T5 Converting G to Speed

[Reban]

T5 gives us VehicleMaker vehicles that can reach orbit and function in space that have a Speed rating, and ACS gives us ships and smallcraft that can function on worlds and have a G rating.

My question is; Is there a simple way of calculating the Speed rating for an ACS craft if I know its G rating for Ground/Personal Combat and vice versa for a Lift or Grav Flyer operating in Space Combat conditions?

I know p295 has the Vehicle Speeds chart with an entry for Lifters, G Drives and M Drives but that doesn't tell me what the maximum speed for a 9G Airframe Fighter should be when it enters the atmosphere.

Equally a GravFlyer may achieve orbit and engage in space combat with an orbiting ACS craft, but if it has Speed 5 or a cruising speed of 50kph, what is its acceleration?

Anybody given some thought to this?

 

[Reban]

There is a chart on p.364 called "MANEUVER SPEEDS NEAR WORLD SURFACES" but again its not related to the G rating of an ACS craft.

 

[tjoneslo]

Any craft capable of reaching orbit has a effective top speed over 40,000 kph.

Based upon other design systems the top speed of an airframe craft in an atmosphere is in the thousands of kph, and with a Gdrive, the pilot can effectively choose any speed from 0 to top speed.

Conversely, the acceleration of most ACS is less than 1G for purposes of space combat. Some of the really fast craft may have an acceleration of 1G, but that's a limit. For a quick rule of thumb, you could assume 0.5G.

- edit -
After looking through the rules I would give a top speed to a cluster, braced, planetoid, unstreamlined hulls a top speed of 100 kph (speed 6). This is the limitation of the hull, and causes damage to the hull or attachments for faster speeds. A streamlined hull is 500 kph (speed 8), with an airframe at speed 2000 (speed 11) and lifting body at 5000 (speed 13).

In reverse, building a grav craft with a speed of less than 8 should have a G rating of 0.5 G, Speed 8 or greater would be 1 G. If you want a small craft with a higher acceleration, build it using the starship rules.

 

[Murdoc]

The biggest limit of a Traveller spacecraft in an atmosphere will be on its hull configuration, not its engines. The current rules currently ignore this by suggesting that unstreamlined ships will encounter friction, but the fact is that they will break up due to air pressure long before they can go fast enough experience any friction. At the very least there is a little thing called the sound barrier. It is because of this that I have been making house rules to take care of this and other atmosphere related problems in T5. I'm actually about done with making all the rules actually, but I still need to test-balance some of the numbers, like how much damage a craft may receive due to air pressure and the like. Hopefully I'll be able to post it in the next few days or so, but I am working on other projects too. So to answer your question, yes, I've been putting a lot of thought into this (and research).

Unfortunately I have not given any thought into VehicleMaker and acceleration. I'm not that fond of those rules in the first place, but I assume that the abstracted combat rules make acceleration largely irrelevant. I'm not sure how you'd do vehicle to ship combat.

Conversely, the acceleration of most ACS is less than 1G for purposes of space combat. Some of the really fast craft may have an acceleration of 1G, but that's a limit. For a quick rule of thumb, you could assume 0.5G.

Where are you getting this from?

 

[HG_B]

A streamlined hull is 500 kph (speed 8), with an airframe at speed 2000 (speed 11) and lifting body at 5000 (speed 13).

In reality a lifting body would have MORE drag than a plain streamlined (missile/tube shaped) hull. Trav rules have made that mistake from its earliest days...

 

[Reban]

Any craft capable of reaching orbit has a effective top speed over 40,000 kph.

Based upon other design systems the top speed of an airframe craft in an atmosphere is in the thousands of kph, and with a Gdrive, the pilot can effectively choose any speed from 0 to top speed.

The general limit of the rules system is 5000 kph or trans-sonic.

Conversely, the acceleration of most ACS is less than 1G for purposes of space combat. Some of the really fast craft may have an acceleration of 1G, but that's a limit. For a quick rule of thumb, you could assume 0.5G.

