Forced orbit.

[Rath]

Would it be possible to use engines pointing downwards and constantly thrusting to orbit a planet faster than escape velocity?

i ask, because science.

[DrLicor]

Basicly, no. But everything is possible in the books and maths. Here's a great video that explains how much it takes to:

 

[steve_v]

10 minutes ago, DrLicor said:

Basicly, no. But everything is possible in the books and maths. Here's a great video that explains how much it takes to:

What has de-orbiting a moon to do with anything?

16 minutes ago, Rath said:

Would it be possible to use engines pointing downwards and constantly thrusting to orbit a planet faster than escape velocity?

Yes, it' totally possible to exceed escape velocity while using an engine to maintain orbit*... Physics says yes, not sure why you'd want to though...

Did something like this for a jet speed challenge at some point, required 'downward' thrust to stay in the atmosphere.
*but then it's not an orbit, it's a circular path maintained with constant thrust, and it's not particularly usefull.

Edited November 7, 2016 by steve_v

[DrLicor]

2 minutes ago, steve_v said:

What has de-orbiting a moon to do with anything?

Yes, it' totally possible to exceed escape velocity while using an engine to maintain orbit*... but then it's not an orbit, it's a circular path maintained with constant thrust, and it's not particularly usefull.
*Physics says yes, not sure why you'd want to though...

I tought that he meant he wanted to strap some engines to a planet to let it orbit faster:

 

15 minutes ago, Rath said:

Would it be possible to use engines pointing downwards and constantly thrusting to orbit a planet faster than escape velocity?

 

Which is basicly the same as deorbiting a moon, since a planet is kind of a moon of the sun.

[steve_v]

10 minutes ago, DrLicor said:

Would it be possible to use engines pointing downwards and constantly thrusting to orbit a planet faster than escape velocity?

Depends on how you read it - I got: use engines (attached to a craft) to orbit (the craft around) a planet faster than escape velocity (of the planet in question)

Making the planet exceed escape velocity of its parent body (Kerbol) is indeed impossible in this game, though only (highly) improbable IRL.

Edited November 7, 2016 by steve_v

[The Aziz]

And I understood it different too.

You ask if you could orbit a planet at certain altitude but at much higher speed.

No, because orbital velocity is connected with altitude. Also as you should know, pointing downwards at planet does not slow you down, it changes the eccentricity (right? Or another word I'm not aware of, it's just like dragging the blue marker on maneuver node) So you probably would end up entering the atmosphere at escape velocity after some time.

Edited November 7, 2016 by The Aziz

[steve_v]

8 minutes ago, The Aziz said:

Also as you should know, pointing downwards at planet does not slow you down, it changes the eccentricity.

Yes, when applying said thrust at any given point in the orbit... but thrusting anti-radial constantly will effectively add to the force of gravity, increasing the velocity required for any given orbit... so curved path just like an orbit, only with 'augmented gravity' keeping it circular. Note this means continually adjusting your thrust vector to match gravitational acceleration too.

Edited November 7, 2016 by steve_v
Apply spilling checker.

[DrLicor]

Oh indeed, when reading it several times I think what he wants haha. But indeed that should be possible. 

A circulair orbit basicly means that the force you have forward is equal to the force you're pulled down by gravity. So let's take some easy numbers, let's say escape velocity is 100m/s, the current speed for a certain altitude is 10m/s. What you want is to go 100m/s. but you also want your craft to stay in that certain orbit, so you have to thrust those remaining 90m/s down, to help the gravity a little bit. 

Edited November 7, 2016 by DrLicor

[steve_v]

7 minutes ago, DrLicor said:

you also want your craft to stay in that certain orbit, so you have to thrust those remaining 90m/s down, to help the gravity a little bit.

Eggzactically. Orbit is just a straight line bent by gravity

Edited November 7, 2016 by steve_v

[Rath]

I'm wondering if I had a ship going at escape velocity through a planets SOI, would it be possible to thrust downwards (towards the planet), and give gravity some help.

 

or, could I have engines pointing towards the planet that are always on, but to stop my ship from entering a ballistic drop, I am also accelerating forwards, keeping my alt and inclination, but going speed?

[The Aziz]

9 minutes ago, Rath said:

I'm wondering if I had a ship going at escape velocity through a planets SOI, would it be possible to thrust downwards (towards the planet), and give gravity some help.

