Starships in Atmosphere

[apoc527]

Sorry to repeat this question, but I definitely need the answer.

How does one compute the performance of starships (and small craft) in atmosphere? I need to know acceleration (though less important) and max speed (quite important). The book addresses atmospheric flight only briefly, and does not provide max speeds. Thanks!

 

[Ishmael]

the way I use is pretty complex and I wrote my own design rules in part to support it.

base drag coeff divided by sqrt( hull's fineness ratio ) is a ballpark drag coeff.
streamlined base coeff is .3
part streamline ( blunt aft ) is .6
part streamline ( blunt forward ) is .9
non-streamlined is 1.2

with that and the cross sectional area of the hull and the atm density, just set the drag equal to thrust

thrust=(dens/2)*xarea*coeff*V^2
solve for V

I'll go back and take a look at my notes, but the 2-3% of the total energy from the drag is absorbed as heat by the hull...so using black body equation, you should be able to figure the re-entry maximum assuming the hull can withstand 2500C or so. ( I'm pretty sure thats how much the shuttle takes...I ignore hot-spots )

xarea=width*height*.7854
total volume = width*height*length*Q
total surface area = 2*((w*h)+(w*l)+(h*l))*Q
Q= .5236 for sphereoid up to 1.0 for a cube

while Q will affect the base coeff, I choose to ignore that for now and simply mumble stuff about suction slots, pressure slats, dams, base-bleeds and winglets and spoilers and other aero helper stuff to make the craft streamlined or not.

this is basing volume and area on an approximate sphereoid of equal dimensions. hey..its about the same idea as how MT guesses at Area and volume ( compare to a sphere )..I just make the designer state the length/width/height of the craft to figure the volume instead of the other way around as MT does.

see?..sometimes obsessive detail can be helpful

 

[-Daniel-]

Ishmael, you make my head hurt. :wink:

I thought I was back in school for a moment reading the set up for a major word problem. :lol:

Daniel

 

[crazy_cat]

I don't think you can generally go too badly wrong, and will in the main find things usually work out best, if to paraphrase JMS (of B5 fame) - things move at the speed of plot.

 

[Rikki Tikki Traveller]

All of the spaceships are going to be able to fly supersonic (they have to for reentry and launch).

BUT, to really be MANEUVERABLE, I would say use the speed of sound as a good guide. 99% of current airplanes are subsonic. Most commercial jets fly at about 600 mph (ish). That is a good number for a streamlined ship.

Partial streamlined ships are probably limited to about 200 mph. Above those speeds, it is pretty much straight line flight.

Rough, but it will work as JMS said "At the speed of Plot".

 

[Ishmael]

well..the x-43 hit mach 7, iirc
fastest powered flight ever
but it was a relatively small model and thus had far less drag than a larger plane

x-15 hit mach 6 in sustained flight...shock waves sliced off a titanimu pylon though

for manuvering...I think the limit will be the ship's structure.
if a ship is stressed to handle 3 G's...then a Tomcat pulling 9 G's will out turn it.
But thats if it has an "airframe" to allow aero manuvering....I consider that different from streamlining...an frisbee is not an airframe, yet it is streamlined....a WW1 Voison bomber is not really streamlined, yet it is an airframe....

200mph is low....a Japanese Zero was limited to that speed for manuvering, not because it was ( in game terms ) partially streamlined, but because without power assist controls, the ailerons locked up due to aero forces.

one other thing to consider is the sonic boom...a starship is pretty big and moving at mach 4-6 will have a sonic boom that could flatten a building....the shock wave would be pretty powerful...have to fly fast up high.

( I remember as a kid in Albequerque the fighter planes ( tiny compared to a ship ) have booms that rattled the glasses in the cupboard and window-panes....and they were flying much slower than a ship could.

 

[simonh]

If you look at the stats for modern supersonic jet fighters, you'll see that their thrust to weight ratios result in about a 1G acceleration rate. This is why they generaly need to use afterburners to do a straight vertical climb from a standing start.

They achieve their high speed through having extremely low drag coefficients. Most starships will not be able to achieve the same drag coefficients so a streamlined ship probably can't go supersonic if it has a thrust of 1G. As a rule of thumb I'd suggest that the maximum MACH rating for a streamlined ship should be one less than it's thrust rating. Thus 1G ships are subsonic, 2G ships can reach MACH 1, 3G ships MACH 2, etc.

Standard hull ships should definitely be subsonic, but the main differentiation at 1G thrust between them and streamlined ships will be atmospheric manoeuvrability.

All very rule of thumb but I'd think it will do.

Simon Hibbs

 

[FallingPhoenix]

All of the spaceships are going to be able to fly supersonic (they have to for reentry and launch).

Actually, if they've got some kind of contra-gravity, can't they go pretty much as slow as they want for launch and re-entry?

 

[apoc527]

The design rules in T20 have some pretty good rules of thumb. A ship has to have an "airframe" hull (probably similar to streamlined hulls in MGT) to go faster than 1G speeds in Atmosphere. If you have the book, the table on pg 261 is interesting. However, 1G accel drives gets you 3500kph max speed, which is supersonic.

 

[simonh]

Actually, if they've got some kind of contra-gravity, can't they go pretty much as slow as they want for launch and re-entry?

It doesn't have to be contra-gravity. The only reason rockets accelerate to such high speeds at takeoff is for fuel efficiency purposes, otherwise they'd run out of fuel before reaching orbit. If that's not a problem, then you can go as slow as you like. The same applies for re-entry. We currently use atmospheric braking because it doesn't require expenditure of fuel, but if you've got power to burn then a hot re-entry isn't necessary.

