[Ships] Atmospheric Speeds?

[Kilgs]

Has anyone come up with an idea or mechanism to determine atmospheric speed of a ship? Some of the speeds listed are so great as to burn the atmosphere and vaporize water so there should be a drag effect. I know somewhere in CT they discussed it but I can’t find it.

 

[klingsor]

Streamlining might limit it as much as thrust - you can go faster but bad things start to happen - as in "This may get interesting".

Or as the makers of the Firefox model said "you can fly a brick if you put big enough engines on it".

 

[Ishmael]

At the risk of being too gearheaded....

If you know the dimensions of your ship ( length width and height ), you can estimate the parasitic drag coefficient. You can also estimate the frontal cross-section area.

Choose your base Cp based on streamlining. I use the following...
Streamlined base Cp= .2 similar to a sphere
Partial Streamlined- blunt tail Cp=..5 similar to hemisphere_rounded forward
Partial Streamlined-blunt nose Cp=.8 similar to hemisphere-rounded aft
Unstreamlined Cp=1.2 similar to a flat plate

This gives the base Cp
actual Cp is (base Cp) / (length/greatest_diameter)^.5

Given air density ( 1.025kg.m^3 for 1 atm ) and actual Cp and frontal area, find aero drag
if you use water density (1000kg/m^3) you can find the top underwater speed ( ignoring any cavitation effects anyways, but for really big stuff, the hull would probably crumple first... I'll work it out someday I suppose...cavitation number similar to mach number???... just remember that cavitation is very noisy and makes props inefficient )

drag=1/2(density)*Cp*Area*V^2

maximum V will be at the point where drag=thrust
If drag is greater than what the ship is stressed for, it crumples up and fails

if you want to know about aero heating,
figure how much energy is absorbed by the drag and radiate about 2 or 3 % back to the ship as a blackbody and figure your ship's hull temp...
if its greater than about 1500K or so, then the ship fails in a pretty fashion

This assumes zero lift.
If you want the ship to have lift...just assume a Cl of 1 to 3 ( typical lift to high lift STOL) and use the lifting area (which is probably related to length*width) and figure the added drag caused by lift...
(same as above, but use Cl in place of Cp)

this is all approximations
base on old aerodynamic texts from the 30's
2-3% of the re-entry heat is about what the space shuttle absorbs during re-entry ( iirc )

 

[Kilgs]

NASA called... they want their chief engineer to stop checking the forums during work hours :shock:

Thank you very much Ishmael but that's going a bit far for me. I don't like the math involved in figuring out mortgages...

 

[EDG]

Thank you very much Ishmael but that's going a bit far for me. I don't like the math involved in figuring out mortgages...

In all seriousness, you can't really expect reasonable answer to your question unless someone actually attempts to quantify the problem - otherwise you may as well pluck a number out of thin air. You yourself mentioned drag and heating, after all.

The problem is: how fast can a ship travel in an atmosphere?

Qualitatively at least (I'm a scientist, not an engineer), I would expect it to depend on (a) the ship's internal strength, (b) the speed of the ship, (c) the stresses acting on the ship in the atmosphere (possibily filed under "drag"), (d) the density of the atmosphere, (e) the shape of the ship. Which seems to correspond roughly with what Ishmael was calculating.

 

[captainjack23]

Has anyone come up with an idea or mechanism to determine atmospheric speed of a ship? Some of the speeds listed are so great as to burn the atmosphere and vaporize water so there should be a drag effect. I know somewhere in CT they discussed it but I can’t find it.

Me I limit it to the SR-51 known limits and call it good to go. Mach 3-4 at high altitude, less as you get lower. Probably max speed of 1 mach every 10,000 feet including the first) is a good rule. there are lotsa practical reasons to not be flying to damn fast in an atmosphere, heat is just one of them.

Plus, mach 5+ starts being orbital velocity IIRC -but someone should check that.

