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Old 06-02-2020, 07:13 AM   #1
hal
 
Join Date: Aug 2004
Location: Buffalo, New York
Default Piloting rolls in atmosphere for GURPS TRAVELLER

Hello folks,
I was curious - how many GM's or players have bothered with the Max Air Speed rules from GURPS VEHICLES and GURPS TRAVELLER?

Without saying what got me interested in investigating max speed for starships, I was wondering what happens when the speed is HIGHER for a space craft entering an atmosphere of any kind (whether a normal world atmosphere or a Gas Giant for wilderness refueling) than what the craft can handle.

My answer was found on the sidebar of GURPS VEHICLES page 164.

The gist of the answer is that the orbital speed of a planet is the speed required to get into orbit from the planet (not necessarily escape velocity, just what is needed to get into orbit). For Earth, the example was 17,800 mph.

So, let's say you have a starship with Fair streamlining (the first level of streamlining above none). That gives you a max airspeed of 600 miles per hour. Such a spacecraft, entering into Earth's atmosphere and attempting to aerobrake - would require (17,800 mph - 600 mph)/1,500 minutes to execute. This works out to 11.47 minutes. The rules state the ship takes 4 points of damage per minute, for each of the following Areas:

Underside of its body,
Its wings
Any subassembly under its body

1/4 of that applied damage is also applied to all other locations.

The fun part is this: Roll against Piloting - if the roll fails, the damage done is equal to the amount the skill roll failed by x the damage. So a pilot who fails his skill roll by 3, will inflict three times as much damage to his starship than had he succeeded. A critical failure will inflict 20x as much damage as a successful roll.

See sidebar pg 164 of GURPS VEHICLES 2nd edition for the full particulars.

What I intend to do is treat various "hull configuruations" in Traveller as being varying types of streamlining. Configuration Four for instance (See Classic Traveller) - will be treated as "Fair Streamlining" when I design ships for my campaign.

Just thought I'd share this with people.
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Old 06-02-2020, 10:37 AM   #2
malloyd
 
Join Date: Jun 2006
Default Re: Piloting rolls in atmosphere for GURPS TRAVELLER

That doesn't look unreasonable, but will probably never matter. Almost nobody in the TU would even consider aerobraking - thrusters are so good that anything considered space-worthy should be able to kill its entire orbital velocity in less time than it would take to fall from a sustainable orbit to a part of the atmosphere thick enough to matter, even without turning on the contragravity.

Sure various official design sequences have included rules for more primitive drives, but realistically if there is enough contact for the PCs to get here, it's orders of magnitude cheaper to smuggle in an air-raft than to reach orbit by trying to build something less capable.
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Old 06-02-2020, 11:13 AM   #3
dcarson
 
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Default Re: Piloting rolls in atmosphere for GURPS TRAVELLER

Lets a really good PC pilot do a we lost all power from damage and will have to dead stick land scene.
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Old 06-02-2020, 02:21 PM   #4
malloyd
 
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Default Re: Piloting rolls in atmosphere for GURPS TRAVELLER

Quote:
Originally Posted by dcarson View Post
Lets a really good PC pilot do a we lost all power from damage and will have to dead stick land scene.
I suppose in a suitably cinematic game.

In a more realistic one, well, if you don't have any power, you can't do a deorbit burn to get into a trajectory into the atmosphere either. And most spacecraft would lack any sort of hypersonic aerodynamic surfaces or flight stability anyway. Why would you pay a lot extra and cut into useful volume, for a feature that would be useless in anything other than this incredibly unlikely circumstance?
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Old 06-02-2020, 08:47 PM   #5
hal
 
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Default Re: Piloting rolls in atmosphere for GURPS TRAVELLER

Quote:
Originally Posted by malloyd View Post
That doesn't look unreasonable, but will probably never matter. Almost nobody in the TU would even consider aerobraking - thrusters are so good that anything considered space-worthy should be able to kill its entire orbital velocity in less time than it would take to fall from a sustainable orbit to a part of the atmosphere thick enough to matter, even without turning on the contragravity.

Sure various official design sequences have included rules for more primitive drives, but realistically if there is enough contact for the PCs to get here, it's orders of magnitude cheaper to smuggle in an air-raft than to reach orbit by trying to build something less capable.
I kind of have to wonder at the Statement above...