Not according to the RAW. Band changes are based on Gs and T5 goes up to 9G

In reverse, building a grav craft with a speed of less than 8 should have a G rating of 0.5 G, Speed 8 or greater would be 1 G. If you want a small craft with a higher acceleration, build it using the starship rules.

This is a reasonable rule of thumb and should work for my purposes.

 

[Reban]

The biggest limit of a Traveller spacecraft in an atmosphere will be on its hull configuration, not its engines. The current rules currently ignore this by suggesting that unstreamlined ships will encounter friction, but the fact is that they will break up due to air pressure long before they can go fast enough experience any friction. At the very least there is a little thing called the sound barrier. It is because of this that I have been making house rules to take care of this and other atmosphere related problems in T5. I'm actually about done with making all the rules actually, but I still need to test-balance some of the numbers, like how much damage a craft may receive due to air pressure and the like. Hopefully I'll be able to post it in the next few days or so, but I am working on other projects too. So to answer your question, yes, I've been putting a lot of thought into this (and research).

I look forward to seeing what you've come up with

Unfortunately I have not given any thought into VehicleMaker and acceleration. I'm not that fond of those rules in the first place, but I assume that the abstracted combat rules make acceleration largely irrelevant. I'm not sure how you'd do vehicle to ship combat.

In Personal Combat yes acceleration is irrelevant but if I put a ground based Grav Fighter in orbit I'll want to use Space Combat, hence the reason I'm interested in this subject.

 

[Ishmael]

This is one way of doing this. But its a bit more involved than most people would care for....
http://forum.mongoosepublishing.com/viewtopic.php?f=89&t=56475&start=38

 

[HG_B]

This is one way of doing this. But its a bit more involved than most people would care for....
http://forum.mongoosepublishing.com/viewtopic.php?f=89&t=56475&start=38

This should help. Most trav ships designed to enter atmosphere (and skim at supersonic speed aka "streamlined") will probably have a Cd of ~ .29 - .25 Take the bullet shape and add a "boat tail" and you lower that quite a bit.

 

[tjoneslo]

Conversely, the acceleration of most ACS is less than 1G for purposes of space combat. Some of the really fast craft may have an acceleration of 1G, but that's a limit. For a quick rule of thumb, you could assume 0.5G.

Where are you getting this from?

Opps, "ACS" is the wrong term. For vehicles build with the VehicleBuilder rules that are space capable, assume an acceleration of 0.5G in space combat. This is, admittedly, simply made up. But it is based upon experience with real world vehicles and building or reviewing many designs of vehicles in all the Traveller design systems.

Vehicles designed for use on a world (I.e. Vehicle Builder rules) are designed with a world's gravity already present, which puts a much greater limit on how much acceleration you can put into a vehicle.

Space craft (using the space craft rules) are generally assumed to have the handy grav plates which provide the artificial gravity and compensate for the acceleration of the craft. Grav plates is something that the Vehicle Builder vehicles don't have, and are rarely provided for in any Traveller vehicle design rules.

Real world limits for sustained accelerations on normal people, those not trained for acceleration tolerance (e.g. astronauts and fighter pilots) is around 2G. I've seen a few sports cars capable of 1G acceleration, and a several modern day fighter jets can manage 1.2G or so. But those are specialized designs not operated by normal people.

Even with the military equipment the question becomes how often do you expect the craft to be operating outside it's expected environment. For example, driving a grav fighter into orbit. If I, as the designer, don't expect this, I won't include the grav plates, or special grav harness for the pilot, crew, and passengers. And limit the acceleration of the craft to something the crew can stand, about 0.5G to 1.5G. With the latter being the specialized fighter aircraft designs.

This is the one thing I dread about digging through the split vehicle/starship rules. I found the split between the two in the T20 rules to be uncrossable chasm.

 

[tjoneslo]

This is one way of doing this. But its a bit more involved than most people would care for....
http://forum.mongoosepublishing.com/viewtopic.php?f=89&t=56475&start=38

I think I'll stick to GURPS Vehicles...

 

[esampson]

. . .Any craft capable of reaching orbit has a effective top speed over 40,000 kph. . .