 

[GypsyComet]

Under a prior edition, if it wasn't an "airframe", you were limited to 300kph for non-ballistic (ie. controlled) flight in breathable atmospheres. Obviously things like gas giant dips can get much higher than that, but that's also a lot less atmospheric pressure than you might think. If you are taking a ship into the murky depths of any planet, SLOW DOWN.

Only airframes were able to make full use of their drive ratings.

 

[captainjack23]

Under a prior edition, if it wasn't an "airframe", you were limited to 300kph for non-ballistic (ie. controlled) flight in breathable atmospheres. Obviously things like gas giant dips can get much higher than that, but that's also a lot less atmospheric pressure than you might think. If you are taking a ship into the murky depths of any planet, SLOW DOWN.

Only airframes were able to make full use of their drive ratings.

Oh yes, that too. I think the main issue is the streamlining. an airframe makes flight more efficient (lift) and easier to control. Lift is good; but, as the F4 and F104 proved, a brick will fly with a big enough engine*.

IIRC, there is a base -2 to piloting rolls in atm, even with stramlining -you need the airfoils option to overcome this.


Or, as with the 104, if one straps a saddle onto a big honkin engine, and adds some missile pilons disguised as wings...

 

[EDG]

but, as the F4 and F104 proved, a brick will fly with a big enough engine*.

Or, as with the 104, if one straps a saddle onto a big honkin engine, and adds some missile pilons disguised as wings...

The F104 seems like a very meticulously designed "brick" to me...
http://en.wikipedia.org/wiki/F-104_Starfighter

That thing was designed for high speed, and built to accomodate that requirement (sure, it apparently wasn't all that safe with it, but I think calling it a "brick" is doing it somewhat of a disservice).

EDIT: And now I remember that this was my favourite Airfix model I had when I was a kid too .

 

[Colin]

My father worked on 104s for many years. "Manned missile" was a common epithet, as was "Widowmaker". He used to tell me that on Red Flag exercises down in San Diego, the only way that some American pilots could track the Canadians was from the rooster tails they created as they flew 50 feet above the deck in the desert. I spent my childhood listening to them scream across the sky. Nothing like that noise.

I think captainjack's solution is a good rule of thumb, but I would modify it for higher tech levels and/or hull materials. +10% per tech level over 8, +10% for crystaliron, +25% for bonded superdense.

Oh, and it's an SR-71...

 

[saundby]

Basically you can travel faster higher up. For ships of 1 TL over present that were built for high speed atmospheric flight I assume active thermal management for the skin and the use of a boundary layer as a sort of aeroshell. I let these go 12,000kph at altitudes above about 30km for a standard atmosphere.

If you want to go lower, you have to go slower.

Also, the vehicle needs to be "clean." No battle damage, etc. to cause turbulence in the flow or disrupt the boundary layer. Otherwise there'll be dice rolling.

As to airfoils--what a quaint idea. With active thermal management and boundary layer management comes flight controls without moving parts.

BTW, this is based on tech I've worked on IRL. We're close on it. Just need time and money...

 

[EDG]

Also, the vehicle needs to be "clean." No battle damage, etc. to cause turbulence in the flow or disrupt the boundary layer. Otherwise there'll be dice rolling.

Well, just look at what happened to the space shuttle Columbia - one tiny hole in the wing was enough to tear it to bits when it re-entered.

 

[simonh]

Just a general observation, but there's a lot of talk here about high-speed re-entry. I don't htink many, if any Traveller ships will be designed for high speed re-entry. They'll shed most of their orbital velocity well before they re-enter the atmosphere. After all the atmosphere's only a few hundred km thick so it won't take all that long to come to a landing at relatively moderate speeds.

The only reason the shuttle and other orbiters nowadays don't just slow down to a reasonable speed before comign down is that it would take far too much fuel, but that's not a problem with gravitic drives.

Simon Hibbs

 

[Ishmael]

Just a general observation, but there's a lot of talk here about high-speed re-entry.