In order to escape the world's gravity well, you have to reach escape velocity. This is regardless of whether you use reactionless drives or reaction engines. Even GURPS goes on to mention the following from sidebar on page 164 of GURPS VEHICLES 2nd edition:

"To put something into orbit, it is necessary to fly at orbital velocity. For planets, this is 17,800 mph x the square root of (M/R) where M is its mass in Earth masses, and R is its radius in Earth radii. Escape velocity is 1.414
times the orbital velocity.
If air speed is higher than orbital velocity, a vehicle can fly into orbit. If top speed is lower, it can still reach orbit if it has a sAccel rating and its space propulsion system can operate for the necessary time; the required time (in seconds) is [orbital speed (in mph) - top speed (in mph)]/[21.8 x sAccel (G)]."

Here is the problem however: You are already moving at excessively high speeds on your approach to the world in question. Using Earth as an example, its movement vector is 18.5 miles per second as it trundles happily through space. Just to catch up to it from any location in the solar system would require that you are at least going FASTER than Earth if you're catching up, or - if intercepting it from ahead of where it will be in its future, you have to at least position yourself where you will be met by the moving Earth. Problem is - if you go slower than 18.5 miles per second, the sun's own gravity will be pulling on you towards it.

In short? Regardless, the moment you're moving INTO the atmosphere at high velocity, you have to slow down until you are below Maximum air speed. Aerobreaking is the only formula i can find that utilizes the Max Speed. Since Areo-braking is a function of starting at high speeds and slowing down, one would think that regardless of the mechanism used to slow down, you're going to take damage.

Reversing the formula given above from the side bar on page 164 (ie, instead of reaching orbit, you want to descend from orbit) and using a 1 G drive to slow down the descent, the formula for a hull that can only accept 600 MPH as its top speed, would require (17800-600)/(21.8 * 1) or 789 seconds, or roughly 13 minutes.

To me? That's STILL at least 11 minutes of moving FASTER than 600 miles per hour when already moving at 17,800 miles per hour on your initial "dip" into the atmosphere.

The real problem is - if you are going faster than 600 miles per hour for that level of streamlinging - what happens?
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Old 06-02-2020, 09:45 PM   #6
Fred Brackin
 
Join Date: Aug 2007
Default Re: Piloting rolls in atmosphere for GURPS TRAVELLER

Quote:
Originally Posted by hal View Post

The real problem is - if you are going faster than 600 miles per hour for that level of streamlinging - what happens?
Your vehicle will be destroyed if any significant part of it sticks out through the hypersonic shockwave. So you don't do that.

With a typical Traveller M-drive there's no reason doing this would be at all difficult. One hour (3600 seconds) at 1 G (32 ft per second per second) gives you 21.8 miles per second of Delta-v per hour of accel/decel.

Shedding unwanted velocity is no harder than putting it on was when you have no fuel limitations. You can easily slow down to less than 600 mph before you enter the atmosphere and keep your velocity that low or lower as you drop.

You can reverse this too and climb straight up for an unlimited period of time at an airspeed of less than 600 mph until there is no significant atmosphere and the 600 mph barrier doesn't matter.

It's not just at making c fractional missiles tha reactionless drives are very potent. They make virtually anything problematic with reaction drives trivially easy.
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Old 06-04-2020, 11:00 AM   #7
sjard
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Default Re: Piloting rolls in atmosphere for GURPS TRAVELLER

Quote:
Originally Posted by hal View Post
I kind of have to wonder at the Statement above...

In order to escape the world's gravity well, you have to reach escape velocity. This is regardless of whether you use reactionless drives or reaction engines.
Traveller has contragravity; escape velocity could be as low as you want it to be as long as it's positive lift, if you don't mind taking days, weeks, or months to get to orbit. Assuming you're only going up at the rate of inches per hour etc.
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Old 06-04-2020, 03:45 PM   #8
Anaraxes
 
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Default Re: Piloting rolls in atmosphere for GURPS TRAVELLER

Doesn't even have to technically be contragravity. If you have a reactionless thruster that can produce more thrust than local gravity with (minimum) weeks of endurance, you can fly straight up through the atmosphere at any reasonable speed you like. You don't have to use your max acceleration if you're not in a hurry. If your hull generates any aerodynamic lift, then you don't even need full local gravity, yet still don't have to fly through the atmo any faster than normal aircraft do to generate that lift.

You have to reach orbital velocity to stay in an unpowered orbit. But the >1G ship can hover, whether that's 1m off the ground or 100,000.
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