In-atmosphere speed of manoeuvre drives

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paltrysum
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In-atmosphere speed of manoeuvre drives

Postby paltrysum » Wed Jan 10, 2018 6:59 pm

I recall CT having a list of in-atmosphere manoeuvre drive speeds. I don't think MgT has published anything like that. Is there a resource on this subject? For example, how fast can a free trader with 1G of thrust fly in atmosphere? I would assume aerofins would enhance the speed.
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Re: In-atmosphere speed of manoeuvre drives

Postby baithammer » Wed Jan 10, 2018 9:42 pm

Covered under MGT 2ed MB under Dog Fighting Vehicles.
Spacecraft are capable of any Speed Band listed in
the Vehicles chapter, and will typically be going at
Hypersonic speeds when entering an atmosphere.
So 1g = speed band 10 Hypersonic.

Aero-fins simply add +2 DM to pilot tests.
Last edited by baithammer on Wed Jan 10, 2018 10:30 pm, edited 1 time in total.
AnotherDilbert
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Re: In-atmosphere speed of manoeuvre drives

Postby AnotherDilbert » Wed Jan 10, 2018 10:09 pm

CT Striker has a table with speeds for Gs, repeated in MT RM p86.
Condottiere
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Re: In-atmosphere speed of manoeuvre drives

Postby Condottiere » Wed Jan 10, 2018 10:46 pm

Spaceship manoeuvre drives tend to generate a great deal of thrust.

Probably to the point where the atmospheric friction turns the hull skin to plasma.
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Re: In-atmosphere speed of manoeuvre drives

Postby Ishmael » Thu Jan 11, 2018 11:30 am

Condottiere wrote:
Wed Jan 10, 2018 10:46 pm
Probably to the point where the atmospheric friction turns the hull skin to plasma.
This might be helpful for determining that....
https://physics.stackexchange.com/quest ... mperatures
Reynard
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Re: In-atmosphere speed of manoeuvre drives

Postby Reynard » Thu Jan 11, 2018 11:57 am

MegaTraveller Referee's Manual craft design section has a table to convert maneuver thrust to maximum speed. A M1 drive for a streamlined craft gives 1200kph while M6 is 4200kph. That's top speed. Ships more often travel at cruising speed (75%) or Nape of the Earth (25%). A ship's avionics is the big limiting factor no matter how powerful the engines.
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Re: In-atmosphere speed of manoeuvre drives

Postby Condottiere » Fri Jan 12, 2018 1:23 am

1. In a vacuum world, speed should be faster.

2. If I remember correctly, speed tends to be determined by thrust and weight.

3. Hull material should determine melting point; and heat shielding.

4. I assume breaking the sound barrier would be frowned upon by planetary authorities.
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Re: In-atmosphere speed of manoeuvre drives

Postby Reynard » Fri Jan 12, 2018 1:29 am

" If I remember correctly, speed tends to be determined by thrust and weight."

Only for grav and thrust based craft. Ships with maneuver use the Gs to determine speed.

For #3 and #4, ships can slow down when necessary to avoid burning up or causing damage from intense speed bow waves.
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Re: In-atmosphere speed of manoeuvre drives

Postby AnotherDilbert » Fri Jan 12, 2018 8:27 am

Reynard wrote:
Fri Jan 12, 2018 1:29 am
" If I remember correctly, speed tends to be determined by thrust and weight."

Only for grav and thrust based craft. Ships with maneuver use the Gs to determine speed.
It's the same thing.

From Newton's Second (and FFS) we know that F = m × a or a = F / m, i.e. acceleration is thrust / mass.
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Re: In-atmosphere speed of manoeuvre drives

Postby Linwood » Sun Jan 14, 2018 3:13 am

Condottiere wrote:
Fri Jan 12, 2018 1:23 am
1. In a vacuum world, speed should be faster.

2. If I remember correctly, speed tends to be determined by thrust and weight.

3. Hull material should determine melting point; and heat shielding.

4. I assume breaking the sound barrier would be frowned upon by planetary authorities.
Speed of sound is dependent on atmosphere density, as is frictional heating of the hull. So the lower you go or the denser the atmosphere the slower the speed limit is to avoid violating local noise ordinances.

Note as well that proper design can reduce the intensity of the sonic boom generated - but I doubt many starship designs would incorporate those features.
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Re: In-atmosphere speed of manoeuvre drives

Postby Ishmael » Sun Jan 14, 2018 6:20 am

Actually, the speed of sound is dependent on temperature, not density.

----------------------
speed of sound = sqrt ( 1.4 * 8.3144 * T * 1000 / M )

1.4 is the adiabatic index for diatomic molecules ( that's over my head )
8.3144 is the gas constant
T is temperature in kelvin
M is molar mass in g/mol of your atmosphere

1.4 is the specific heat ratio of dry air. Change this value to whatever the specific heat ratio of your world's atmosphere is.
--------------------

Density is important in working out drag and thus, for top speed and for re-entry heating. (link to useful equations for atmospheric heating posted earlier in thread... Stagnation temperatures may be considerably higher though)
---------------------
Den = ( M * P ) / ( 8.3144 * T )

M is the molar mass of the atmosphere and can change depending on composition, so this method will work for any world, from Venus to Mars and even Gas Giants. Traveller's UWP usually assumes that the atmosphere is an oxygen-nitrogen mix. For Earth, 29 can be used. this is pretty close to 79% N2 and 21% O2 ( .79 * 28 + .21 * 32 = 28.84 ). Other worlds will have different numbers, but always greater than the minimum molecular weight retained ( Jean's Escape ).

