Does brute force rendezvous (surface to target) save on dV ?

[lextacy]

Do you save fuel by doing a brute force rendezvous vs the mechjeb way ? The mechjeb seems to time consuming as you need to :

1. make a huge orbit

2. hohmanns transfer

3. match velocities at closest

4. "get closer" burns

WIth brute force you just insert your orbit at same time as rendezvous by using a special algorithm that times when you should launch. Thats just 1 maneuver. But the big question is .....do we really save fuel?

Edited December 19, 2014 by lextacy

[Superfluous J]

I'd have to do it, but my gut says you would save a little fuel but not very much. In the end, you'll essentially need an orbit the same as your targets', which means you will need to pull your periapsis up to your targets' periapsis and your apoapsis to your targets' apoapsis.

The one thing I don't know for sure is if getting into a lower orbit, then raising your apoapsis up to touch the target's orbit, and then raising your periapsis up so the orbits match uses any more fuel than just launching up to get your apoapsis up to the target and then raise your periapsis. I am very sure that (if it *is* more efficient) compared to the fuel you just spent getting into orbit, the fuel you'd save from meeting this is a pittance.

[EDIT]

The more I think about it, the more I suspect that actually reaching orbit first could be better. My thinking goes like this: There is a single best way to reach orbit, and if you are trying to hit a certain point at a certain time you will deviate from that single best way by some amount. Any fuel you may have saved from doing the meeting with only 2 maneuvers (which I still don't know if it's actually any savings at all) could easily be eaten up in the fact that you are wasting some fuel overcoming your failure to do a really efficient launch-to-orbit.

Edited December 13, 2014 by 5thHorseman

[Guest]

I think the path used to get into orbit is the biggest factor. Things like where to begin the gravity turn and ship design can have big effects on the delta-v needed to achieve orbit. But all things being equal, a direct rendezvous should be the most efficient. It's just very difficult to time it right.

[LethalDose]

Well, I'm not familiar with just how big an orbit is when you state:

1. make a huge orbit

Yeah, if MJ is doing it dumb, then it's inefficient and almost anything else will be better.

But Otis hit the nail on the head:

I think the path used to get into orbit is the biggest factor.

When you say "brute force rendezvous", if you mean go straight up to the target altitude, then go straight sidewise to orbit, that's also horridly inefficient

Doing a direct rendezvous well (meaning circularizing at the target's altitude at the target's position) should save a very small amount of dV compared to going into a lower parking orbit first, then performing a Hohmann transfer to the target's altitude and position.

However the scant efficiency gains are almost meaningless compared to the greater ease of going into a lower parking orbit first.

How this relates to MJ's method, I have no idea since I don't use MJ and you haven't given us enough information to explain it.

[Starman4308]

I'd have to do it, but my gut says you would save a little fuel but not very much. In the end, you'll essentially need an orbit the same as your targets', which means you will need to pull your periapsis up to your targets' periapsis and your apoapsis to your targets' apoapsis.

The one thing I don't know for sure is if getting into a lower orbit, then raising your apoapsis up to touch the target's orbit, and then raising your periapsis up so the orbits match uses any more fuel than just launching up to get your apoapsis up to the target and then raise your periapsis. I am very sure that (if it *is* more efficient) compared to the fuel you just spent getting into orbit, the fuel you'd save from meeting this is a pittance.

My gut instinct is that you might shave a few m/s off with a very perfect ascent which goes straight from the launchpad to rendezvous without bothering to circularize into a parking orbit first.

It's unlikely to save dV in practice, though, because unless you're a robot, you'd have to correct your course a bit, whereas with the circularize-first approach, you can make very accurate maneuver nodes and spend basically all of your dV on going towards the rendezvous.

The important things, as I see them:

#1: Launching into the correct orbital plane. The closer you are, the less dV you have to spend on inclination changes in orbit.

#2: Correcting remaining inclination changes in high orbit when practical.

#3: Performing your Hohmann transfer from low orbit, where you can best take advantage of the Oberth effect.

#4: Nailing the rendezvous distance as closely as possible, and not accidentally over-burning when matching velocities.