As I said, if you look at your trajectory, you would burn up in atmo.

 

10 minutes ago, Rath said:

or, could I have engines pointing towards the planet that are always on, but to stop my ship from entering a ballistic drop, I am also accelerating forwards, keeping my alt and inclination, but going speed?

Depending on their power, but it would be very hard to keep altitude, if not impossible at all.

Sandbox + cheat menu, some could try do it and post results.

[eddiew]

So... you want to do donuts in a space ship?  

Technically yes. If you pass Kerbin at, say, 6km/s but throw out a really long cable to anchor yourself to the planet, you'll loop at 6km/s. In this case, substitute engine burns for long cable.

However... if the goal is a specific ejection vector, it's not going to be an efficient option.

[The Aziz]

5 minutes ago, eddiew said:

 If you pass Kerbin at, say, 6km/s but throw out a really long cable to anchor yourself to the planet, you'll loop at 6km/s. In this case, substitute engine burns for long cable.

But cable has limited length. After a time you'll be moving on spiral lower and lower.

Seriously, can we just get from theory to practice?

[eddiew]

Just now, The Aziz said:

But cable has limited length. After a time you'll be moving on spiral lower and lower.

I assumed a large reel of cable so that you could pay it out as the other end wound around the planet. Apologies to any kerbals on the orbital plane, you're gonna have bad days   

[The Aziz]

That still does not apply to my imagination of physics, I mean it would have to be short enough to keep orbital altitude, but long enough to allow for orbiting, then it would bend around the planet on the surface rather than in orbit, ah so many unknowns my brain is overheating.

[DrLicor]

1 hour ago, Rath said:

I'm wondering if I had a ship going at escape velocity through a planets SOI, would it be possible to thrust downwards (towards the planet), and give gravity some help.

 

or, could I have engines pointing towards the planet that are always on, but to stop my ship from entering a ballistic drop, I am also accelerating forwards, keeping my alt and inclination, but going speed?

first, yes you have to thrust 'downwards' to give the gravity a help if you want to stay in the same orbit, and you have to burn constantly, if you stop burning, you'll slingshot yourself. 

Burning downwards to get the same force downwards as forward, doesn't effect your speed, inclination or altitude. As long as you keep your force downwards the same as the force thats pushing you forward. Note that the force forward isn't provided. Since you don't have any friction, your speed forward doesn't need to be maintained by an engine or any other factor that provides thrust. Although, you won't get any engines that provides that many thurst to let you stay the same altitude. Not for kerbin.

But you could try to do it at a planet with a very low mass. Which means low gravity, low escape velocity. The real problem is, you need 2 engines, one for reaching escape velocity, one for keeping you close to the planet, lets say the one to reach escape velocity can provide 3m/s of thrust. That means that the one who tries to keep you close to the planet need to thurst 3m/s downwards the first sec, 6m/s of thrust the 2nd sec, 9m/s of thust the 3rd sec. etc. etc. So basicly, you need an engine that gains thrust over time, with the following formula:

[F]=[x] * [T]

Where [F] = the thrust of your engine tries to keep you close to the ground 

[x] = the thrust of the engine that wants to reach escape velocity 

and [T] is the time that the engine that want to reach escape velocity burns.
 

edit, so you can see, if the engine that burns foward stops, you still have that centain time that it has burned. Let's say it was 10 seconds with a thrust of 3m/s, you have to help gravity with 3m/s*10 = a thrust of 30m/s 

[Superfluous J]

1 hour ago, The Aziz said:

But cable has limited length. After a time you'll be moving on spiral lower and lower.

Seriously, can we just get from theory to practice?

He went from theory to practice with:

1 hour ago, eddiew said:

Substitute engine burns for long cable.

In short, yes you can. It's not an orbit but you can circle a planet in such a way until fuel runs out.

[eddiew]

1 hour ago, The Aziz said:

That still does not apply to my imagination of physics, I mean it would have to be short enough to keep orbital altitude, but long enough to allow for orbiting, then it would bend around the planet on the surface rather than in orbit, ah so many unknowns my brain is overheating.

I think you've overthought this a tad...