"Top speed" in space is something of a misnomer. The only real top speed in space is C although as you begin to approach C you can encounter other problems with things like the resistance being generated by the trace amounts of hydrogen that exist in what we call "vacuum".

I'm going to assume that by "top speed" what you mean is "top speed at 1 atmosphere" and if that's the case then you are suffering from a common fallacy. A craft with a top speed of 1kph can reach orbit as long as it is capable of maintaining that speed as it leaves the atmosphere (which is why blimps can't go into orbit). I'm guessing that your 40,000 kph is based on escape velocity which is the velocity that is needed for an object to leave Earth orbit from the surface of the Earth and without the input of more energy.

These two factors are majorly important. The further away you are from the center of mass the lower the gravity becomes so the lower escape velocity becomes. Ultimately if you can sustain the 1kph speed you'll reach a point where escape velocity is low enough that even at 1 kph you can go into orbit (however, see my comment to Reban).

Incidentally, it isn't possible to really go into orbit with any amount of velocity without the expenditure of more energy at some point later on. The reason for this is pretty simple. If your speed is too low your periapsis will be below the surface of the planet (meaning you crash into the surface). If your speed is too high you will simply escape orbit. If you are anywhere in between those numbers then you will have a theoretical orbit but when you've completed 1 orbit you end up back where you started with your initial vector. This means that the inverse vector can't be below the planet's surface (meaning you have to fire pretty much tangential to the surface) and even then when you come back around if you have any atmosphere at all the drag it induces will slow you down and cause you to crash into the surface.

. . .In Personal Combat yes acceleration is irrelevant but if I put a ground based Grav Fighter in orbit I'll want to use Space Combat, hence the reason I'm interested in this subject.

While a grav vehicle can reach orbit it is pretty limited in that capacity. Grav drives can only reach 1D away from the center of the planet (which translates to 1R away from the surface). On Earth that would mean they would have a maximum altitude of 6,300 km, which translates to Mid Earth orbit but they are only about 1/6 of the way to High Earth Orbit. The weapon ranges on such ships is also typically highly limited since they have to be smaller than a ton and don't have a ship's power supply to back them up. In most cases a true spaceship with a maneuver drive, power plant, and 1 ton+ weapon should be able to pretty safely stand off and blast an armed grav vehicle that is coming out to attack them.

A grav vehicle could probably extend its ceiling by orbiting at close to its maximum and then diving down to just 300-400km above the surface, giving it a highly elliptical orbit. This would be useful for reaching stations outside the grav vehicles normal reach but since the grav vehicle would be unable to make any maneuvering changes once its orbit carries it above the 1D limit it would not be an effective combat manuever (the 1% efficiency would probably be enough to allow for docking manuevers as long as the apoapsis of the orbit dropped the ship off pretty much right on the doorstep of its destination but for combat it would be a sitting duck).

 

[Ishmael]

This should help. Most trav ships designed to enter atmosphere (and skim at supersonic speed aka "streamlined") will probably have a Cd of ~ .29 - .25 Take the bullet shape and add a "boat tail" and you lower that quite a bit.

Not really, no. ... not for a more complete examination of the problem.
I'd say that ships capable of supersonic flight would have a Cd quite a bit lower than ~.29 - .25.
That's the approximate Cd for Totyota Corolla as opposed to ~.05 for a modern day supersonic jet.
That does not explain the relationship between Cd and a fuselage's fineness ( which is often a pure abstract along with x-sectional area in most design systems ).



THIS is a greater source of help for this sort of thing.

http://www.amazon.com/Theory-Flight-Dover-Aeronautical-Engineering/dp/0486605418

 

[HG_B]

I'd say that ships capable of supersonic flight would have a Cd quite a bit lower than ~.29 - .25.
That's the approximate Cd for Totyota Corolla as opposed to ~.05 for a modern day supersonic jet.

Naw, the AGM-45A Shrike has about a .40 and is supersonic.

