Starship Speed In Atmosphere

[rust]

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

According to "canon" even an air/raft, a vehicle much slower than
4,200 kph, can reach orbit, thanks to its contragrav drive which
counters the planet's surface gravity - so it should be no problem
at all for a starship. And once the starship has reached the orbit,
there ts no more atmospheric drag, and its maneuver drive enab-
les it to accelerate as long as it takes to reach the desired velo-
city. Traveller magic makes it possible ... :wink:

 

[spidersrepublic]

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.

Some really great ideas all, thanks for the interesting discussions. I'm a total n00b for not remembering that Mach changes depending on medium density/pressure. It seems silly to use it now, but the idea of -1 mach for dense Atmo and +1 mach for thin Atmo is still pretty cool and simple...

We do need a chart...

Also... does this contragrav system run on handwavium :wink: ?

 

[Ishmael]

top speed will end up being proportional to the square root of thrust and inversely proportional to atmo-density, a number associated with the hull form ( profile drag ) and the 2/3 root of volume.

MT's top vaccuum drag was siimply a step in determining top atmo speed, not an actual speed limit.

 

[F33D]

MT's top vaccuum drag was siimply a step in determining top atmo speed, not an actual speed limit.

Not according to the rules.

 

[F33D]

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.

Exactly. I found a really good 3D of a Free Trader https://7chan.org/tg/src/134953594119.jpg

With 1G in a Standard Atmos. I don't see this easily going super sonic. What do you think?

 

[rust]

Also... does this contragrav system run on handwavium :wink: ?

It depends on the Traveller version, some use handwavium
while others use unobtainium or implausibilium ... 8)

 

[rust]

I don't see this easily going super sonic. What do you think?

I agree, although this is actually a Far Trader, not a Free Trader. :wink:

 

[F33D]

I don't see this easily going super sonic. What do you think?

I agree, although this is actually a Far Trader, not a Free Trader. :wink:

Oops! Yeah, a Free Trader is a little more streamlined but also has those huge scoop vents that would act as speed brakes. http://ecosenelvacio.files.wordpress.com/2010/03/beo-damage4.jpg

 

[spirochete]

top speed will end up being proportional to the square root of thrust and inversely proportional to atmo-density, a number associated with the hull form ( profile drag ) and the 2/3 root of volume.

MT's top vaccuum drag was siimply a step in determining top atmo speed, not an actual speed limit.

Top speed is reached when the aerodynamic drag force equals thrust.

Fd = pv²CdA/2

Where
- Frontal area (cross-section) A
- Drag coefficient Cd
- Velocity v
- Atmospheric mass density p

See http://en.wikipedia.org/wiki/Drag_equation

 

[Ishmael]

top speed will end up being proportional to the square root of thrust and inversely proportional to atmo-density, a number associated with the hull form ( profile drag ) and the 2/3 root of volume.

MT's top vaccuum drag was siimply a step in determining top atmo speed, not an actual speed limit.

Top speed is reached when the aerodynamic drag force equals thrust.

Fd = pv²CdA/2

Where
- Frontal area (cross-section) A
- Drag coefficient Cd
- Velocity v
- Atmospheric mass density p

See http://en.wikipedia.org/wiki/Drag_equation

what I said...
rearrange that equation, and.....

v = sqrt ( (2*Fd) / (p*Cd*A) )

Fd = Thrust such that they balance each other out, however F=ma and I don't believe the rules cover mass for ships/vehicles
G's of acceleration alone isn't enough to determine Newtons of force.

A = frontal x-sectional area, but the rules don't cover this at all, but it can be shown to be proportional to the 2/3 root of the volume.

Cd = profile drag coefficient. We won't bother with induced drag, wave drag, or parasitic drag. These can change depending on Reynolds numbers of the flow.
Cars typically go from ~.25 and up, planes range from .02 and up

p is density and should probably be in a table for differing UWP surface pressures; scale heights and lapse rates for different altitudes. A spreadsheet is better.

You might also use Prandtl–Glauert transforms as correction factors for compressible flows as mach 1 is approached and surpassed. The cool vapor cones around planes breaking the sound barrier are Prandtl–Glauert Singularities

This all assumes some form of grav /thruster tech, otherwise you would have to factor in aerodynamic lift and thus, induced drag ( proportional to Cl^2/(pi*AR) ( for elliptical lift distribution )

( yes, I like airplanes )

 

[spirochete]

A = frontal x-sectional area, but the rules don't cover this at all, but it can be shown to be proportional to the 2/3 root of the volume.

