic lift. Even this logic, of course, ignores the reality that a planet’s gravitational pull decreases at a much lower rate than its atmosphere does as you move away from the surface, so atmospheric lift can’t be the entire answer. A fat trader would have to accelerate to incredible veloicity within the atmosphere (which, or course, it can’t do because of atmospheric resistance, no matter how streamlined it is) in order to have momentum necessary to escape the planet’s gravity, while a 2G capable ship just leisurely takes off. Despite this, 1G is the standard for civilian ships. Yeah, right.

If you think taking off in a 1G ship seems hard, try Gas giant skimming in that same ship.

Welcome to Traveller rules absurdity. I hope no one actually tries to play the game this way.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby AnotherDilbert » Sun Jan 27, 2019 3:51 pm

Moppy wrote: Wings take-off with thrust less than gravity needs runway. Possible but i never heard of it within the setting for regular spacecraft. Assuming no counter-gravity.
It's mentioned in some early adventures, e.g.:
The Traveller Adventure, p74 wrote:A shuttle flight from Aramanx Starport brings the party to Seniek, the capital of the Union, where an airstrip has been converted for use by incoming small spaceships carrying visitors and occasional cargos.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby AnotherDilbert » Sun Jan 27, 2019 3:58 pm

Sigtrygg wrote: They don't have to have wings - a streamlined lifting body with TL9+ computer controlled control surfaces will suffice.
OK, not wings but aerodynamic lift.

I'm personally not fond of calling the Scout a lifting body, but T5 is what it is...
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Condottiere » Sun Jan 27, 2019 4:21 pm

I'm not a rocket scientist, but I assume there's a reason they need to control a massive explosion that creates that much thrust to get to orbit that propels it so fast that passengers feel that extra gravitation force.

As for the trick of escaping the atmosphere, I would think it's constant acceleration, though jets and wings need said atmosphere to operate efficiently, and then have to switch to a rocket motor.

I would think that the higher anti gravtitational motors go, the more relative power/speed they have potentially, having to contend with less gravitational force; if lifters are overfactored relative to the gravitation, the extra can be used for propulsion.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby AnotherDilbert » Sun Jan 27, 2019 4:55 pm

Old School wrote: You’re taking the combat movement rules, which are entirely different from those that govern normal ship movement, and applying them to whether or not a ship can take off.
Yes, those are the rules for normal ship movement. They are the basis for the travel time formulas.

Old School wrote: Even this logic, of course, ignores the reality that a planet’s gravitational pull decreases at a much lower rate than its atmosphere does as you move away from the surface, so atmospheric lift can’t be the entire answer. A fat trader would have to accelerate to incredible veloicity within the atmosphere (which, or course, it can’t do because of atmospheric resistance, no matter how streamlined it is) in order to have momentum necessary to escape the planet’s gravity, while a 2G capable ship just leisurely takes off.
I'm not an expert in Aerodynamics. I'm assuming aerodynamic lift can make up for a small shortfall in thrust, say a 1 g ship taking off from a 1.1 g planet.

As far as I know surface gravity increases linearly with radius, so even a size A planet would only have a surface gravity of ~1.25 g.

Old School wrote: Despite this, 1G is the standard for civilian ships. Yeah, right.
1 g is cheaper and can handle most planets. Shuttles can handle special cases.

Old School wrote: If you think taking off in a 1G ship seems hard, try Gas giant skimming in that same ship.
I'm thinking in terms of a high speed pass through the upper atmosphere, not landing on the "surface" of a gas giant.

Note that gas giants are not very dense compared to rocky planets, so don't have all that high gravity. Saturn has a "surface" gravity of 1.07 g and Jupiter 2.5 g.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Moppy » Sun Jan 27, 2019 7:52 pm

If they use aerodynamics to lift they need to be moving first. I have never seen wheels on a Free Trader so I think that rules out runways. Either there are many worlds you can't take a "standard" 1G ship (which runs counter to lore but not T5 rules?) or they must have inherent anti-grav that somehow counter-acts planetary gravity. I am leaning towards anti-grav. I'm not going to consider tug or catapult launches.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby AnotherDilbert » Sun Jan 27, 2019 9:00 pm

Old School wrote: Despite this, 1G is the standard for civilian ships. Yeah, right.
This is of course a good question. Why isn't, say, the iconic Free Trader more capable?

Let me illustrate why I think M-1 makes sense:

The standard Free Trader (with custom hull):
Image
It just about breaks even if we can fill it to 80% of capacity on average.


Now give it an M-2 drive and see what happens:
Image
Price is up, operating cost is up, and cargo capacity, hence revenue, is down. The ship is now losing money hand over fist.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby phavoc » Sun Jan 27, 2019 9:22 pm

AnotherDilbert wrote:
Sun Jan 27, 2019 8:51 am
Ships don't necessarily have A/G drives in addition to M-drives.

That ships have anti-grav lifters is often assumed, but not specified in CT, MT, or MgT. TNE ships do have contragrav, a different system with similar effects.

That ships are affected by the full force of gravity is specified in CT (e.g. LBB2, p29) and MgT1 (HG, p86).

So, how does ships take-off from hi-grav planets? Either they have wings and can fly like airplanes (cf. Subsidised Merchant) or they simply don't land (and use a highport instead).