EDIT: Ninjas. Ninjas everywhere. Also, while I use MJ, I've never used it's auto-rendezvous option: I've used it to set up inclination match, Hohmann transfer, and velocity match maneuvers, but never went to full autopilot.

Edited December 13, 2014 by Starman4308

[lextacy]

to clarify a bit, what I do for "brute force" is this

1. I use MJ's acent guidance timer that tells me to launch at a given time. Just tick the "launch to redezvous" button

2. I then launch manually getting into the targets orbit , lets say is 100km

3. As im climing to reach the potential Ap I watch 2 items, distance to target at closest, and distance to target. If these numbers keep decreasing, im doing something right.

4. Engine cutoff when poAp reaches 100km. I then look at the map and if things performed properly you will see 2 orange closest approach ticks being close to each other.

5. Then I fire up MJ and just do the "match vel at closest approach". This burn will happen at the Ap. This also doubles as the circularization burn to get into orbit. Thus the fuel savings I suspect im getting.

6. What cool about the Ap is your taget should be visible and should wizz by you. But dont freak out. Your match velocities burn will catch up to it.

7. Sometimes you need to make a few target +- burns afterwards to correct the inclination,radial sways , the dV for these would be the same as if you were going to use MJ for the entire mission.

Ive had 3 successful redezvous with this approach. Its amazing it works and saves tons of time. But the fuel saving is still in debate

[LethalDose]

to clarify a bit, what I do for "brute force" is this [words]

I believe you're basically describing what is widely described as a "direct rendezvous", and it actually requires substantially more finesse than other styles, so I wouldn't really call it a "brute force" method. it's also probably about as efficient as you can get.

However, I'm not sure how to compare this to MJ's method you described in the original post, or even if that's a comparison you're still looking to make.

[MarvinKitFox]

WIth brute force you just insert your orbit at same time as rendezvous by using a special algorithm that times when you should launch. Thats just 1 maneuver. But the big question is .....do we really save fuel?

"Brute force" is a very funny name to use for what you describe.

Like asking if the Brute Force method of playing golf is better. (brute force, in your terms, meaning driving a hole-in-one at each course)

The most efficient way to rendezvous with an orbiting ship is to launch to exactly that orbit, at the exact moment when you will be in the same place as your target when you enter the orbit.

This is about as easy as winning the lotto, three time in a row.

The best compromise, that sacrifices virtually no delta-v yet makes the job infinitely easier, is to time your launch to exactly match the inclination and argument of perigee of the target orbit, but to enter into as low an orbit as can be done efficiently.

Then hohmann transfer to the orbit, then match velocity.

[bandi94]

Mechjeb has a very funny way doing it , when i was building my Mun Orbital station he wanted to wait almost 100 day's to make the first step so yes LOL. I did manually in only 2 orbits ( max 2 Kerbin days and not 100 ...).

I don't use Mechjeb rendezvous function and i can't say that i use who knows how much fuel doing it manually in 2-3 orbit's , after i am in almost the same orbit as the target or lower and doing hohmann transfer, it takes only small burns to adjust things ( 100-200m/s at most for hohmann transfer and somewhere around 10-50m/s for adjusting things) .

[GoSlash27]

In theory, direct rendezvous is the most efficient profile because it inserts you exactly into the correct position in the correct orbit immediately. Doing a Hohmann transfer means you have to first

1) establish orbit

2) burn normal/anti-normal to correct your inclination

3) burn prograde/ retrograde to alter your eccentricity for intercept

and then

4) do the opposite of 3 to restore eccentricity.

3 and 4 are a waste of fuel because 4 erases the changes made by 3 and both maneuvers cost fuel.

2 is a waste of fuel because it's far more efficient to launch into a perfectly inclined orbit from the outset than it is to do it up there.

And also doing it all at once maximizes the Oberth effect.

Now... this is in theory. In practice, I can't say because I've never used MechJeb and therefore don't know how that mod does it. I have managed to do it by hand on occasion, and it did save me fuel. I would do it that way every time if I could, but the window to pull it off is small compared to my time to orbit variations and I'm not good enough to pull it off with that sort of precision every time.