Not really...just discussion of top speed through an atmosphere for a Traveller ship. The same principals also apply to planes trains and automobiles.
This can cover re-entry, but leaving the world and gas giant skimming have the same considerations. In the latter cases, orbital velocities will have to increase while the ship is in the atmosphere in order to attain orbits above the atmosphere and even to reach escape velocity.

Then there's grav tech....but that's no fun!
OTU grav tech is a killjoy by allowing any ship and any configuration to reach the world's surface by letting it float down slowly. For this reason and others ( grav focused lasers<?> and making all other forms of transport obsolete<bleh> ), I feel OTU grav-tech puts Traveller too far into the space opera camp, but thats just my opinion. I make energy_in times efficiency proportional to altitude, mass and surface G's ( potential energy equation ).

something else to consider...sonic booms!
think how powerful they might be if a 200-400dton ship goes blasting by 50m above the deck at mach 4.

" In other news today... Rogue Imperial traders made an illegal hi-speed, low-altitude fly-by and flattened the shanty-town west of the city where refugees from the recent wars eke out a living. Medics treat dozens for injuries and hundreds are left homeless. Protests have been filed with the Imperium's Ambassador and promises of prompt legal action have been made."

 

[simonh]

This can cover re-entry, but leaving the world and gas giant skimming have the same considerations. In the latter cases, orbital velocities will have to increase while the ship is in the atmosphere in order to attain orbits above the atmosphere and even to reach escape velocity.

There's no law of physics that says you have to do much of the acceleration in the atmosphere. You can leave the atmosphere and then accelerate.

Simon Hibbs

 

[EDG]

If you're entering the atmosphere slowly (a controlled grav descent), won't that mean you're going to be more vulnerable to crosswinds and so on? And obviously it would take a lot longer to actually get from edge of atmosphere to surface too?

 

[rust]

If you're entering the atmosphere slowly (a controlled grav descent), won't that mean you're going to be more vulnerable to crosswinds and so on?

I would think so, here on earth jetstreams can reach speeds of more than
500 km/h.

This is why I decided that in my setting jetstreams are a navigational ha-
zard, which requires a skill roll when crossing a jetstream, and pilots on
planetary flights have orders to stay below the jetstream height.

This makes atmospheric flights somewhat slower, with speeds of rarely
more than 3,500 km/h, but also somewhat safer, I think.

 

[Ishmael]

There's no law of physics that says you have to do much of the acceleration in the atmosphere. You can leave the atmosphere and then accelerate.

Simon Hibbs

True...
You just need to accelerate a bit more than surface g's pull you back
I guess it all depends on what sort of thrust agencies you use in your game.
reactionless thrusters don't have to worry about reaction mass running out before orbit is achieved. grav tech just floats upward, I suppose.
I tend to avoid reactionless thrusters and grav tech as Traveller uses them.
I guess mtu looks more like 2300.

 

[captainjack23]

but, as the F4 and F104 proved, a brick will fly with a big enough engine*.

Or, as with the 104, if one straps a saddle onto a big honkin engine, and adds some missile pilons disguised as wings...

The F104 seems like a very meticulously designed "brick" to me...
http://en.wikipedia.org/wiki/F-104_Starfighter

That thing was designed for high speed, and built to accomodate that requirement (sure, it apparently wasn't all that safe with it, but I think calling it a "brick" is doing it somewhat of a disservice).

EDIT: And now I remember that this was my favourite Airfix model I had when I was a kid too .

Contrite and sincere apologies: An editing failure. I meant to contrast the F4 and the 104 - and neglected to edit it out of the first sentence.

The 104 is hardly a brick, and one of my faves too -despite it having complicated my fathers job immensely (EOD/crash recovery)

B-58 is the only US plane that rivals it for awsomecoolness*.... And if you think the 104 is loud.........

*Well, okay the SR 71. But the 58 is really the ultimate NASCAR/muscle car approach to air warfare....whereas the 71 is a dedicated race car.

 

[captainjack23]

Oh, and it's an SR-71...

Jeeeze, looks like that was one beer too many. Yes, of course.