Starship Speed In Atmosphere

[spidersrepublic]

How fast can a Starship move while planetside? Is there a formula for converting Thrust into Km/h


if a G-Carrier can move 620 km/h, and a Grav Floater only 40 km/h, could these represent the extremes of thrust? So a G-Carrier might be a Thrust 6-9 and a Grav Floater only Trust 1?

From this i could extrapolate a formula, but I'm sure this has been done properly somewhere in Traveller literature!

Thrust X 40 = Km/h Planetside

So:

Thrust 1 = 40 km/h

Thrust 2 = 80 mk/h

Thrust 3 = 120 km/h

Thrust 4 = 160 km/h

Thrust 5 = 200 km/h

Thrust 6 = 240 km/h

...

Thrust 9 = 360 km/h


Only problem with this formula is it never really reaches the 620 km/h that the G/Carrier can move.

Any ideas?

 

[rust]

GURPS Traveller uses Drag and Thrust to calculate the atmospheric
speed of a starship, but unfortunately Mongoose Traveller cannot
use that formula. However, it may help that for example the Scout
has an atmospheric top speed of 2,357 mph and the Free Trader of
1,700 mph in GURPS Traveller.

 

[spidersrepublic]

GURPS Traveller uses Drag and Thrust to calculate the atmospheric
speed of a starship, but unfortunately Mongoose Traveller cannot
use that formula. However, it may help that for example the Scout
has an atmospheric top speed of 2,357 mph and the Free Trader of
1,700 mph in GURPS Traveller.

Interesting, thanks!

so that means 3793 km/h for a Scout, and 2735 km/h for a Free Trader.

Ships Manoeuvre rating:

Scout: Thrust 2

Free Trader: Thrust 1

ok. Those are some extremely fast speeds for low man. craft. From this we could show:

Thrust 3 : 3174

Thrust 4: 4232

etc.

We could say: 1 Ships Thrust = Mach 1.5

So a Thrust 2 Scout would be 3675 km/hr (Mach 3)

Thrust 3 would be 5512 km/hr (Mach 4.5)

Thrust 4 would be 7350.264 km/hr (Mach 6)

One would need to adjust for dense or thin atmosphere. 1 mach or so either way.


For example, my PCs in their Thrust 3 SC Courier. They need to go from Brava Docks to the Research Facility on Phaedrus.

(Phaedrus IMTU world map here)
http://postimg.org/image/6noc48mox/

This trip is 12 hexs, 13,680 km.

With Thrust 3, the SC Courier could go 5512 km/hr.
Phaedrus has a Dense Atmosphere, so I reduce 1 Mach from their speed.
total speed: 4287km/hr

Meaning the trip would take 3.1 hours. Not bad.

any Ideas on this/suggestions?

 

[Infojunky]

How fast can a Starship move while planetside? Is there a formula for converting Thrust into Km/h

Succinctly No, as we are missing chunks of information Such as mass, etc. etc... All we are given is said ship's gross acceleration. And honestly aeronautic calculations go farther into maths than most of us are willing to go into for a game.

But with that a table in Striker for Grav vehicles gives us some raw numbers, which have some of that info buried behind the chart. (Note to all I went with a CT source as that is the general base MgT was written from) I also assumed that all of a ship's thrust was available for forward flight.

So from the chart.
1G = 1200 kph
2g = 2120 kph
3g = 2850 kph
4g = 3400 kph
5g =3840 kph
6g = 4200 kph

Using this data, These are "Maximum" speeds for streamlined craft, figure cruising speed is 75% of these and Nap-of-Earth (NoE) speed is at max 25% (NoE speed is really dependant on the ship's electronics package, as such commercial ships will be slower than the computed speed as the pilots primary sensor is the MK 1 Eyeball)

 

[Infojunky]

Interesting, thanks!

so that means 3793 km/h for a Scout, and 2735 km/h for a Free Trader.

Ships Manoeuvre rating:

Scout: Thrust 2

Free Trader: Thrust 1

ok. Those are some extremely fast speeds for low man. craft.

Ok, might I point out that a F-14 Tomcat has a Thrust to weight ratio of 0.88 or Less than 1g.

We could say: 1 Ships Thrust = Mach 1.5

So a Thrust 2 Scout would be 3675 km/hr (Mach 3)

Thrust 3 would be 5512 km/hr (Mach 4.5)

Thrust 4 would be 7350.264 km/hr (Mach 6)

One would need to adjust for dense or thin atmosphere. 1 mach or so either way.


