Technical Liftoff Question

[Terquem]

I get a little chuckle

One of the things about science fiction, and by extension science fiction games, is that you start with this thing people call "handwavium", which I love, and then for some reason find the need to start adding bits and pieces of actual science to these simplified explanations.

My favorite one is the concept of "anti-gravity" which sounds great in "handwavium" terminology, but if you look at it too closely it becomes bizarrely impossible.

Your ship is grounded on the surface of the world, near the equator, say. You want to lift off, so you fire up the "gravity nullifiers", when suddenly your ship is imparted with an initial velocity of near 1000 mph. The planet spins below you at near the same speed, and you get sucked into the vortex of the relationship between the air that is being dragged by the planet you just told your ship to ignore the gravity pull of.

There are a lot more things than gravity at work, so I prefer to stop at the handwavium, and call it good.

 

[sudnadja]

One of the things about science fiction, and by extension science fiction games, is that you start with this thing people call "handwavium", which I love, and then for some reason find the need to start adding bits and pieces of actual science to these simplified explanations.

My favorite one is the concept of "anti-gravity" which sounds great in "handwavium" terminology, but if you look at it too closely it becomes bizarrely impossible.

Your ship is grounded on the surface of the world, near the equator, say. You want to lift off, so you fire up the "gravity nullifiers", when suddenly your ship is imparted with an initial velocity of near 1000 mph. The planet spins below you at near the same speed, and you get sucked into the vortex of the relationship between the air that is being dragged by the planet you just told your ship to ignore the gravity pull of.

There are a lot more things than gravity at work, so I prefer to stop at the handwavium, and call it good.

And you could leave it on and you also get the motion of the planet around the star, and star around the galaxy. You can reach galactic escape with nothing more than that.

I think Traveller can be done without reactionless thrusters and anti-grav without changing the game very much, if you want an aspect of plausibility, presume reaction thrusters with extremely high exhaust velocities. You do need to design the ships with propellant then, but presume some kind of photon rocket for the thrusters, with matter-antimatter providing the photon. Or something like The Expanse rockets, with Isp > 10 million seconds.

The item that does have to be completely handwaved is the Jump drive, but I think everything else could be done with plausible physics.

 

[kilemall]

Well, gravitics IS kind of a mess as it does seem to have about 4 differing properties depending on situation, and that's not counting unexplored things like probable use in material fabrication or fusion power.

Best be a separate thread.

 

[esampson]

And you could leave it on and you also get the motion of the planet around the star, and star around the galaxy. You can reach galactic escape with nothing more than that. . .

The gravitic drives used by things like air/rafts or just to assist with liftoff only work in close proximity to the mass whose gravity is being neutralized (which is why you can't put on a vacc suit and fly an air/raft to another planet). Thus, while you may have neutralized the interaction of gravity between the object and the planet you have not neutralized it between the object and the star the planet is currently orbiting. The object will now be in an independent orbit around the star with an initial vector that is very largely based on the planet's orbital velocity with a slight modification coming from the planet's rotation (if you were on the equator of Earth you would have an orbital vector of 65,700 mph from Earth's orbit modified by a pretty measly 1000 mph by Earth's rotation).

This means the initial vector will be really, really close to the planet's orbital vector which will put the ship in an almost identical orbit to the planet. I haven't run all the numbers but I suspect that you might drift out of the atmosphere as the two orbits diverge (the gravitational attraction between the star and the planet will be larger by an infinitesimal fraction than the attraction between the ship and the star due to the ship's lower mass and their orbital vector will be slightly different) but in the end the tiny, tiny differences in the gravitational forces and orbital vectors will result in an orbit that is only a tiny, tiny bit different from the planet's orbit. You aren't leaving the orbit of the star, much less anything else.

Addendum: I just realized that since the neutralizing effect only works fairly close to the planet you won't even drift that far away from the planet. As you get to whatever the limit of the drive is the planet will once again exert force on you, pulling you back into the planetary orbit around the star.

 

[sudnadja]

The gravitic drives used by things like air/rafts or just to assist with liftoff only work in close proximity to the mass whose gravity is being neutralized (which is why you can't put on a vacc suit and fly an air/raft to another planet). Thus, while you may have neutralized the interaction of gravity between the object and the planet you have not neutralized it between the object and the star the planet is currently orbiting. The object will now be in an independent orbit around the star with an initial vector that is very largely based on the planet's orbital velocity with a slight modification coming from the planet's rotation (if you were on the equator of Earth you would have an orbital vector of 65,700 mph from Earth's orbit modified by a pretty measly 1000 mph by Earth's rotation).

