Tutorial docking not enough fuel

[Marple]

Hi,

I tried to search for an answer on the forum, but couldn't find one.

I am working my way through the training tutorial of the KSP 1.04 and get stuck with the tutorial on docking. I manage to get in to a coplanar orbit and plan the rendez-vous at less than 5km. But somehow my burns are apparently not efficient, because when I start my brake when nearing the target I run out of fuel. I have tried it now at least 10 times and same thing keeps happening.

Any ideas as of how to make my burns more efficient?

Thanx

[Wemb]

Well - you're not going to be able to do much to a Hohmann transfer to make it more efficient (unless you're doing something that really isn't a Hohmann transfer) - my guess would be that if you're having to do an inclination change in LKO, that'll be taking up most of your fuel - that or your launch is very inefficient.

Once you're in a circular, co-planar orbit, moving from it to the orbit of your target, and then matching velocities/orbit with it shouldn't really cost a lot of fuel at all. But inclination changes in low orbit are -very- expensive.

Wemb

[Marple]

Well - you're not going to be able to do much to a Hohmann transfer to make it more efficient (unless you're doing something that really isn't a Hohmann transfer) - my guess would be that if you're having to do an inclination change in LKO, that'll be taking up most of your fuel - that or your launch is very inefficient.

Once you're in a circular, co-planar orbit, moving from it to the orbit of your target, and then matching velocities/orbit with it shouldn't really cost a lot of fuel at all. But inclination changes in low orbit are -very- expensive.

Wemb

Thank you Wemb for your answer.

Because it is a tutorial built in in KSP I cannot change the sequence of things nor build a better spacecraft. Both the inclination change and changing orbit cost around 400 dV, and then when the rendez-vous is nearing, I have to slow down from approx. 700m/s relative to the target and I only manage to slow down to approx. 200m/s and then my fuel is gone.

[Wemb]

Thank you Wemb for your answer.

Because it is a tutorial built in in KSP I cannot change the sequence of things nor build a better spacecraft. Both the inclination change and changing orbit cost around 400 dV, and then when the rendez-vous is nearing, I have to slow down from approx. 700m/s relative to the target and I only manage to slow down to approx. 200m/s and then my fuel is gone.

I'm afraid I can't help at this stage without playing the tutorial myself - which I will do tonight - but intercepting the target and then ending up with a relative speed of 700m/s seem very high to me. And if the rest of it is 'on rails' and doesn't allow you to change anything then there must be a problems somewhere...

Can you do a screen shot of the sort of rendezvous manoeuvre you're flying before you get your intercept?

Wemb

[diomedea]

Marple, believe there is some inaccuracy in the way you maneuver. The ship in the docking tutorial comes with 1562 m/s of DV to start with.

If you follow the tutorial, you'll be doing the following burns:

- plane alignment (do at the AN, not the DN), for a cost of 335.1 m/s (29s burn duration);

- raise apoapsis to intercept the target, for a cost of 285.6 m/s (23s burn duration);

- zero the relative speed at the closest approach , for a cost of 226.7 m/s (19s).

After zeroing relative speed, you would still have 715 m/s of DV, of course you need only RCS from there to dock.

However, Wemb is right that the tutorial isn't giving the most efficient procedure. If you do the following instead (without caring for the tutorial text):

- raise apoapsis to intercept target, at a cost of 280.5 m/s (25s burn duration);

- plane alignment then costs much less, only 260.5 m/s (22s);

- another burn to match the R/V position (or even some correction burns), that took me 91.4 m/s;

- zero relative speed at closest approach, for 145.7 m/s (11s);

resulting in 786 m/s of DV still available (quite close to the amount saved by making the plane alignment maneuver at a higher altitude).

Can't say from what you wrote why those maneuvers cost you so much more. You may not be keeping the maneuver node centered on the navball at all times (get help from the autopilot) or you may be burning not at the node, but too early or too late. For greater efficiency, each burn has to be done starting at "Burn_duration"/2 seconds before the node. And definitely, don't overburn, when you come close to 10m/s DV left with the current burn, cut throttle and proceed only with very small adjustments.

Edit: if it could come useful, few pics showing some of the maneuvers I did (and the mandatory end result= docked).

Edited July 23, 2015 by diomedea

[Wemb]

Hello again - had a quick look at the tutorial, and while I've not played it all the way through, I can tell you my Hohmann transfer cost me 170 dV - if yours is costing 700 then whatever it is you're doing isn't a Hohmann transfer...

You aim with those is the end up - at the point of closest approach - to be in teh same orbit as the target. You won't be going at the same speed - (if you were you wouldn't need to match velocities) - but your orbit should be tangential to the targets when you meet.

For instance, after you've completed your transfer burn, but before your circularise when you meet the target, your orbit should be eliptical and look something vaguely like this.

