Fun with Orbits?

[GungaDin]

I am finally getting the feel of space navigation but boy it can be disorienting at times.  I'm getting to be a bad ass though like when I manage to adjust my inclination and apoapsis with a single burn....

Here are a few basic questions I'm still playing with :

1 - Is this an inefficient procedure?  My lazy way of quickly getting into a stable, circular orbit.  Once I punch most of the way out of the atmosphere I begin to tilt over to 45^ and then all the way to 90^.  Then I start to keep track or my periapis and apoapsis.  Once my apoapsis is where I want it I turn off my engine and glide up to it - then i resume thrust at 90^ until my periapsis matches.  This works but I'm sure that I lose efficiency while gliding up to apoapsis.  Question is - what is a better solution to save fuel?

2 - I'm still struggling with the concept of rendezvous in the following example.  I am in a stable, circular orbit with no eccentricity, at velocity X and altitude Y.  My target is in my exact orbit and inclination 1 Km in front of me.  By definition we are moving at the same velocity. (If we were not at the same velocity we would NOT be at the same height and eccentricity right?)  I want to start to close the distance between us.  Am I not in a 'you can't get there from here' situation?  If I speed up (by thrusting toward target)  I add altitude.  If I slow down I lose altitude.  Must I do this in 2 moves by diving down first and then thrusting forward - sorta like a 'Fokker bounce'?  Excuse my dog fighting reference but I don't know how else to describe it. 

Thanks for any answers here.

[AHHans]

1 hour ago, GungaDin said:

1 - Is this an inefficient procedure?

The way you described it, it probably is. Not so much because you need to coast to apoapsis, but because by first going straight up and then tilting over by 45 deg you are not thrusting in the direction that you are already going, so you are fighting against the velocity that you already gained. The most efficient ascent is a gravity turn, where you tilt over a bit shortly after leaving the launchpad and then lock prograde, thus letting the gravity pull your velocity vector more and more horizontal. The exact parameters for an optimal gravity turn depend on your launch vehicle: its TWR and drag during the ascent.

A typical gravity turn for me looks like this: when the I get to 20 - 30 m/s it tilt over by 10 - 20 deg, when I get to 120 - 150 m/s I lock to prograde, I'll stay there until the apoapsis is where I want it, and then I'll coast to apoapsis and circularize. If I have high TWR I tilt over soon(er) and more and lock to prograde earlier, with low TWR I tilt over less and later, and with high drag (but high-ish TWR) I first go up quite a bit before I tilt over. I kind of try to keep the coast phase, i.e. the time to apoapsis when my apoapsis gets out of the atmosphere, at about 1 - 2 min: if the time to apoapsis increases too fast then I need to tilt over more (or reduce thrust), if the time to apoapsis decreases during ascent, then I tilted over too much.

1 hour ago, GungaDin said:

My target is in my exact orbit and inclination 1 Km in front of me. 

At 1 km distance I usually go for the direct approach (even if it isn't the most efficient). I.e. I thrust towards the target, and then use the method for "Fine tuning the rendezvous" from @Snark's Illustrated guide to docking, to keep my prograde vector pointed to the target.

 

[Empiro]

2 hours ago, GungaDin said:

I am finally getting the feel of space navigation but boy it can be disorienting at times.  I'm getting to be a bad ass though like when I manage to adjust my inclination and apoapsis with a single burn....

Here are a few basic questions I'm still playing with :

1 - Is this an inefficient procedure?  My lazy way of quickly getting into a stable, circular orbit.  Once I punch most of the way out of the atmosphere I begin to tilt over to 45^ and then all the way to 90^.  Then I start to keep track or my periapis and apoapsis.  Once my apoapsis is where I want it I turn off my engine and glide up to it - then i resume thrust at 90^ until my periapsis matches.  This works but I'm sure that I lose efficiency while gliding up to apoapsis.  Question is - what is a better solution to save fuel?

The optimal flight path is a pretty difficult problem to solve. Generally speaking, the most efficient ascents will balance between minimizing gravity losses, minimizing drag, and minimizing mass (not carrying too many engines). An efficient ascent has you thrusting at full power for most of the burn (if you're not at 100%, you might as well take fewer engines or more fuel). You also want to point at prograde as much as possible to minimize steering losses. So the ideal ascent would look like:

1. Launch,
2. 50 m/s or so, tilt slightly to the east
3. Lock to prograde, continue to burn. Your nose will come down naturally (gravity turn)
4. At some point, you want your time-to-AP to start decreasing, even as you burn at 100%. You typically do this by staging into efficient, low-thrust vacuum engines.
5. Just as you reach AP, you circularize your orbit

In practice, you'll never be able to get #5 perfectly (I typically will need to cut thrust or coast), but the fortunate thing is that even if you don't the difference in delta-V is minuscule. Getting 1-4 down will be the biggest difference.

2 hours ago, GungaDin said:

2 - I'm still struggling with the concept of rendezvous in the following example.  I am in a stable, circular orbit with no eccentricity, at velocity X and altitude Y.  My target is in my exact orbit and inclination 1 Km in front of me.  By definition we are moving at the same velocity. (If we were not at the same velocity we would NOT be at the same height and eccentricity right?)  I want to start to close the distance between us.  Am I not in a 'you can't get there from here' situation?  If I speed up (by thrusting toward target)  I add altitude.  If I slow down I lose altitude.  Must I do this in 2 moves by diving down first and then thrusting forward - sorta like a 'Fokker bounce'?  Excuse my dog fighting reference but I don't know how else to describe it. 