This means the initial vector will be really, really close to the planet's orbital vector which will put the ship in an almost identical orbit to the planet. I haven't run all the numbers but I suspect that you might drift out of the atmosphere as the two orbits diverge (the gravitational attraction between the star and the planet will be larger by an infinitesimal fraction than the attraction between the ship and the star due to the ship's lower mass and their orbital vector will be slightly different) but in the end the tiny, tiny differences in the gravitational forces and orbital vectors will result in an orbit that is only a tiny, tiny bit different from the planet's orbit. You aren't leaving the orbit of the star, much less anything else.

Addendum: I just realized that since the neutralizing effect only works fairly close to the planet you won't even drift that far away from the planet. As you get to whatever the limit of the drive is the planet will once again exert force on you, pulling you back into the planetary orbit around the star.

I hope motion gifs work:

.. and it appears that motion gifs do not work. Direct link instead:
gravity lift

1000 days are simulated. The earth remains centered in that, and you can see that the craft appears to be in a steadily widening orbit for a time. Since it will never be attracted to the earth, eventually an interaction with the moon (which is still computed) will boost or drop its orbit.

I ran the numbers with a star the mass of the sun, planet and moon the mass of the earth and moon, with locations and velocities as they were on October 14, 2017 midnight UTC. A ship was placed on the surface of the earth, opposite to the sun and with earth's equatorial rotation (inclination wasn't accounted for in this case).

The spacecraft does go into a different solar orbit:

However, this was with Earth's gravity completely turned off (for the ship, the moon is still obviously in orbit of the earth and the earth+moon barycenter is in orbit around the system barycenter) rather than turning off at a particular distance.

I started to model the effect if it were only effective at a distance with no other factor and some things came up: how does the anti-gravity know to work against earth's gravity rather than that of a person standing near the ship? Does it only cancel a particular amount of force, and if so, relative to what?

 

[mike wightman]

If general relativity and spacetime are the final word on how gravity works then null grav modules and reactionless maneuver drives require a major re-handwave.

 

[sudnadja]

Addendum: I just realized that since the neutralizing effect only works fairly close to the planet you won't even drift that far away from the planet. As you get to whatever the limit of the drive is the planet will once again exert force on you, pulling you back into the planetary orbit around the star.

Atmospheric effects are not computed, but in the scenario that a anti-grav ship turns on its antigrav at the surface and is lifted by centrifugal motion, and the anti-grav becomes ineffective at 1.1 radii from the center but cannot reengage until 1.09 radii from the center, the ship will crash in the absence of thrust.

If it's a razor sharp interface, it will bounce around but that's difficult to model. Once gravity turns back on the ship will have some residual velocity that will bleed off, then start accelerating toward the planet -

 

[esampson]

I hope motion gifs work:

.. and it appears that motion gifs do not work. Direct link instead:
gravity lift

1000 days are simulated. The earth remains centered in that, and you can see that the craft appears to be in a steadily widening orbit for a time. Since it will never be attracted to the earth, eventually an interaction with the moon (which is still computed) will boost or drop its orbit.

I ran the numbers with a star the mass of the sun, planet and moon the mass of the earth and moon, with locations and velocities as they were on October 14, 2017 midnight UTC. A ship was placed on the surface of the earth, opposite to the sun and with earth's equatorial rotation (inclination wasn't accounted for in this case).

The spacecraft does go into a different solar orbit:

However, this was with Earth's gravity completely turned off (for the ship, the moon is still obviously in orbit of the earth and the earth+moon barycenter is in orbit around the system barycenter) rather than turning off at a particular distance.

I started to model the effect if it were only effective at a distance with no other factor and some things came up: how does the anti-gravity know to work against earth's gravity rather than that of a person standing near the ship? Does it only cancel a particular amount of force, and if so, relative to what?

Which software was this, because widening orbits make no real sense, at least not at that scale. When you have an object with a given orbital vector around a mass and no further energy put in there's really only two possible solutions, a parabolic orbit or a hyperbolic orbit (an impact is still a parabolic orbit. It is simply one in which periapsis is beneath the surface of the major body).

As for how the drive 'knows', it 'knows' the same way a jump drive 'knows' it is too close to a body. It's just an aspect of the physics. I assume the drive works off of any sufficiently massive body that is within range. The problem is that the range is really very short (I think the limit is 10D) and so you almost never get two objects of sufficient mass that are close enough to one another for a drive to be in range of both.