It's just the raw cross-section; it should be easy to approximate by placing a frontal silhouette over a deckplan grid and counting the squares. The streamlining and skin friction are incorporated into the drag coefficient.

 

[locarno24]

According to "canon" even an air/raft, a vehicle much slower than 4,200 kph, can reach orbit, thanks to its contragrav drive which counters the planet's surface gravity - so it should be no problem at all for a starship. And once the starship has reached the orbit, there ts no more atmospheric drag, and its maneuver drive enables it to accelerate as long as it takes to reach the desired velocity. Traveller magic makes it possible ...

Exactly. Even a personal grav belt can theoretically make orbit. "Escape Velocity" is only really a concept if you're being fired out of a cannon, less so if you have an engine capable of sustained (and certainly indefinitely sustained) thrust. If you can keep flying upwards at an inch an hour, eventually you must reach high orbit.

With 1G in a Standard Atmos. I don't see this easily going super sonic. What do you think?

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.

With 1G in a Standard Atmos. I don't see this easily going super sonic. What do you think?

Obviously, without knowing the mass, we don't know how big the 1G is as a force in N and hence can't match it proportionately to the drag in N. But yeah.... that thing could be worse but you're going to have the mother of all shock waves at the throats of the fuel scoops (I assume there's a matching one on the other side?)

Of course, you aren't dependent on aerofoils for control, and in theory a brick can go supersonic with enough force behind it (or technically ahead of it*, I guess).

A big part of it is if you believe a 'partially streamlined' ship actually does a shuttle-style re-entry as seems to be implied by the rules (in which case it must be both survivable and vaguely controllable at supersonic speed), or just parks above the downport and lowers itself downwards into the gravity well at a relatively low speed.



* Now that's going to cause some wierd aerodynamics, thinking about it... A grav drive generates a gravity well ahead of the ship that it can 'fall into', right? Wouldn't matter too much in water, but what happens when you put a localised 4-6 G gravity pocket flying ahead of you through a compressible medium?

 

[Ishmael]

It's just the raw cross-section; it should be easy to approximate by placing a frontal silhouette over a deckplan grid and counting the squares. The streamlining and skin friction are incorporated into the drag coefficient.

yes, you could find frontal area that way, but that requires actually drawing a front silhouette for every ship you design and it'd be an approximation either way.
When I get home from work, I'll post the way I do it and how I get an approximate Cd as well.

And skin friction is not factored into the Cd for form drag. It is separate.
Profile drag is the sum of form drag and skin friction.

I may have confused the issue by calling form drag, 'profile drag'.
That was how the term was used in a couple of aerodynamic texts I have and has been synonymous in the past. Sorry if that caused some confusion.

 

[F33D]

yes, you could find frontal area that way, but that requires actually drawing a front silhouette for every ship you design and it'd be an approximation either way.
When I get home from work, I'll post the way I do it and how I get an approximate Cd as well.

Thanks I'd be interested in the Cd of any common trav ship. Given the shape of most of the smaller common ships, the Reynolds number would be very high so I'll take that as a given unless I'm using one of those torpedo shaped small craft.

 

[spidersrepublic]

so we need a formula involving:

Drag (could we use a ships dtons or a factor of it?)
Thrust (in Gs)
Speed (in kph)

Streamlined ships would have a static drag modifier. (say -30% drag?)
Atmosphere Type would modify the Drag factor too.
Altitude would also modify Drag. (and depend on a planets size...)
Nap of the Earth flying would have a modifier too... perhaps a maximum speed unless piloted by a tech level 15 AI

Would a Distributed ship also modify the drag factor? or just make it harder to fly?

+10 Traveller points to whoever maths this out first and comes up with an equation.

 

[F33D]

+10 Traveller points to whoever maths this out first and comes up with an equation.

Too many variables to have a formula that would be workable.

 

[Sigtrygg]

ok, bowing to real world physics, the factor for the acceleration rating will be based on the sqrt of said rating in m/s2.

10m/s2 - 3.1, this becomes our benchmark
20m/s2 - 4.5, 1.5 times faster than 1g
30m/s2 - 5.5, 1.8 times faster than 1g
40m/s2 - 6.3, 2.0 times faster than 1g
50m/s2 - 7.1, 2.3 times faster than 1g
60m/s2 - 7.8, 2.5 times faster than 1g

So now we have to think about the basic maximum speed based on hull configuration, multiply that by the above factor, then multiply by a factor based on atmospheric density. As altitude increases density will drop and maximum speed with increase.

