Reaching orbit with Air-raft

[MrBackman]

Originally Posted by malloyd

Actually, you can't tell that, because for the larger structures, like say our supercluster, you have nothing fixed to measure against. Everything we see could be gravitationally accelerating at a billion gravities in some direction and we couldn't tell, and in any case have to account for the as yet not well defined repulsive effect at very large scales, which may or may not be be altered by contragravity..

Eh no, gravity is caused by masses attracting each other and no cosmological theories posit such insane masses as to generate the huge accelerations you pluck from thin air (huge solid sheets of black holes or what?). The 'as yet not well defined repulsive effect' is just cosmologists attempts to save their theories from the WMAP data. I refuse to take the current state of cosmology into account when rationalising my Traveller handwavium.

For the Sol system the effects we can account for are fairly low (the centripetal force of the rotation of the Earth is about 0.03 m/s^2, for the orbit about the sun 6x10^-3 m/s^2, for the galaxy about 2x10^-10 m/s^2; and 3x10^-5 m/s^2 for the moon) but of course that doesn't have to be the case in other star system - many worlds in Traveller have shorter days or are a lot closer to their primaries. And isn't really negligible here on Earth either, leave your contragravity on an hour and 0.03 m/s^2 has given you a 250 mile per hour velocity in some direction you didn't intend to go. This isn't terrible for a vehicle, though you have to have an engine you can run constantly and which can thrust in all directions, on a continuously changing basis to compensate for it, but it isn't negligible. And it does pretty well simplify the reach orbit with the air-raft problem. Turn off the compensating thruster and in a couple hundred hours you'll have reached a point your inherent velocity puts you in some sort of orbit, then you turn off the contragravity. Admittedly it'll probably be a pretty elliptical orbit.

Well thanks for doing the calculations but there are some overlooked points here: A balloon is subject to the exact same centripetal forces but we do not see them zoom off at 250 mp/h after an hour in the air, right? Air drag against such a minute force would act exactly the same on our Floater and a balloon. Or, let us say that our air-raft is using a reactionless engine (that mr Lloyd propsed would give much less scientific problems) that produces a thrust that completely opposes the pull of gravity, very much like a hovering helicopter, balloon or zeppelin, wouldn't they experience exactly the same cetripetal force. What is it with my floaters that all of a sudden turn these minute forces into something to bother with?

Reaching orbit from centripetal forces alone won't work either. The air-raft would have the same rotational speed as it had on the ground so it would be in orbit in the same sense as you and I are in orbit ie not at all. The air-raft would simply fall back onto the planet when turning off the Floater. Circular orbit means the kinetic energy is equal to the potential energy and this tell us that the higher up you are the MORE velocity is needed to stay in orbit.

If my take on Floaters are bothersome (negating gravity completely), just posit them as gravthrusters that can only thrust along the gravity gradient vector. That way they'd behave like my Floaters yet be immune to the insane million gravity homogenous fields posited by mr Lloyd. They would still behave the same visavi centripetal forces of course. So, at TL 10 we can make gravthrusters that thrust along the gravity gradient only, at TL 11 we can have gravthrusters that can thrust in any direction as long as there is a gravity gradient and at TL 13 we get the reactionless thrusters that can thrust without relation to gravity (the relativistic rock generators, if you will).

The lesson here: A drive that magically nullifies gravity in all directions through electric power breaks no more laws of physics than a drive that magically creates thrust from electricity alone; conservation of momentum and conservation of energy are broken in both cases. Breaking the two most fundamental laws of nature means that we know for certain there will be problems as there always is when one adds this amount of handwavium but I think I have shown that we are equally frakked with the gravthrusters. We also learn that the small forces values of centripetal acceleration are present on all bodies; ballooons, helicopters, floaters and gravvehicles alike and as we don't bother with them for the real owrld versions we don't have to bother with the fictious ones.

[malloyd]

Originally Posted by MrBackman

Well thanks for doing the calculations but there are some overlooked points here: A balloon is subject to the exact same centripetal forces but we do not see them zoom off at 250 mp/h after an hour in the air, right?

Yes. So? So is a rock sitting on the ground. But both of those are also subject to gravity. Balloons don't float because they are not affected by gravity, but because air is heavier. The force of gravity on anything is greater than the centripetal force of the Earth's rotation by exactly the same ratio - both are linear in the mass of the object. Contragravity changes that. In effect it's equivalent to lowering the mass of the Earth while keeping everything else constant. And if you do lower the mass of the planet, then yes, eventually stuff is thrown off centrifugally. Small enough objects spinning fast enough *do* rip themselves apart, though it's not a familiar sight, since engineers try to keep that from happening.