For example, my PCs in their Thrust 3 SC Courier. They need to go from Brava Docks to the Research Facility on Phaedrus.

(Phaedrus IMTU world map here)
http://postimg.org/image/6noc48mox/

This trip is 12 hexs, 13,680 km.

With Thrust 3, the SC Courier could go 5512 km/hr.
Phaedrus has a Dense Atmosphere, so I reduce 1 Mach from their speed.
total speed: 4287km/hr

Meaning the trip would take 3.1 hours. Not bad.

any Ideas on this/suggestions?

I also would point out that Mach numbers are dependant on the speed of sound at a given atmospheric density, as such Mach One is slower in KPH as the atmosphere get's thicker. Also that doubling the amount of thrus doesn't correlate in doubling the speed as the inverse square is involved in the equation.

 

[Sigtrygg]

The maximum speed of a ship in an atmosphere is going to be determined by its streamlining and the density of the atmosphere.

Higher thrust will let it get to that maximum speed faster.

As the ship gains altitude maximum speed increases as air resistance drops off.

 

[Ishmael]

The speed of sound in an ideal gas is independent of frequency, but does vary slightly with frequency in a real gas. It is proportional to the square root of the absolute temperature, but is independent of pressure or density for a given ideal gas. Sound speed in air varies slightly with pressure only because air is not quite an ideal gas.

 

[Wil Mireu]

I think this is another problem with the way Traveller grav tech works - it's not based on mass and it doesn't have a fuel limitation, so anything can just keep accelerating until it hits a speed where the stresses of atmospheric drag will tear it apart or burn it up. The thrust rating (whether 1G or 6G) would only determine how fast it reaches that maximum speed.

 

[F33D]

I think this is another problem with the way Traveller grav tech works - it's not based on mass and it doesn't have a fuel limitation, so anything can just keep accelerating until it hits a speed where the stresses of atmospheric drag will tear it apart or burn it up. The thrust rating (whether 1G or 6G) would only determine how fast it reaches that maximum speed.

IRL there was at least one airplane could not use full thrust for much time because it would melt. So the problem isn't confined to Trav spacecraft using G drives.

 

[Nathan Brazil]

Other versions of Traveller had other computations. Megatraveller had a different system. "Vacuum speed" was computed, then atmosphere speeds were computed based on streamlining. Basically start G's of speed:
Vacuum
G kph
1 1200
2 2120
3 2850
4 3400
5 3840
6 4200
"Airframe" was 90% Vacuum speed
Streamlined were 1000 kph max.
Unstreamlined were 300 kph max.

Not sure what the relation to Streamlined, Unstreamline, and Dispersed are in this version of the game.

 

[F33D]

Other versions of Traveller had other computations. Megatraveller had a different system. "Vacuum speed" was computed, then atmosphere speeds were computed based on streamlining. Basically start G's of speed:
Vacuum
G kph
1 1200
2 2120
3 2850
4 3400
5 3840
6 4200
"Airframe" was 90% Vacuum speed
Streamlined were 1000 kph max.
Unstreamlined were 300 kph max.

Not sure what the relation to Streamlined, Unstreamline, and Dispersed are in this version of the game.

The vacuum speeds are incorrect as it is a continuous thrust mechanism (for space craft). An error in the rules. Thus, all other speeds are meaningless.

 

[Sigtrygg]

Yup, at those speeds no ship could achieve escape velocity from a large moon, let alone a planet

Like a lot of MT stuff, it looks good but when you think about it it turns out to be pants.

 

[F33D]

Yup, at those speeds no ship could achieve escape velocity from a large moon, let alone a planet

Like a lot of MT stuff, it looks good but when you think about it it turns out to be pants.

And, it invalidates MT's own interplanetary travel figures.

 

[Galadrion]

Part of the problem is that velocity is a linear measure, while thrust is quadratic. Any formula to convert from thrust to velocity is going to need to involve exponents.

On the good side, that means that a vehicle capable of 9G acceleration is going to have on the order of 80 times the top speed of a vehicle with only 1G acceleration. Which means that if that 1G floater has a top speed of 40 km/h, then a 9G vehicle should have a top speed on the order of 3240 km/h. As a check, that also puts that (approximately) 640 km/h G-Carrier as a 4G thrust vehicle - which seems appropriate.

WARNING!

The formula I just "proposed" is extremely simplistic - basically, top speed = 40 km/h x vehicular thrust in G's, squared. Far too simplistic to be even remotely realistic. But it does illustrate the approximate order of the necessary formula.