I do realize that Diameters is not really a good way to deal with gravity but it's how Traveller does it for the sake of convenience. If you really want to get nerdy we could assume that 10D is relative to Earth and so what really happens is the drive loses effectiveness when the force of gravity is less than .01 G's.

 

[esampson]

Atmospheric effects are not computed, but in the scenario that a anti-grav ship turns on its antigrav at the surface and is lifted by centrifugal motion, and the anti-grav becomes ineffective at 1.1 radii from the center but cannot reengage until 1.09 radii from the center, the ship will crash in the absence of thrust.

If it's a razor sharp interface, it will bounce around but that's difficult to model. Once gravity turns back on the ship will have some residual velocity that will bleed off, then start accelerating toward the planet -

Most likely it isn't that the drive simply turns off. Instead what happens it there is a sharp decrease in efficiency as you near the limit.

You also need to understand that grav modules are not simply null gravity. If they were a ship wouldn't have much ability to move and while the centripetal force would give the ship some upward velocity it would only be about .03 meters per second, not really enough to do anything. Instead grav modules appear to take a substantial portion of the gravitational force and redirect it. Since the portion is over 50% you can redirect the force down and back so that the 'downward' portion of the vector balances the remaining gravity and the 'rearward' portion of the vector pushes the object forward.

 

[sudnadja]

If general relativity and spacetime are the final word on how gravity works then null grav modules and reactionless maneuver drives require a major re-handwave.

Yes, and it does allow other neat tricks. For example, if you wanted to move an asteroid around you could just put a nul-drive mass on the surface, turn it on, allow the mass to be flung away from rotational velocity.

At some point the grav drive becomes ineffective and the asteroid and mass mutually attract each other. The mass is pulled back to the asteroid and can come to rest on the surface. Once the asteroid has rotated again to the proper position, turn the gravity drive on and again the mass gets flung away.

You're able to convert asteroid rotational inertia into forward inertia "for free" (in fact, it's not clear you lose rotational inertia at all, you are just creating energy).

Even better, a dual lobe asteroid where each lobe is the same mass as the other. Put gravity drive on each, turn off the gravity on one lobe and it breaks apart and flies away from the other - same effect as with the mass above but faster.

You should also be able to use a gravity drive to escape from inside the event horizon of a black hole, which also breaks the real universe.

It should also act as a null-inertia field, or something like that. Say you have a grav belt on inside an accelerating ship - after turning on the belt you won't feel any gravity, even though that gravity was due to acceleration instead of mass, meaning you could do things like wrap the fuel modules and cabin in anti-grav and the ship should be able to accelerate faster on the same amount of thrust.

 

[sudnadja]

You also need to understand that grav modules are not simply null gravity. If they were a ship wouldn't have much ability to move and while the centripetal force would give the ship some upward velocity it would only be about .03 meters per second, not really enough to do anything. .

You're mixing dimensions there, centripetal force would provide an acceleration, rather than a velocity. It'd be 0.03369 m/s^2, presuming earth radius and a rotation rate of 2 pi radians / 24 hours. That would be a velocity of ~600 meters/second after 5 hours, relative to the point on the surface you started from, with a distance of ~5460 km.

But it's true I don't understand how the grav modules really work.

 

[esampson]

You're mixing dimensions there, centripetal force would provide an acceleration, rather than a velocity. It'd be 0.03369 m/s^2, presuming earth radius and a rotation rate of 2 pi radians / 24 hours. That would be a velocity of ~600 meters/second after 5 hours, relative to the point on the surface you started from, with a distance of ~5460 km.

But it's true I don't understand how the grav modules really work.

Yes, it's an acceleration, but once you are no longer touching the surface of the planet and you are countering the force of gravity from the planet it would no longer be accelerating you. It will be similar to having a weight at the end of a string that is being spun and then you cut the string. As soon as you do that the object takes off with its imparted energy and does not continue to accelerate (other than downwards to the floor).

I suspect that might be what is wrong with the simulation you created where the ship spiraled outward. You were continuing to add energy to it from the rotation of the planet which it would not be receiving.

 

[sudnadja]

Yes, it's an acceleration, but once you are no longer touching the surface of the planet and you are countering the force of gravity from the planet it would no longer be accelerating you. It will be similar to having a weight at the end of a string that is being spun and then you cut the string. As soon as you do that the object takes off with its imparted energy and does not continue to accelerate (other than downwards to the floor).