 

[rust]

+10 Traveller points to whoever maths this out first and comes up with an equation.

mph = square root [(thrust / drag*) x 15 million]

*Drag = Total surface area / 5

Credit and criticism to GURPS Traveller, please.

 

[Ishmael]

What I do is beyond what most people do and much is not really based on Mongoose, or any other version. Its a bit heavier on math than many would want; I hate tables and charts. Besides, I kind of like to 'roll my own'.

This will take a couple of posts, I'm afraid.
First is density
Next is hull dimensions
Last is Cd

First off, The density of the atmosphere has to be determined. Using the ideal gas law, the density is easy to find for a wide variety of worlds. This is also useful for world-building wonks.

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
I won't bother working out here how pressure, and density vary with altitufe. Just put in numbers appropriate for different atm levels ( dense, std, thin, etc. ) and it should be good enough.

T is Temperature in Kelvin. Again, Mongoose doesn't produce this, but there are a number of methods from world-building that can generate this.

For P and T from world generation, I use my own rules here;
https://sites.google.com/site/moukotiger/files
"UWP_article.txt" near the bottom *

My UWP stuff here works for primarily habitable worlds and assumes O2-N2 atmosphere composition and water hyrographics.

example; dens = ( 29 * 101,325 ) / (8.3144 * 288 * 1000 ) = 1.227 Kg/m^3
the "* 1000" in the denominator is to convert the molar mass from g/mol to Kg/mol; its the same as dividing the molar mass by 1000.
A you can see, it is good approximation for Earth when using Earth's P and T and the molar mass of the atmosphere.

Another useful thing from this equation is ( density * velocity * scoop inlet area ) gives the mass flow rate ( Kg/s ) during scoop-refueling in a Gas Giant.
-------------------------------------------
* I didn't feel it nice to try to make people download a file without see where its coming from first

 

[Ishmael]

Next is hull dimensions...

The hull is based on a sphere inside a bounding box with the acceptance that it is an approximation. I won't even worry about configurations directly as much of that sort of thing can be looked at sideways with my use of hull proportions.
Given a hull displacement and the hull's proportions, the rest can be figured relatively easily. The hull proportions are ratios of length to width to height, such as l:w:h. For example, a 100 dton hull would be 1400 m^3 and let's use fineness ratios of l:w:h of 5 : 3 : 1 .
I divide the actual volume by .5 to be an approximation of the size of the hull's bounding box*, and then divide that volume by the different ratios to get the size of the cubes that make up such a bounding box. Taking the cube root of that number gets the length of the cubes that compose the bounding box.

( 1400 /.5 ) / ( 5 * 3 * 1 ) = 186.887 m^3
186.887 ^ ( 1/3 ) = 5.715 m

If I multiply the result by the length, width and height proportions, I can have an approximation of the overall dimensions for the hull.

length = 5 * 5.715 = ~28.5 m
width = 3 * 5.715 = ~ 17.15 m
height = 1 * 5.715 = ~ 5.715 m

A wedge like the typical 'S' class scout is a worst case example because a wedge, takes up a much smaller volume inside its bounding box than a stretched out sphere takes. I can live with it as it would be an extreme case that is not overly common compared to most of the other ships/vehicles that can be made, and I would sacrifice that small bit of 'detail' in favor of ease in use. As the dimensions are approximations, I can fudge when drawing deckplans. The important thing is that performance stats will work out assuming I use the same procedure for all ships and vehicles to maintain an acceptable amount of consistency within my universe.

Having an idea of basic dimensions will be used to limit spinal mounts as hull length limits spinal mount tunnel length, and to limit agility through the longest dimension affecting the hull's moment of inertia.

the hull's surface area is proportional to the surface area of the hull's bounding box. As it is based on a sphere/ellipsoid within a bounding box, stretched as the box is stretched, That proportion is .5, the same as the area of a sphere to the area of that sphere's bounding box *.

(( 28.5 * 17.15 ) + ( 28.5 * 5.517 )+( 17.15 * 5.517 ) * 2) * .5 = ~750 m^2

frontal x-section can be approximated by width * height * .7854, for use in estimating atmospheric speeds
lifting area can be approximated by length * width * .7854, for use in estimating aero performance.
Surface area can also be used as a measure of space for hardpoints, radiators, antennae, etc. on the hull although I'd think each such thing would add to drag unless retractable into the hull itself ( takes up volume ).


Last is Drag coefficient
----------------------
* the proper number should be .5236 to be more accurate, but let's keep the numbers simple...its all a rough approximation anyways.