Thanks for any answers here.

At only 1 km, I'd just burn directly toward your target and then slow down. If the difference is much greater, there are a number of approaches, all with different fuel costs and time to intercept. What you described would indeed work -- by thrusting down (radial in), you're "rotating" your orbit so that you'll traverse the next part of your orbit at a lower altitude and faster before slowing down. This will catch you up to your target. If you do it just right, you'll intercept your target at your initial altitude.

The most efficient way (that requires a lot of time) is to burn retrograde (even a minuscule amount). This put you in an orbit with a shorter period, and when you make a full orbit and come back around to where you are now, you'll be slightly closer to your target. After enough orbits you'll be right on top of your target, and you'll only need to burn the same amount to match velocities. In practice, I tend to use this method most often, though I will burn more so that I can get an encounter more quickly.

[Snark]

3 hours ago, GungaDin said:

Is this an inefficient procedure?

Yes, very.  It's not the coasting to apoapsis that's the problem, it's the "going straight up" part.  As @AHHans points out, it's a lot more efficient to do a gravity turn.  Basically, what you do is to start your eastward turn practically right off the launch pad.  Just a smidgeon-- how much is "right" will depend on what your TWR is.  If you have a low TWR (e.g.1.5 or below), you only want a degree or two.  If it's high TWR, like 2.0 or close to it, you'll want a bigger angle, like 5-10 degrees.

Having done the initial eastward nudge right off the pad, you then just follow all the way up.  No steering needed.  Keep burning full throttle, staging when needed, until your Ap gets up to where you want it.  Then coast up to Ap and circularize.

Judging the "correct" angle to nudge it off the pad takes a bit of practice, but you'll find that you quickly develop a feel for it (especially if you tend to build your rockets with a consistent TWR).  A good way to judge whether you've nailed it is, look at what angle from the vertical you are when you reach 300 m/s.  At that point, you should be tipped probably about 45 degrees from the vertical.  If you're much more vertical than that, your initial nudge wasn't big enough; if you're much more horizontal than that, it was too much. So, if you get to 300 m/s and see that you're way off 45 degrees, just revert to launch at that point and try again.

(Also, note that there's nothing "wrong" with how you're doing it now, if you like it that way.  It's just somewhat wasteful of dV, is all.  A good gravity turn can save you several hundred m/s of dV compared with an inefficient ascent profile, so it's up to you how much you want to fine-tune your ascent.)

3 hours ago, GungaDin said:

My target is in my exact orbit and inclination 1 Km in front of me.

If you're only 1 km apart, then for all practical purposes you might as well be in "flat space", i.e. you don't need to think about orbital mechanics, just treat it as if you were both floating out in deep space with no gravity involved.  So you can just point your nose directly at the target and thrust to a reasonable closing speed that will get you there in a minute or less (say, 10-20 m/s), then take it from there.

The reason that works is that with such a short separation, 1. there's not much difference in the gravity vector that the two craft experience, since the separation is tiny compared with the radius of the body you're orbiting, and 2. if the closing time is only a minute or less, you won't have orbited much of a fraction of an orbit in that time, so motion is pretty close to the classic inertial case.

If you were significantly farther apart-- for example, if the target were 50 km away instead of just 1 km-- then you wouldn't want to do this; it would be better to take the orbital-mechanics approach, unless you're in a big hurry and have lots of dV to spare.

[GungaDin]

Hazaaa!  Thanks so much to all of you.  My first dock and I had no RCS system....  Now I have to start building my gas station in the sky.  LOVE THIS SIM! 

(As I approached my target I had to make adjustments at several different moments.  From my point of view it was almost like docking with a stationary submarine underwater but not quite.  Correct me if I'm wrong but I thin that as I make these smaller adjustments I am basically putting the final touches on matching my orbital altitude, inclination and eccentricity an exact match with my target.)

 

[Guest]

Yup, that's entirely correct. You'll be drifting slightly off target because both you an the target aren't actually stationary, both of you are in ever so slightly different orbits. So your target retrograde burns are, in fact, equalising them.

In case you aren't already doing it, note that docking is tons easier if you periodically skip to the other ship to line up its docking port with yours. Select "Control from here" on the docking port on both craft and line up the target markers on each of them, and they will magically click every time.

Edited November 21, 2019 by Guest

[GungaDin]

Boom! This was not easy.  I put a starboard and port hatch on that central piece and the last ship to dock has a few rcs thrusters on it which I just used for brakes occasionally.  Then I tried to get into a parking orbit like 100 meters off port or starboard of the mother ship.  If I got close to stable there I would switch to the mother ship and maneuver it to be at 90 degrees...then switch back and close in at like .2m/sec praying all the way.  I bumped a few times at that speed with no apparent damage and eventually caught hold.  I know this is primitive construction so any building tips would be appreciated.  PS you can see that I have pasted more and more solar panels on these ships in an attempt to meet a contract requirement but now I realize that it is batteries that are required...  next few flights I guess.

Edited November 26, 2019 by GungaDin