I suspect that might be what is wrong with the simulation you created where the ship spiraled outward. You were continuing to add energy to it from the rotation of the planet which it would not be receiving.

No, the spiral outward is just because the ship and the planet are both in orbit of the sun. To be clear: I did not apply any acceleration to the ship at all, the only accelerations it experiences are due to sun, moon and venus gravitation. It had an initial velocity vector of that of earth + earth rotational velocity, but no accelerations. It just appears to spiral because I centered the entire system on the earth. The spiral is a side effect of the ship being in an independent orbit around the sun. That's why I posted the picture of the sun-centered system as well, the black (ship) and red (earth-moon) were both in stable orbits around the sun.

To make it a little more clear, I plotted out for 100 years and added Venus (blue) to the system: Note the spiraling pattern that Venus also appears to have, when viewed in an earth-centric system. The spacecraft, in black, appears to spiral out further from the earth to eventually reach a maximum distance, and then spiral back in. This is representative of the spacecraft very slowly getting farther from Earth, because they are almost in the same solar orbit, eventually reaching a maximum, and then drawing nearer again.

And of course, as soon as "anti grav" is turned on, the spacecraft is in an independent solar orbit, even though it is very close to the surface of the earth.




This is the exact same data, simply plotted so that it is sun-centered, and zoomed out to about 2 au radius from the sun.

 

[sudnadja]

I suspect that might be what is wrong with the simulation you created where the ship spiraled outward. You were continuing to add energy to it from the rotation of the planet which it would not be receiving.

And remember, the acceleration (as I noted) is relative to the point that it was separated from. If you are in zero g, swinging a bolt attached to a string around your head and the bolt detaches, it will have an instantaneous velocity at the point of separation but the point of the string it departed from will still appear to be accelerating relative to it, as it completes the circle around your head.

At any rate, I added no energy at all to the system, as I stated above. My EOM for the spacecraft is:


x[4] is the position of the spacecraft, x[1] is the sun, x[2] the earth (which you will note is not accounted for in the EOM for x[4]), x[3] the moon and x[5] venus.

x' and x'' are with respect to time, of course. The ICs are rolled into this one string.

I should note that x[n] are all vectors, and if you provide a vector as an IC in mathematica, it will automatically generate one equation for each dimension for you, thus you can think of that x[4]''[t] as really {x[4][1]''[t], x[4][2]''[t], x[4][3]''[t]} = ... , in case you aren't familiar with mathematica.

I'm not quite sure how to accomplish the same thing in Maple, by the way.

 

[sudnadja]

I'm looking for visualization and tactical realities for players to deal with, so again, it's that first 20 km out of the downport.

Now if we are headed straight up as per rocket launches, wouldn't the gravity cancel most of our lift for the first few minutes and miles at 1-G, 9.8 mps vs. our accel of 10 mps and therefore we are accelerating at .2 mps until we get further up?

I did approximately that. Two ships, one that can accelerate at 1G and one at 4G (where G = 9.806 m/s^2). Placed on the surface of an earth like planet (in terms of mass and radius) the 1G ship would not be able to lift off at all, so I gave them both an initial upward bump of 20 meters/second. Presume explosives or some kind of chemical rocket assistance for a fraction of a second.

The left hand plots are for the first 30 seconds, and the top plots are for position while the bottom plots are for velocity. x axis is in seconds, y axis is in meters and meters/second on the left hand plots, and planetary radii for the top right plot.

After 30 seconds the 4G ship is already at 13832 meters and moving at 902 meters/second - already supersonic. The 1G ship is at 591 meters altitude moving at 19.37 m/s. Net acceleration is still negative at this point, -0.02007 m/s^2. In fact, net acceleration is negative until 416.26 seconds. Altitude is only 7101 meters at that point, but even that tiny reduction in earth's gravity pull at that altitude is enough that the 1G ship is net positive.

(It should actually be net positive at even 1 cm over the earth's surface, so I presume one of my constants is slightly off, either mass or radius of earth).

At any rate, now that net acceleration is positive the ship starts to make progress. At this point, incidentally, the 4G ship is already at 0.415 earth radii over the surface.

At 8000 seconds, the 4g ship is just about at 200 radii (100 diameters) and ready to jump. The 1g ship, as can be seen, has gotten high enough above the gravity well that it really no longer is a factor, but it will still take ages to get to 100 diameters.



I've tried this before with a simple atmospheric model and presuming simple aerodynamic lift and drag, and it is possible to escape from a 1G planet with a 1G drive presuming a lifting body, but if at all possible it's more simple from a math perspective to launch straight up.