I have generally assumed that all streamlined vessels have gravitic systems, as part of their 10% hull space used for streamlining, that ease landing and takeoff.

Interestingly in classic Traveller 'basic' hulls could land and takeoff routinely but in the Mongoose edition they require elaborate facilities to take off again. It's a reasonable change. Book2 and High Guard didn't really pay enough attention to this area.

Simon Hibbs

 

[Infojunky]

Interestingly in classic Traveller 'basic' hulls could land and takeoff routinely but in the Mongoose edition they require elaborate facilities to take off again. It's a reasonable change. Book2 and High Guard didn't really pay enough attention to this area.

Er? No, in CT standard hulls are unstreamlined, though there is no tonnage fraction just an extra cost to streamline a standard hull. That is 1Mcr per 100 tons. It is about step 10 in the design sequence.

 

[rje]

Interestingly in classic Traveller 'basic' hulls could land and takeoff routinely but in the Mongoose edition they require elaborate facilities to take off again.

Granted, hulls always needed streamlining to land and take off in atmo, but that aside, my players' favorite habit was to use their starship as the ultimate Recreational Vehicle: why park it at the starport when you could just land it near your destination?

That worked fine in frontier situations, well fine for them, but annoying for the referee. Nowadays I tend to make starship hulls somewhat more vulnerable than before. Falling boulders can damage your ship... so can artillery fire. Or PGMPs.

 

[Xoph]

I suppose RVing would be fine on many unoccupied planets, but most occupied ones would restrict landing the downport. Besides you can't just land anywhere, you need a flat piece of land. Ships are tough but they still shouldn't be rolled down a mountain into a lake... Especially a burning acid lake.

 

[rje]

I suppose RVing would be fine on many unoccupied planets, but most occupied ones would restrict landing the downport. Besides you can't just land anywhere, you need a flat piece of land. Ships are tough but they still shouldn't be rolled down a mountain into a lake... Especially a burning acid lake.

Quite. And then we get to situations like:

A: "Okay, we fly to the village and land nearby."

R: "You can't."

A: "Why not?"

R: "The village is surrounded by burning acid lakes."

A: "That's stupid!!"

 

[Xoph]

Seriously... In that case you say sure. Unless it was a populous planet, then planetary air control might have an issue with ships flying all over the place. Or for some reason the village is in a valley.
Besides I don’t think it is a problem for the players to fly where they want normally. Although after the first armed boarding party takes over their ship after a deal goes bad the players them selves might want to keep their ship a bit out of site, unless it is acting as air support.

 

[Ishmael]

RV starships?..bah

Think how much they mass
Think how much contact area the gear will have ( if picture like big airplane landing gears )
Figure ground pressure...and tell players they sank into the dirt and bits and pieces on the bottom got smushed and broken.

Starships land at starports because of runways/pads that are stressed to handle the weight

now maybe if the crew can find a patch of bare bedrock...thats flat

mtu != ytu

 

[Xoph]

I wouldn't go that far.
Lots of heavy buildings were built before foundations and they don't sink. Ships don't use spikes as landing gear, Im sure they use some sort of struts that spread the mass over a large area so it won't sink.
Besides how would ISS survey a planet if they can't land. not everything is done by shuttle.

 

[Infojunky]

Here is a trick I have used;

Take your players out and have them help you measure out the ground foot print of their ship.

Even a scout boat is much larger than most people think. Finding a place to safely set down is not as easy as they think.

The other part is tell them to find a "safe" place to look using Google Earth.

 

[zozotroll]

Currently, large aircraft land at expensive airports because they have to. And those airports are so expensive in part because of the very think runways they need to have.

I am not a pilot, but I have spent most of my adult life moveing armored vehicles around various sorts of terain, and grounsd preasure is a major issue. Unless you have some way of telling what is under what you see on the surface, sink to the belly is a very real likelyhood.

It may act like an RV, but the mass difference is pretty huge. I bet scouts check for a good week, and do a ground recon with the air/raft before setting down. Lord knows I have walked many a mile leadiung my track to insure it didnt get stuck, and even that did not alway work.

And perhaps scout ships where designed by actual scouts, rather than designers that never actualy did any field work, and the full belly is solid hull armor with nothing breakable sticking out. Merchants are not likely to be so constructed, unless you order one special, for a premium price of course.

 

[PFVA63]

Hi,

Although Mongoose Traveller doesn't really appear to address space ship weight, other versions of Traveller have, and that info could be of use here.

In one of these other versions of Traveller they provide designs with weight data for a 100dt scout type ship and a 200dt free trader type ship. The Scout has a listed weight of 680 tons empty or 740 tons full load. The Free Trader has an empty weight of 240 tons empty or 720 tons full load.

From Wikipedia it suggests that the ground pressure under a normal standing human male might be on the order of 8 psi (55kPa), under an M1 tank it would be about 15 psi (103kPa), under a passenger car about 30 psi (207kPa), and under a wheeled ATV about 35 psi (241kPa).

As such, for a scout or free trader type ship, if they were to have 3 landing pads with a total area of about 330 square feet (30.6 square meters), or 110 square feet per pad (10.2 square meters), fully loaded it would have about the same ground pressure as a wheeled ATV. This would equate to a pad about 10ft x 11 ft (or 3m x 3.35m). If you had four landing pads, or operated the ship at less than full load, you could attain a 35 psi load with smaller landing pads.

Anyway, as such, although you probably wouldn't be able to land the ship everywhere, you could probably at least operate it anywhere that you could operate a wheeled ATV.

Regards

PF