Best,

-Slashy

[Trentendegreth]

As iv used both i will say, a direct launch is possibly irrelevant if:

while doing a Hohmann transfer setup your orbit is less than your target by 10km

while doing a Hohmann transfer your inclination is less than .1 off target (allowing you to bypass this step)

while doing a Hohmann transfer setup your willing to sacrifice time over fuel, possibly days until your in your transfer window, you would still need a transfer burn.

while doing a Hohmann transfer setup you still launch within a given window, not necessarily for a direct launch, but close enough

while building a rocket for the launch you tend to give your self a DV margin of error, this can apply to the argument of both,

a direct transfer in this instance would only save a few hundred DV at most,

however if that few hundred DV is necessary or the orbit of the target is "Kerfuffled" then a direct launch would be ideal, provided your into that kinda thing, though it doesn't leave any breathing room that establishing an orbit first, gives.

Edit: side thought

Mechjeb isnt the most efficient pilot anyway, manual "can" squeeze out that extra DV anyway, but im Lazy and when your on your umpteenth mission, you (I) stop caring...

Edited December 13, 2014 by Trentendegreth

[How2FoldSoup]

I haven't ever ran the numbers...but I've done it a few times. It was certainly more efficient on time and it felt like it was more efficient on fuel due to having to make less burns through the process. I made my intercept course...then circularized at the rendezvous.

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At any rate it certainly feels pretty cool to do it as well...no mech jeb though! It spoils all the fun!

[psyper]

I use mechjeb for its readouts rather than letting it fly - if my target is about 80km in orbit I use the distance from target to get it about 450Km away from my craft and then launch - do a normal launch, drop tanks and do a gravity turn at 10km - I also have closest distance to target and time to closest distance set up on a readout too so as I come in I watch my closest distant slowly drop as I speed up and get my ap close to 80km - at around 10km closest distance from the target I switch the navball to target mode and start to push my retrograde close to the retrograde of the target doing any inclination corrections along with this - keep your time to redezvous about two minutes - if its increasing, throttle down. Then when you're about 2km and physics kick in then its just a straight redezvous - I've done all this in about 8 minutes in game time. Your launch window does depend on how slow your launch is and how high the target is so you might need to revert and try a different distance to get it just right.

[Ziff]

http://imgur.com/a/OaIHD

What exactly is going on with this craft. How'd you make the wings fold in like that?

[herbal space program]

Echoing what everybody else has said, while a direct, perfectly efficient orbital ascent to a rendezvous would shave a little bit of deltaV off because you'd be circularizing your orbit at a higher altitude, achieving that perfect profile manually is going to be quite difficult. You are far more likely to waste more deltaV than you would have saved making course corrections to time your rendezvous perfectly. Another possible issue with doing it that way, especially if you are trying to reach a fairly high orbit, will be that the larger deltaV required to match velocities at the rendezvous point will require a longer burn, which will make timing everything perfectly so that you don't over or undershoot that much more difficult. Optimizing your ascent to the lowest, best-aligned pre-transfer parking orbit possible is a much easier and likely more fruitful exercise.

Edited December 15, 2014 by herbal space program

[lextacy]

I use the distance from target to get it about 450Km away from my craft and then launch

do you wait untill the target has already passed you overhead? Ive been launching about 150-250km (depending on craft) from target before it gets to the overhead point. I do get variances. Sometimes the target gets too far ahead when its Ap time and im forced to hohmann it up. Of course its not a waste of fuel because Id be getting into orbit anyway. So in direct target approach there is a nice cop out to do the second (long) method of rendezvous. I guess to sum it up, I think all ground launches should "try" to do a direct approach, while keeping in mind if you dont meet up within 50 km of target at Ap time, you can always continue with the standard way without loss of dV.

Edited December 20, 2014 by lextacy

[tyrant264]

unfortunately "brute force" seems to be the best way to rendezvous yes. I often have to use this method to dock and align orbits. Because let's AINT NOBODY GOT TIME for waiting for the two vessels to come within range naturally

[FlyingPete]

I'm starting to get better at timing my launches for a rendezvous. Usually though I launch slightly too early and end up going above the target altitude and making the rendezvous on the way back down, with a few correction burns here and there. If you do it right, you can do a direct ascent from Minmus with a Kerbal's jetpack and meet up with the orbiting ship.