Reaching orbit from centripetal forces alone won't work either. The air-raft would have the same rotational speed as it had on the ground so it would be in orbit in the same sense as you and I are in orbit ie not at all. The air-raft would simply fall back onto the planet when turning off the Floater.

Actually, if you jump you are in an orbit, it happens to be a really eccentric elliptical one that intersects the surface of the Earth. If you turn off gravity then the rotational velocity of all the objects on the Earth throws them off tangentially. I suppose arguably this is not an orbit either, it's the maximally dengenerate case of a hyperbola with the center of the Earth at a focus, and never loops back on itself, but these kinds of things are often called "grazing orbits", and if you turn the gravity back on before the object gets too far out, you'll pull it into an elliptical path. And yes, it will be very elliptical. If the planet had a radius of zero, then you'd be right, you would fall straight back after launching this way.

If you lower the gravity of the Earth to that 0.03 m/s^2 (or equivalently had a "partial contragravity" device that reduced the effect of gravity by 99.7% so that weight was reduced to that) then objects at the Equator *would* be in a circular orbit - try it youself, figure the orbital velocity around something with the necessary mass to reduce gravitational acceleration to that and compare it to the speed of the rotation of the Earth. This pretty well ought to prove to you that you can reach an orbit with a contragravity device capable of 100% negation, if you pulsed it on and off on say a 99.7/0.3 millisecond duty cycle, it turns into approximately a 99.7% weight reduction and you are in a circular "orbit" right away.

Circular orbit means the kinetic energy is equal to the potential energy and this tell us that the higher up you are the MORE velocity is needed to stay in orbit.

No. Close, but it's potential energy relative to infinity, not the surface of the Earth. That is, it's zero at infinite distance, so the kinetic energy (and hence velocity) you need to stay in orbit infinitely far away is likewise zero, and the potential energy gets *more negative* as you drop toward the Earth, and your orbit speeds up. Another way of thinking about it is the circular orbit velocity is always square root (1/2) times the escape velocity (the one you need to reach infinity, and hence compensate for the potential energy you lost falling toward the planet from there). Yes, it's a weird convention from the standpoint of the way planetbound people think of potential energy.

The lesson here: A drive that magically nullifies gravity in all directions through electric power breaks no more laws of physics than a drive that magically creates thrust from electricity alone; conservation of momentum and conservation of energy are broken in both cases

Sure. But the practical consequences, in terms of how easy it is to understand the results, and how easy it is to set up an abusive exploit that will allow you to derive vast energies from nothing, are quite different.

[MrBackman]

Originally Posted by malloyd

<snip>Actually, if you jump you are in an orbit<snip>

No, not at all. Being in 'orbit' is NOT the same as being in 'freefall', being in orbit is having such a velocity vector that you will be able to do at least one complete ORBIT around the object you are orbiting. Hitting the retros on the shuttle so its path will take it into the atmosphere is called 'de-orbit' for a reason.

No. Close, but it's potential energy relative to infinity, not the surface of the Earth. That is, it's zero at infinite distance, so the kinetic energy (and hence velocity) you need to stay in orbit infinitely far away is likewise zero, and the potential energy gets *more negative* as you drop toward the Earth, and your orbit speeds up. Another way of thinking about it is the circular orbit velocity is always square root (1/2) times the escape velocity (the one you need to reach infinity, and hence compensate for the potential energy you lost falling toward the planet from there). Yes, it's a weird convention from the standpoint of the way planetbound people think of potential energy.

Correct, my bad.

This pretty well ought to prove to you that you can reach an orbit with a contragravity device capable of 100% negation, if you pulsed it on and off on say a 99.7/0.3 millisecond duty cycle, it turns into approximately a 99.7% weight reduction and you are in a circular "orbit" right away.

I hope you see for yourselves that you are a bit ridiculous when you have to write "orbit" with quotes as you very well know that this isn't really orbit in the same sense everybody else uses it. Stutter-orbit maybe :)
Also note that the gravthrust vehicle would be able to do EACTLY the same feat.

My problem basically was that using an air-raft to dock with an orbiting ship would be very hard to do in the reality of Traveller, something the short, canon, paragraph about air-rafts had led me and probably other referees to believe. I'm sure most other agree as the air-raft design for GURPS Traveller lack the neccessary instrumentation, I don't even thiink they have paid for vaccuum proofing the vehicle. I stand by my statement that Floaters and Gravthrusters have the same problems with reality.