 

[Wil Mireu]

As has been pointed out - there is no 'top speed' in a vacuum (other than 'c') . A ship can accelerate in vacuum as long as it likes and go as fast as it likes - the only thing limiting that are (a) the amount of fuel it has (not an issue with grav drives, unless the reactor shuts down), (b) dust/micrometeoroid impacts and (c) relativistic effects (only very close to the speed of light) and the speed of light itself. The only difference between a 1G and a 6G ship in that regard is how long it takes to get to a given velocity.

In atmospheres, none of that matters. What matters is aerodynamics - the density of the atmosphere, the shape of the ship, etc. The top speed isn't so much dependent on the output of the drive as what the ship is made out of, what it looks like, how it's staying in the air, what the air is made of (and how much there is), and what Tech Level the ship was made at.

 

[Nathan Brazil]

Yup, at those speeds no ship could achieve escape velocity from a large moon, let alone a planet

Like a lot of MT stuff, it looks good but when you think about it it turns out to be pants.

In MT, starships were assumed have for lack of a better word, a "extra G" which was assumed to exist and used to counteract the 1G of a world. So you could get to orbit no problem. Still bad from a physics point of view, but that was their story..

 

[locarno24]

In atmospheres, none of that matters. What matters is aerodynamics - the density of the atmosphere, the shape of the ship, etc. The top speed isn't so much dependent on the output of the drive as what the ship is made out of, what it looks like, how it's staying in the air, what the air is made of (and how much there is), and what Tech Level the ship was made at.

Agreed. The maximum speed (or at least the maximum speed you'd want to travel at without shock waves and aerodynamic heating becoming an issue) will be driven by armour type first ad distributed/standard/streamlined hull form second.

Whether you've got the engine power to maintain that speed will depend on air density and hull form versus engine power.

if a G-Carrier can move 620 km/h, and a Grav Floater only 40 km/h, could these represent the extremes of thrust? So a G-Carrier might be a Thrust 6-9 and a Grav Floater only Trust 1?

A grav floater is ultimately little more than a flying skateboard - someone couldn't be expected to stay on at a speed of more than 40 km/h. Yes, as a grav vehicle ultimately you could express the engine's power in G (and it must be capable of more than 1G to lift off) but I doubt the lateral acceleration is ever anything like a full G's power.

Equally, a G/Carrier is a combination attack helicopter/apc/sci-fi flying tank. It's not a proper aircraft per se. If you can do 6G thrust I'm sure supersonic flight isn't a problem...

I'd suggest a quick rule of thumb, based on limited science:

Basic atmospheric speed is your thrust score in 1000 Km/H (approximately 'mach number' for standard temperature and pressure earth atmosphere).

As suggested, kick it up by one mach (ore more accurately 1000 Km/h as mach speed varies) for a thin atmosphere or for flying at high altitude (so by two for flying at high altitude in a thin atmosphere). Drop it by one for a thick atmosphere.

Decrease by one for a non-streamlined hull.

If you're left with a value of less than one - then it's subsonic flight only.

 

[Sigtrygg]

Yup, at those speeds no ship could achieve escape velocity from a large moon, let alone a planet

Like a lot of MT stuff, it looks good but when you think about it it turns out to be pants.

In MT, starships were assumed have for lack of a better word, a "extra G" which was assumed to exist and used to counteract the 1G of a world. So you could get to orbit no problem. Still bad from a physics point of view, but that was their story..

It can't get you to orbit with a maximum vacuum speed of 4200kph. The maximum speed rule is broken.

 

[Sigtrygg]

If you have the magic of 1g continuous thrust your maximum sped is limited by your ship's aerodynamics an atmospheric density.

The ship is more than capable of standing up to the stresses of hypersonic flight during atmospheric re-entry.

If you have a 2g or a 6g drive your maximum atmospheric speed is still limited by your hull aerodynamics, not your acceleration/thrust rating. All that does is get you to your top speed faster.

What is needed is a table that gives you the maximum speed at different atmospheric pressures (this should be based on world size an atmosphere type) versus hull configuration.

 

[Infojunky]

It can't get you to orbit with a maximum vacuum speed of 4200kph. The maximum speed rule is broken.

Only if you assume a ballistic entry path. With Contragrav or simular removing the need to lift against gravity any thrust will get you there. You are making statements without stating your assumptions. I only quoted a previous source, which was mostly concerned with horizontal movement in respects to the plain of gravity. Also note the higher you get the less atmosphere you have to produce drag and a lessening effect of gravity, thus top speed increases.