 

[tjoneslo]

I'm going to assume that by "top speed" what you mean is "top speed at 1 atmosphere" and if that's the case then you are suffering from a common fallacy. A craft with a top speed of 1kph can reach orbit as long as it is capable of maintaining that speed as it leaves the atmosphere (which is why blimps can't go into orbit). I'm guessing that your 40,000 kph is based on escape velocity which is the velocity that is needed for an object to leave Earth orbit from the surface of the Earth and without the input of more energy.

You are confusing speed (or velocity) with acceleration here. A craft capable of a top speed of 1kph will never reach orbital height, orbital velocity, or escape the earth's gravity. To reach orbit (or anywhere else) the vehicle needs to have 1G + epsilon acceleration. If the craft has a physics violating cancel gravity machine (like the Traveller GDrive), it still needs an acceleration to achieve orbit or escape from the planet.

 

[HG_B]

If the craft has a physics violating cancel gravity machine (like the Traveller GDrive), it still needs an acceleration to achieve orbit or escape from the planet.

Yes, any accel beyond 1G (for Earth) would suffice.

 

[esampson]

You are confusing speed (or velocity) with acceleration here. A craft capable of a top speed of 1kph will never reach orbital height, orbital velocity, or escape the earth's gravity. To reach orbit (or anywhere else) the vehicle needs to have 1G + epsilon acceleration. If the craft has a physics violating cancel gravity machine (like the Traveller GDrive), it still needs an acceleration to achieve orbit or escape from the planet.

It will if it can sustain that top speed indefinitely (and vertically). Eventually you will reach a point where escape velocity becomes less than 1 kph.

If it helps think of it this way. The minimum distance between Earth and Mars is 54.6 million KM. If you timed it right then you could make that trip at 1 kph in about 6228 1/2 years. Unless something occurs which slows the vehicle down or it goes off course (both of which invalidate the premise) it will get to Mars. There's no way around it.

At some point along that trip you will have escaped Earth's gravity. You can prove this because if you turn off the engine the vehicle will fall towards Mars and not Earth.

How does such a magical device work? Irrelevant. The point was simply that there is no required velocity that must be reached in order to leave a gravity well (or to go into orbit).

 

[Murdoc]

I look forward to seeing what you've come up with

In Personal Combat yes acceleration is irrelevant but if I put a ground based Grav Fighter in orbit I'll want to use Space Combat, hence the reason I'm interested in this subject.

Ah yes, I see your point. Kind of the COACC thing. No idea how to do that.

Opps, "ACS" is the wrong term. For vehicles build with the VehicleBuilder rules that are space capable, assume an acceleration of 0.5G in space combat. This is, admittedly, simply made up. But it is based upon experience with real world vehicles and building or reviewing many designs of vehicles in all the Traveller design systems.

Ok, that makes more sense, thanks. And yes, I do seem to remember MT vehicles generally having fairly low Gs for the most part, unless they were designed specifically for orbital combat of course.

 

[Licheking]

Megatraveller did 1000kph Times the G-rating for in atmosphere flight.

1 G is 10m/s^2 which is 100m/second which means 360kph or am i missing something?

 

[esampson]

. . .1 G is 10m/s^2 which is 100m/second which means 360kph or am i missing something?

Kind of. First off, only the 's' is squared, not the entire equation, and even then that is a short hand from 'per second per second'. What that means is that if the acceleration of an object is 9.8 m per second per second then each second it accelerates by 9.8 meters per second. You have to have that second 'per second' (or first depending on how you are looking at it) in there because 9.8 meters is a distance, not a velocity.

You can't say that after 1 second the velocity is 9.8 meters because that makes no sense. 9.8 meters over what amount of time? A second? A minute? A week?

Likewise you are limited in your ability to talk about an acceleration of 9.8 meters per second. Does it take 1 second to reach 9.8 meters per second? 1 minute? 1 month? Without the second 'per second' it's kind of like saying that a certain car can go from 0 to 60. It's great that the car can reach 60 mph, but really doesn't tell you very much (there are some things where you actually can use simply the fact that its final velocity is 9.8 meters per second but a lot of the time you need to know how quickly it reaches that speed).

So usually the acceleration of an object is expressed in m/s² but that doesn't mean you are actually suppose to square the number of seconds any more than you are suppose to divide the meters by the number of seconds.