 

[mike wightman]

Rockets do not fly vertically into orbit, neither do streamlined Traveller ships that have lift. You have to have a curved path in order to enter an orbit. A streamlined ship can fly at subsonic speeds and climb to a great height, higher than an air breathing engine can manage since the magic 1g drive provides thrust regardless. Once you are 25 km up you can change to a more 'rocket' like flightpath since local gravity has decreased. Assuming a subsonic speed of 500kph for easy maths that's 3 hours to get to rocket mode altitude.

Now with a magic Traveller constant 2g drive you can just blast to the 100D limit and then jump. Or if you head straight up and achieve escape velocity you are now in orbit around the star (if you switch off the m-drive), achieve the star's escape velocity and you are now orbiting the supermassive black hole at the centre of our galaxy. Achieve galactic escape velocity and you are travelling in a straight line to where ever you wanted to go

 

[sudnadja]

Yes, this is assuming that both the 1G and 4G ships want to launch directly for 100 diameters without entering orbit. Entering orbit is also possible for either, the 1G ship would start thrusting directly upward and then transition the fraction of acceleration that it has in excess of the local gravity field laterally as it gains height (something like a gravity turn)

 

[esampson]

Rockets do not fly vertically into orbit, neither do streamlined Traveller ships that have lift. You have to have a curved path in order to enter an orbit. A streamlined ship can fly at subsonic speeds and climb to a great height, higher than an air breathing engine can manage since the magic 1g drive provides thrust regardless. Once you are 25 km up you can change to a more 'rocket' like flightpath since local gravity has decreased. Assuming a subsonic speed of 500kph for easy maths that's 3 hours to get to rocket mode altitude.

Now with a magic Traveller constant 2g drive you can just blast to the 100D limit and then jump. Or if you head straight up and achieve escape velocity you are now in orbit around the star (if you switch off the m-drive), achieve the star's escape velocity and you are now orbiting the supermassive black hole at the centre of our galaxy. Achieve galactic escape velocity and you are travelling in a straight line to where ever you wanted to go

Actually, I would imagine that even streamlined ships fly vertically assuming their engines are powerful enough. There is just no reason for them to fly into an orbit (barring things like the destination being occluded by the planet). Orbiting is only necessary when the engines are not strong enough to counteract the full force of gravity and only possible when there is an atmosphere and the ship is a lifting body (or, as in the case of real life when the engine is strong enough but the specific impulse is so limited that orbit is required to prevent crashing back onto the planet after remass is expended).

At 25 km, however, you are still well within Earth's gravity well (.9922 G's) so switching to a more vertical flightplan would not be possible.

 

[nobby-w]

You're mixing dimensions there, centripetal force would provide an acceleration, rather than a velocity. It'd be 0.03369 m/s^2, presuming earth radius and a rotation rate of 2 pi radians / 24 hours. That would be a velocity of ~600 meters/second after 5 hours, relative to the point on the surface you started from, with a distance of ~5460 km.

But it's true I don't understand how the grav modules really work.

If you want to tinker about with orbital mechanics I can't recommend Kerbal Space Program highly enough. Spend a bit of time figuring this out and you, too, can become a miserable old grognard like me and get to complain about the lack of verisimilitude. You can contemplate launch trajectories and orbital mechanics from having some idea about how they work - and when that gets boring you can play 'let's crash the rocket'.

It also does space planes so you can see what taking off into orbit looks like in a streamlined craft with lift.

 

[esampson]

I did approximately that. Two ships, one that can accelerate at 1G and one at 4G (where G = 9.806 m/s^2). Placed on the surface of an earth like planet (in terms of mass and radius) the 1G ship would not be able to lift off at all, so I gave them both an initial upward bump of 20 meters/second. Presume explosives or some kind of chemical rocket assistance for a fraction of a second. . . .

At 8000 seconds, the 4g ship is just about at 200 radii (100 diameters) and ready to jump. The 1g ship, as can be seen, has gotten high enough above the gravity well that it really no longer is a factor, but it will still take ages to get to 100 diameters.

At 8000 seconds the 1G ship would be approximately 16km above the ground since it has no acceleration. At 16km it is still experiencing .995 G's. It would actually have a very small acceleration at this point of about 5 cm/s² so it would be gaining speed (and as it would have been experiencing small amounts of acceleration before this point it would actually be slightly higher than 16km, but not by much) but it is certainly not to the point where 'the gravity well is no longer a factor'.