[psyper]

do you wait untill the target has already passed you overhead? Ive been launching about 150-250km (depending on craft) from target before it gets to the overhead point. I do get variances. Sometimes the target gets too far ahead when its Ap time and im forced to hohmann it up. Of course its not a waste of fuel because Id be getting into orbit anyway. So in direct target approach there is a nice cop out to do the second (long) method of rendezvous. I guess to sum it up, I think all ground launches should "try" to do a direct approach, while keeping in mind if you dont meet up within 50 km of target at Ap time, you can always continue with the standard way without loss of dV.

Its before the target it over head - pretty much before it comes over the horizon. 450km away from your rocket - if its in a 80km orbit - I'm sure my maths teacher is having a field day when I said when would I ever need Pythagoras theorem in real life!! I'm sure it makes a triangle somewhere that you could work it out. I think if you go in to map view and look down over the north pole with KSC at the 6 o'clock position I think your target needs to be about 9-8 o'clock position. It does matter how long your ship takes to get in to orbit of course so theres that to factor in. Its always best to launch too soon than too late as you can go above your targets orbit and wait for it to catch up to you.

Just do a launch when the target is about 8:30 position and when you reach 10km do your gravity turn as normal. If you don't use mechjeb switch to map view until your rendezvous markers start to show up. The should start to show you the distance at closest approach from one of the markers - once it comes to about 20km you can go back out of map view and then switch your navball to target mode. You need to push your retrograde marker towards the retrograde marker or your target this will close the gap between your closest approach - keep them on top of each other and you will automatically circularise in line with the target - switch back to map view every now and then to check your closest approach time - you want it to be about 2minutes away - if its less keep burning full throttle and once it starts increasing start to drop off your thrust (remember dropping tanks will also drop your twr!).

I'm starting to get better at timing my launches for a rendezvous. Usually though I launch slightly too early and end up going above the target altitude and making the rendezvous on the way back down, with a few correction burns here and there. If you do it right, you can do a direct ascent from Minmus with a Kerbal's jetpack and meet up with the orbiting ship.

this is how I normally do it if I'm not doing a direct approach. Especially handy if your target it in LKO as you don't have to go in to the atmosphere to chase it down! If its too far ahead you can do a Hohmann Transfer where you circularise 10-20km outside your targets orbit - this will allow it to catch up with you. If you're able to - drop a manoeuvre node on your path and pull retrograde until the predicted path hits your targets path - then click on the node and move it around until your closest approach markers meet - thats where to drop down on to the targets orbit (DEATH FROM ABOVE) and dock with it.

[lextacy]

Here is a great video I just saw tonight on some real techniques the Soyuz uses for its redezvous procedures. Some odd ball ones are the bi-eliplical transfer and the spacing technique. I think the bi-eliptical one is the most dangerous, and the spacing manuever is very time consuming (2 days in orbit)

[Coga19000]

Well, if you feel confident enough to attempt doing it and not falling miserably, it saves a ton. Haven't done it myself, but I've run the numbers while watching Scott Manley videos

But I don't see why the Mech Jeb rendezvous method is such big a deal. I've been using it to compensate for my broken hand, and it works like a charm in rendezvous.

[lextacy]

Well, if you feel confident enough to attempt doing it and not falling miserably, it saves a ton. Haven't done it myself, but I've run the numbers while watching Scott Manley videos

But I don't see why the Mech Jeb rendezvous method is such big a deal. I've been using it to compensate for my broken hand, and it works like a charm in rendezvous.

Yes mechjeb is still a good friend . Sometimes I just need to make dinner, wash clothes ect. and mech jeb will rendezvous for me.

[Greep]

Well, if you aerobrake, you don't have to fix your orbit at rendezvous right? So that's a few hundred dV. Brute force sounds like the way to go for duna, what with the atmosphere and near identical orbit inclination. I always "fail miserably" when trying to rendezvous with eve. But then I just bring extra dV xD

Edited December 25, 2014 by Greep