Physics question: Catapult objects into orbit

[funxus]

I'm doing a course in programming, and to complete the course we need to do a project. This year we're supposed to write a program (in Java) in which you can launch "sattelites" from the surface of earth with an initial velocity, and using vectors calculate their orbits. This is done 2-dimensionally and is drawn on the screen. No forces except for the gravitational force is exerted on the sattelite.

I don't have that much problem with the programming, I've managed to write the whole thing. However, all my sattelites seem to crash before completing the first orbit. Just for debugging purposes I've made the earth "transparent", so that all the sattelites can pass through the earth without crashing, and the sattellites always returns to the base, no matter launch direction or speed. My question is therefore if it's ever physically possible to catapult something into orbit, or even just make it circle earth once, without having an adjusting force after it's launched? If so, I either have to accept that they crash, or I have to add a thrust to the sattelites...

 

[Turner]

Well, get enough velocity, and anything can achieve orbit.

 

[Rad Chris]

If your object has no other forces acting on it then it will always return to the original point. So if you are assuming an instant blast of acceleration to orbital velocity then it will go into orbit that takes it back through your launchpad, and hence crash. In reality rockets burn over the course of 30 minutes or so, and as they burn they change orientation and thus change the final orbit.

If you want to use instant acceleration then your best bet is too add a second burst when your object reaches apoapsis.

 

[bhsup]

Space guns! They're the wave of the future! Forget space elevators, space guns are the way to go.

Wiki article

NASA stuff

 

[Leifmk]

Indeed, it is not possible. An object in true (i.e. unpowered) orbit around a primary body (such as a satellite around the Earth) follows a trajectory which is an ellipse (circular orbits being a special case). Unless the orbit is perturbed or adjusted by some means, the object will always come back to the same points on the orbit, on each go-around. If you start the object by just giving it a good kick from the ground, then you've given it an orbit which, well, intersects the ground, and it will therefore undergo aero- or lithobraking (i.e. crash and burn, or rather burn and crash) shortly before completing its first go-around. If you want the object to achieve a stable orbit, it must somehow be given an additional impulse after launch.

 

[The Last Conformist]

Assuming that you're launching it from the groundlevel, Leifmk is right. If your catapult raises a bit above the groundlevel, and you launch it in an horizontal or almost horizontal path, you can get it into a very low orbit in the absence of air friction (assuming no part of the catapult stays at launch height!).

 

[Leifmk]

Assuming that you're launching it from the groundlevel, Leifmk is right. If your catapult raises a bit above the groundlevel, and you launch it in an horizontal or almost horizontal path, you can get it into a very low orbit in the absence of air friction (assuming no part of the catapult stays at launch height!).

Yeah, if there are no higher points on the primary's surface (along the path of the orbit, anyway) and if there is no atmosphere, then you can get a circular orbit like that. If the primary is sufficiently regular wrt. mass distribution and there are no external perturbing influences which will eventually make the orbit intersect a mountain. It might actually be possible to do on some moderately-sized moons.

 

[The Last Conformist]

Yeah, I was assuming we're dealing with a perfectly spherically symmetric "earth". Obviously you'd need to go clear of any inconvenient mountains, windmills, or cows that may litter the surface!

 

[Leifmk]

Or, theoretically, you could have a "catapult" or sling a couple of hundred kilometers long, which when "fired" would reach up out of the atmosphere and release the payload directly into a circular orbit. Given sufficiently strong materials and handwaving away problems with air drag and whatever.

 

[ybbor]

Assuming that you're launching it from the groundlevel, Leifmk is right. If your catapult raises a bit above the groundlevel, and you launch it in an horizontal or almost horizontal path, you can get it into a very low orbit in the absence of air friction (assuming no part of the catapult stays at launch height!).

what if you used a very large trebuchet? that launches with a horizontal path, right?

 

[The Last Conformist]

what if you used a very large trebuchet? that launches with a horizontal path, right?

Trebuchets normally send their payloads on high, howitzer-like trajectories, but they can be configured to release them on a horizontal path if you want to.

I should know - I and some friends built a miniature one last year.

 

[warpus]

funxus

I have an idea for your project.

Give the rocket a small & limited amount of fuel... and then allow the user to decide how much of the fuel he wants to use for takeoff.. and also give him the ability to use the remainder of the fuel at any time during the flight.