Imagine you're jogging down steep hill. You need to make a very sharp turn at the bottom, but the hill is too steep and you can't slow down - so you stick out your arm, grab a streetlight or a fence post, and swing yourself round it, thus changing your velocity vector but not your speed. That's all my cable analogy was meant to be, considering the planet as a fixed point of negligible diameter. I thought it would be a more everyday example than rocket engines in vacuum 

In KSP you'd be using engines to do exactly what the cable (or your arm) does, i.e. apply a force radially inwards, effectively supplementing the gravity of the particular body and making it act like a larger planet with a higher escape velocity. As long as you're within the thrust limit of the engines (or the strength of your arm) you can swing round at whatever speed you like and not escape. 

[Red Iron Crown]

4 hours ago, Rath said:

Would it be possible to use engines pointing downwards and constantly thrusting to orbit a planet faster than escape velocity?

i ask, because science.

Absolutely possible (though infinite fuel is a big, big help). Check out this video from the incomparable Abyssal Lurker:

 

[SchweinAero]

5 hours ago, DrLicor said:

A circulair orbit basicly means that the force you have forward is equal to the force you're pulled down by gravity.

Nitpick: a circular orbit means that the sum of all forces acting upon your spacecraft is

1. directed towards the central body
2. of magnitude (spacecraft's mass)x(speed^2/radius).

How this combination is accomplished does not affect the craft's trajectory. Usually the only major force in play is gravitational, but it does not have to be that way. For example, here's a video showing simulated atmospheric flight at more than escape velocity.

[septemberWaves]

Technically, with the way that centrifugal force works, it could theoretically be done. If you want to travel faster than the natural orbital velocity for a given altitude, you just need to accelerate to your desired orbital speed, and increase the force pointing towards the center of the orbit (by firing the engines towards it, for example). This is similar to how orbiting a body with higher gravity will mean that you travel faster at the same altitude, because the force towards the center is higher.

Accomplishing it would require enormous amounts of propellant and also very precise control, but it is theoretically possible. I'm not sure I could see a potential use for this however, and it's definitely not practical to consider in KSP.

[magnemoe]

13 hours ago, Rath said:

Would it be possible to use engines pointing downwards and constantly thrusting to orbit a planet faster than escape velocity?

i ask, because science.

Yes, I have done this multiple times on Gilly using an jetpack, Kerbal is at escape velocity but burn downward from time to time to keep close to surface.
This is mostly interesting on Gilly as the dV costs is so low and the orbit time is so long. You also want to time it, did an rescue on Gilly surface fly 2 km away from stranded kerbal, I set him up for an 30 ms/ burn so he would overlfy base, the first kerbal braked to low orbit. rescued one overflew base did an braking burn then an pro target one for intercept, entered station, second kerbal then did an 2-3 burns to keep higher than orbital speed before doing intercept and then correction burn. 

[Archgeek]

This seems kinda easy to test on Gilly -- just pick a speed a goodly chunk over its escape velocity (which is tiny so no need to go too fast), pick your desired orbital distance (from Giliy's centre), calculate the mass of a body you can orbit at the target height and speed, then find the gravitational acceleration of the body at that distance.  Next, tune your engines to give that exact accleration, fly towards Gilly, making sure you rampage past a point at the target altitude at your target speed, hit the hold radial-in button, wait for periapse, then punch the throttle and watch the fun!

[ZooNamedGames]

I believe the question is; can a craft thrust downwards towards a planet to accelerate to escape velocities and I can say, that's an easy yes, granted only if you have enough altitude.

I do this to manage my orbit whenever I launch. It's slightly less efficient than a normal launch to target apo and then burning towards the prograde terminator to orbit, but it's realistic considering real engines have a limited number of ignitions but the point is- when I go for orbit, and my apo goes above my target, I aim down towards the planet to continue accelerating towards orbit but lower the impending apo.

[FullMetalMachinist]

55 minutes ago, ZooNamedGames said:

I believe the question is; can a craft thrust downwards towards a planet to accelerate to escape velocities and I can say, that's an easy yes, granted only if you have enough altitude.

Not quite. I'm pretty sure the question is, as others have noted above, is 'can you artificially shorten your orbital period?'  In other words, can you be in an orbit whose period is normally 30 minutes, and thrust in such a way that your altitude doesn't change, but your orbital period drops to, say, 20 minutes? Then answer is yes. 

Scott Manley mentions it in this video of a Mun land-and-return speedrun (start at 11:01). I though he had done a video specifically on the OP's question, but I can't seem to find it.