This makes a M-2 drive very useful.
CT had repulsor fields at ground starports to help heavier ships taking off (more rules miasma...)

However the idea that the wings on some ships provide enough lift to take off violates basic aerodynamics. Traveller ships are too heavy with too small of a wing to allow enough lifting force to take off. Also, a standard scout as a lifting body will not work - notice that the hull is anything but aerodynamic.

Rules don't cover lots of things, but logic dictates they must be present otherwise they won't work. Like much of Traveller the rules have very large holes, or worse, direct contradictions.

Even with M-2 you have to have LIFT to get off the ground. Even putting it in a vertical position isn't without problem. Lift, regardless of your M-drive, is predicated upon mass. Since the rules don't state that mass is negated in any way (not mentioning it isn't the same thing, as pointed out above), lift must exceed mass in order to work. Mass (or weight in a gravity field) therefore matters. Ergo contragravity must be employed for the ships to actually get off the ground.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Moppy » Sun Jan 27, 2019 10:16 pm

phavoc wrote:
Sun Jan 27, 2019 9:22 pm
However the idea that the wings on some ships provide enough lift to take off violates basic aerodynamics. Traveller ships are too heavy with too small of a wing to allow enough lifting force to take off. Also, a standard scout as a lifting body will not work - notice that the hull is anything but aerodynamic.
I agree that the numbers for lift probably don't work due to weight - so you're in no way wrong.

However I'm OK with hand-waving Traveller's hull shapes. Given enough speed, even a brick will provide lift. All you need to do is angle the brick so it deflects air downwards. There's so much that isn't known about fluid flow/turbulence that I'm prepared to accept this in a game setting.

In reality, I would not be astonished if the speeds required meant you set the atmosphere on fire, or you had an unsolvable control/stability problem. There's also how you handle the air/ground transition if you need such speed for lift, and need to not have such speed when you contact the ground. I agree that (absent anti-grav or catapult) any vertical take off requires manever rating > planetary gravity, and any conventional aircraft takeoff requires horizontal speed.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Condottiere » Sun Jan 27, 2019 10:18 pm

Mass driver catapult; or

Image
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Linwood » Sun Jan 27, 2019 10:23 pm

Ummm - not to kick off a new controversy thread, but aren’t ships and ship’s drive ratings based on displacement tons, not mass?

I think I just found my handwavium out if this conversation ever comes up in my campaign.... 🙂
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby AnotherDilbert » Sun Jan 27, 2019 10:28 pm

phavoc wrote: CT had repulsor fields at ground starports to help heavier ships taking off (more rules miasma...)
Since we had Repulsors in HG, that wasn't much of a problem?

phavoc wrote: However the idea that the wings on some ships provide enough lift to take off violates basic aerodynamics. Traveller ships are too heavy with too small of a wing to allow enough lifting force to take off.
Isn't that a question of enough thrust? Enough thrust gives enough speed gives enough airflow over the wings to lift with fairly small wings? (And some artistic licence in the illustrations?)

Even a paltry 1 g thrust is quite a lot by current aircraft standards. E.g. a A380 has a max take off weight of ~575 tonnes and a thrust of 1400 kN, so can achieve flight with thrust to mass ratio of ~0.24 g. Of course it has much bigger wings (per mass) than any Traveller spacecraft I have seen.

A Starfighter (with rather small wings) could take off with a mass of 13 tonnes and a thrust of 44 kN (dry), for a thrust to mass ratio of about 0.34 g.

phavoc wrote: Also, a standard scout as a lifting body will not work - notice that the hull is anything but aerodynamic.
Agreed, but T5 very specifically overrules such petty concerns...
Image
Wings gives the benefit "Wings increase the performance of a ship’s Maneuver or Gravitic Drive +1G if operating in Atmosphere". (T5.09, p277)

phavoc wrote: Even with M-2 you have to have LIFT to get off the ground. Even putting it in a vertical position isn't without problem. Lift, regardless of your M-drive, is predicated upon mass.
Since a ship with M-2 can achieve 2 g in free space it can produce enough thrust to overcome 1 g twice over by definition.

It's just a simplification in the system not to consider mass, but just volume, when dimensioning drives. Not even TNE FF&S dared dimension the thrusters on mass even if that clearly was the basic rule.

phavoc wrote: Ergo contragravity must be employed for the ships to actually get off the ground.
No, not if we have enough thrust from the M-drive. Or aerodynamic lift.

(Only TNE used contragrav; CT, MT, MgT, and even T5 uses anti-grav, there is a small but significant difference.)
Last edited by AnotherDilbert on Sun Jan 27, 2019 10:33 pm, edited 1 time in total.
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Re: A lot of worlds over 1g gravity. How do 1G thrust ships take off?

Postby Moppy » Sun Jan 27, 2019 10:32 pm

Linwood wrote:
Sun Jan 27, 2019 10:23 pm
Ummm - not to kick off a new controversy thread, but aren’t ships and ship’s drive ratings based on displacement tons, not mass?
The mass cancels out if you have an acceleration in gs. That's a measurement of speed change over time, with mass not being a term.

However, changing load should change the acceleration. I can't wait for the new rules that require recalculating acceleration every game turn as you burn off fuel. :-)

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