 

[Esox]

OK I'm confused by something. And I daydreamed through Physics in high school.
If an object can not be catapulted into a permanent orbit around Earth, then why is it that a satellite which, I'm assuming, ceases to be propelled by a rocket while in orbit, doesn't return to Earth? Does it reach a point at which it gets more or less "stuck" in orbit? And if so, why can't a ginormous slingshot accomplish the same thing (in theory)?

 

[warpus]

OK I'm confused by something. And I daydreamed through Physics in high school.
If an object can not be catapulted into a permanent orbit around Earth, then why is it that a satellite which, I'm assuming, ceases to be propelled by a rocket while in orbit, doesn't return to Earth? Does it reach a point at which it gets more or less "stuck" in orbit? And if so, why can't a ginormous slingshot accomplish the same thing (in theory)?

From what I understand, you do need to apply some force once the satellite has left the surface of the Earth, in order to 'nudge' it into orbit.

 

[Sidhe]

Because there not fired in an arch like a trebuchet, they're taken to an orbital height and fired horizontally, fire an object vertically and it will eventually reach a stationary height after which gravity will begin to pull it back down to Earth. Satellites are never stationary so there momentum around the Earth keeps them in orbit, eventually left to it's own devices this orbit decays as the satellite slows and it gradually lowers into the atmosphere where it breaks up and either disentegrates completely or falls in a shower to the Earth. That's my guess anyway, based on what I know of classical physics. The moon is moving away from the Earth but that's a whole 'nother story.

 

[The Last Conformist]

I don't actually know, but in the real world it might be possible to get into orbit with a catapult by cleverly utilizing air friction.

 

[Souron]

I don't actually know, but in the real world it might be possible to get into orbit with a catapult by cleverly utilizing air friction.

You mean like launching an glider from a catapult?

Trouble is, a glider will always fall to earth eventually. The air resistance not only keeps the glider up, it slows it down. After a while the glider will be going too slow to have significant upward pressure on it's wings, and it will fall to earth.

Along simmilar lines, you could create a craft that changed it's tregectory by depoying wings mid flight. The problem with this is the same as before; the new tregectory would still pass through the atmosphere which will slow the velocity, which will destroy the orbit, making the craft fall to earth.

 

[funxus]

Thanks for the responses, good to know that it's not an error in my program. The teacher also wrote that we should add in the mass of the sattelite in the equation as well, so I guess he's knowledge about physics is pretty limited.

Adding a thrust is optional, and wouldn't be too hard, I was thinking of making it a function of the distance from earth and the angle to the surface or something like that, shouldn't be more than a few lines. Apparently though they're only going to check that the program launches something graphically, and then say we've passed, so I'm gonna focus on other stuff instead...

 

[Sidhe]

I don't actually know, but in the real world it might be possible to get into orbit with a catapult by cleverly utilizing air friction.

It think the problem is you'd need to achieve an escape velocity from launch, since there is no propulsion but the imediate velocity of take off, and this means achieving 11.2 km/s at release and that requires explosives, I fail to see how a catapult could store the potential energy required which would not destroy it's structure, even with space age alloys.

 

[The Last Conformist]

You mean like launching an glider from a catapult?

Trouble is, a glider will always fall to earth eventually. The air resistance not only keeps the glider up, it slows it down. After a while the glider will be going too slow to have significant upward pressure on it's wings, and it will fall to earth.

Along simmilar lines, you could create a craft that changed it's tregectory by depoying wings mid flight. The problem with this is the same as before; the new tregectory would still pass through the atmosphere which will slow the velocity, which will destroy the orbit, making the craft fall to earth.

Every low-orbit satellite will be dragged down to earth by aerial friction in short order on an astronomical timescale. The ISS needs regular rocket boosts to regain lost height.

The specific mechanism I was thinking of was having a compact projectile that folds out when its reentering denser layers of the atmosphere, creating an aerobraking effect that throws into a new path, then folds together again for future low passes, minimizing kinetic energy loss. Sure, it'll come down sooner or later, but it would seem realistic to keep it up for a humanly significant time.


Of course, the entire concept is somewhat stupid - you'd just give it a small boost rocket, and you could get it into an orbit stable over astronomical timescales.