When we disagree so much on even fundamental assumptions such as what 'orbit' really means, whether or not there are huge gravitic fields from cosmology pulling on us etc that there is very little knowledge we can gain from each other in this discussion - I think we can bury it by that. We are like fantasy roleplayers arguing what magic system is more realistic.

[Braun]

Okay, just to chime in here...

GURPS Traveller, 2nd Ed. by Wiseman, pg 147 (ISBN 1-55634-408-2)

Air/Raft, Open Topped (TL12)

...This vehicle (and its close-topped cousin) is one of the most commonly produced TL12 vehicles within the Imperium, and innumerable variants are produced....

....Long-range radio comm; low-res imaging radar; PESA; terrain-following radar; small computer, complexity 6....

...Aerial top speed 160 mph, aAccel 2 mph/s...aDecel 20 mph/s...

...sAccel 0.08 Gs. Most air/rafts can reach orbit (this one would require vacc suits be worn), but are rarely used for such.


I know, we aren't talking TL12, but here is a GURPS reference that may aid in the discussion. Looks like this text has enclosed versions with a decent computer and can reach orbit.

Guess it comes down to your hand-waving of the science.

[Anaraxes]

You need to be able to reach orbit with an open-topped air raft so that you can re-create Marc Miller's commute.

(And even though the link-dumping has died down a bit, I'll still add that the link is really to YouTube, the intro scene from Heavy Metal. Thus spoiling the joke, but rules is rules.)

[Darkwalker]

Actually closed and ragtop air/rafts have been around since MegaTraveller and TNE has the default as "ragtop" with a "hardtop availabel"

[Jame]

Originally Posted by MrBackman

Air-rafts according to canon text and illustrations are all open topped vehicles. You'll need a vacc suit to do the orbit thing.

In CT. GT has closed-topped variants (well, okay, it mentions them).

However, my first Traveller which was TNE, which had an illustrated closed-top one. (And I didn't actually read the entry, which states that the top isn't enclosed, until years later. So IMTU closed-top air/rafts are standard, just as closed-top cars are standard in the real world. The difference is that ground-cars would never need to be pressurized.)

[MrBackman]

Air-rafts posed several headaches with real design systems. They were open-topped yet capable of reaching orbit. Unless one had a separate lift system with much less powerconsumption and cost than the one producing thrust, it was really hard to come up with an excuse for why they were so damned slow (110 km/h cruisng speed or something like that).
That was my real excuse for adding the 'gravity nullifier' / 'can only thrust along gravity gradient' lift system. Set the TL for air-rafts so low that one could only build Lift systems and needed to supply thrust by propellers, turbofans etc.
At higher TLs the grav vehicles would be much fatser and therefore also enclosed for the comfort of the occupants.

[Jame]

Originally Posted by MrBackman

Air-rafts posed several headaches with real design systems. They were open-topped yet capable of reaching orbit. Unless one had a separate lift system with much less powerconsumption and cost than the one producing thrust, it was really hard to come up with an excuse for why they were so damned slow (110 km/h cruisng speed or something like that).
That was my real excuse for adding the 'gravity nullifier' / 'can only thrust along gravity gradient' lift system. Set the TL for air-rafts so low that one could only build Lift systems and needed to supply thrust by propellers, turbofans etc.
At higher TLs the grav vehicles would be much fatser and therefore also enclosed for the comfort of the occupants.

I may adopt that if any of my players ask how it works. (Provided if I remember it.)

[Malenfant]

Originally Posted by MrBackman

No, not at all. Being in 'orbit' is NOT the same as being in 'freefall', being in orbit is having such a velocity vector that you will be able to do at least one complete ORBIT around the object you are orbiting. Hitting the retros on the shuttle so its path will take it into the atmosphere is called 'de-orbit' for a reason.

Erm, I'm pretty sure that being in orbit (assuming you're not thrusting or rotating) IS effectively identical to being in freefall. That's why people in orbit (e.g. in the ISS) are in zero gravity when they're only about 100 km above the earth (where the gravitational field strength pulling you down is still pretty close to 9.8 m/s² because that's still very close to Earth)

see http://en.wikipedia.org/wiki/Weightl...n_a_spacecraft

Weightlessness in an orbiting spacecraft is physically identical to free-fall, with the difference that gravitational acceleration causes a net change in the direction, rather than the magnitude, of the spacecraft's velocity. This is because the acceleration vector is perpendicular to the velocity vector.