P is pressure. Surface pressure of the world is a good start, but Mongoose doesn't produce that. However, a decent fit of a pressure vs UWP value from World Builder's Handbook is ( UWP atm^2 ) / 49.
As pressure is actually measure in Pascals, just multiply atm's by 101,325 P
---------------------
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Re: In-atmosphere speed of manoeuvre drives

Postby Condottiere » Sun Jan 14, 2018 7:59 pm

Dirtside starports would be in fairly remote areas, so sonic booms might not be an issue, especially for a near vertical departure or arrival.
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Re: In-atmosphere speed of manoeuvre drives

Postby baithammer » Sun Jan 14, 2018 8:39 pm

Ishmael wrote:
Sun Jan 14, 2018 6:20 am
Actually, the speed of sound is dependent on temperature, not density.
Only when dealing with Ideal gases.
This equation is correct to a much wider temperature range, but still depends on the approximation of heat capacity ratio being independent of temperature, and for this reason will fail, particularly at higher temperatures.
Lower altitude have a higher density which retains higher temperature, so its not solely temperature that is at play.
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Re: In-atmosphere speed of manoeuvre drives

Postby phavoc » Sun Jan 14, 2018 8:42 pm

A 'streamlined' starship would still have antenna's, turrets, airlocks etc. If you look at the design of the free-trader, the cockpit windows are built more like the old Eagle from the Space 1999 series - recessed and flat-screened. Craft that are going supersonic, let alone hypersonic, are going to need to have extremely aerodynamic skins with minimal protrusions and disruptions. There should not be a straight-line comparison of how many maneuver G's a ship has vs. an overall speed in atmosphere. At best it would be a rule of thumb, but it would be dependent upon the air and atmospheric density of a world. No matter how much power you are putting out, the faster you go the more dense the air becomes. And eventually you run into the equivalent of an air brick wall, especially at lower altitudes. You would be better off having a rule of thumb of say 500 kph per G output for a starship within an atmosphere along a sliding scale (say it halves with each additional G).

And starships are not going to be designed to minimize sonic booms - if they go super sonic (and assuming they could), they would produce sonic booms.
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Re: In-atmosphere speed of manoeuvre drives

Postby Sigtrygg » Sun Jan 14, 2018 9:09 pm

Hull configuration and streamlining should be factors.
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Re: In-atmosphere speed of manoeuvre drives

Postby baithammer » Sun Jan 14, 2018 9:58 pm

Hull configuration and streamlining should be factors.
In a simplified way they are in MGT 2ed, with partial streamlining being limited to atmosphere 3 or less and unstreamlined craft making pilots checks -4 for entry and every minute of flight.
And eventually you run into the equivalent of an air brick wall, especially at lower altitudes.
Which would be well over the hypersonic range with streamlining and material strength for traveller. ( IR sensors would have a field day however.)
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Re: In-atmosphere speed of manoeuvre drives

Postby phavoc » Sun Jan 14, 2018 10:12 pm

The rules covering hull configuration and in atmosphere flight are silly and don't follow logic. A sphere is fine to land, as is a cube shaped ship. No Traveller ship can fly without antigravity for lift because they mass too much and they have insufficient lift surface. So they all need to have antigravity - at least all the standard ones. A tail sitter could go up and down, but they would be silly to have once you get antigravity lifters.

The only real difference is that ships with non-aerodynamic shapes would be limited in their speed. A spherical or cube like ship could make 150kph to maneuver, but it would maneuver like the brick it is when in atmosphere. But 150kph is more than sufficient to land or maneuver to orbit without suffering any pilot penalties. Where the penalties should occur is when they try to exceed that rating.
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Re: In-atmosphere speed of manoeuvre drives

Postby SSWarlock » Sun Jan 14, 2018 11:37 pm

phavoc wrote:
Sun Jan 14, 2018 8:42 pm
*snip* Craft that are going supersonic, let alone hypersonic, are going to need to have extremely aerodynamic skins with minimal protrusions and disruptions. There should not be a straight-line comparison of how many maneuver G's a ship has vs. an overall speed in atmosphere. At best it would be a rule of thumb, but it would be dependent upon the air and atmospheric density of a world. No matter how much power you are putting out, the faster you go the more dense the air becomes. And eventually you run into the equivalent of an air brick wall, especially at lower altitudes. You would be better off having a rule of thumb of say 500 kph per G output for a starship within an atmosphere along a sliding scale (say it halves with each additional G).

And starships are not going to be designed to minimize sonic booms - if they go super sonic (and assuming they could), they would produce sonic booms.
Sounds like gas giant skimming brings its own set of limitations. Seems to me that anything designed for that would need to be designed for super/hypersonic flight consequences. However, would this also limit some skimming ships to smaller gas giants (less G forces, less dense atmosphere, etc.)?
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Re: In-atmosphere speed of manoeuvre drives

Postby phavoc » Sun Jan 14, 2018 11:46 pm

SSWarlock wrote:
Sun Jan 14, 2018 11:37 pm
Sounds like gas giant skimming brings its own set of limitations. Seems to me that anything designed for that would need to be designed for super/hypersonic flight consequences. However, would this also limit some skimming ships to smaller gas giants (less G forces, less dense atmosphere, etc.)?
Skimming has never been well defined, you just skim in a few hours and go. Nothing about atmospheric hazards, storms, hitting pockets of rich gas vs. say helium or maybe even oxygen. Oh, and what about that High Guard thing... just why does refueling in a gas giant make you more vulnerable?
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Re: In-atmosphere speed of manoeuvre drives

Postby Condottiere » Mon Jan 15, 2018 1:58 am

Image

Predictable flight path.

Trouble with the nebula, sir, is all that static discharge and gas clouds our tactical display. Visual won't function and shields will be useless.

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