Here the target is Mars from the Earth's orbit around the Sun. If your Hohmann trasnfer doesn't look like this, it's not a Hohmann transfer, and that's where you're spending all your fuel.

A point to note about them is that while they're efficient, they're not the quickest way to get somewhere - usually you have to time them just right with respent to where you and your target are in relation to each other. The timing is everything - the shape of the trajectory, though, will always look like the above.

Wemb

Edited July 23, 2015 by Wemb

[Sharpy]

The tutorial tells you to use RCS for all rendez-vous maneuvers - do it. Probably the amount of fuel is calculated in such a way as to force you to do that. Instead of throttle, use "H", "N" for braking and accelerating when in range of the target.

Also, just like in Wemb's picture - the more similar your transfer orbit to the target's, the less dV it costs to match orbits. Your transfer should end with a tangent - the purple and orange markers should be really close together. If you're crossing the target orbit at a wide angle, there will be a lot of speed change needed.

Edited July 23, 2015 by Sharpy

[Nich]

Why do a plane alignment? I have not done the tutorial in quite a while but normally I just burn to the AN of DN when it gives me an intercept. I will try it in 1.0.4 and see if anything has changed.

[Starwhip]

There's been enough of these that I'm thinking of doing a tutorial of the docking tutorial.

Here's an example of the rendezvous maneuver from my docking tutorial. (Click the image in my signature for the entire tutorial) The intercept is quite far from the maneuver node (location of burn).

EDIT:

I realize that the context for the burn is completely different from the tutorial. No inclination, and it's from a parabolic (suborbital) trajectory, but all I was trying to show was the distance between burn and intercept.

Edited July 23, 2015 by Starwhip

[The Science Bucket]

I could never get the stock docking tutorial anywhere near the target ship. This video really helped me understand all that goes on.

I watched it a couple of times and then built my own ship with waaaaaaay too much fuel and RCS to allow for a lot of first time mistakes. I get into orbit without using barely any fuel from my 2nd stage rescue capsule and have a ton of RCS for close in maneuvers. I'm up to five rescued Kerbals now and getting better/more efficient with every rescue. Just build your own ship and keep practicing!

[Marple]

Thanks guys for all the help. Allthough I have not accomplished the docking itself yet, I have managed to come as close as 200m to the target and still have plenty of fuel left. The main thing I changed was combining the inclination change with the Apoapsis increase which saves about 150-200 dV.

[Sharpy]

BTW, did anyone try *finishing* the docking tutorial? I mean, after you dock, bringing them to Kerbin?

I transferred them to my craft, prepared a deorbit burn, entered the atmosphere, then pressed space to decouple the tank and the engine...

Whoops.

I wonder what kind of psychopath at Squad decided upon that placement of the decoupler. I really didn't notice it until it was too late...

[klesh]

I knew a guy that played KSP for hundreds of hours. He confessed to me at one point that rendezvous and docking was something he could never figure out. If you mange to get it, do it, and understand it, you'll be well on your way to some awesomely fun times in this game.

[Alshain]

FWIW, the tutorial on rendezvous is terrible. You don't need to come to a full stop at 50km. As you start to understand what the craft does and the navball looks like while performing maneuvers, you will realize you can move the markers around without coming to a full stop till you are nose to nose with the docking port.

It's simply a matter of "pushing" your retrograde, and "pulling" your prograde. If you burn slightly ahead of your prograde marker, it will come to you. Likewise, the retrograde will move away from you. All you have to do to rendezvous is either pull the prograde to the target prograde (pink circle) or push the retrograde to the target retrograde. The latter is usually the choice because burning retrograde will also slow you down which you will eventually need to do anyway.

Coming to a full stop and then burning toward the target and then coming to another stop and then burning toward the target again is just a waste of energy. It's like stop and go traffic vs highway driving.

[Scarecrow88]

BTW, did anyone try *finishing* the docking tutorial? I mean, after you dock, bringing them to Kerbin?

I transferred them to my craft, prepared a deorbit burn, entered the atmosphere, then pressed space to decouple the tank and the engine...

Whoops.

I wonder what kind of psychopath at Squad decided upon that placement of the decoupler. I really didn't notice it until it was too late...

I've just done the tutorial to see what it expects you to do. First thing I noticed was that it asks you to correct the orbital alignment at the descending node, when it's the ascending node that is closest to your ship. I also didn't reduce my closing speed to 0 m/s, and as a result of that the tutorial was of no further use as it was stuck on that bit with no opportunity to get it to progress to the next step.

However, I did notice the placement of the decoupler on the rescue craft and decided to quit the tutorial before performing a de-orbit burn.

All I can say to the OP is that the rescue craft has enough fuel to complete the mission, and that at no point did I encounter a required burn of 700 m/s so I can only assume that you are doing something wrong somewhere along the way.

[Kryxal]

You're right, some sick puppy decided that Jeb, Bill and Bob didn't need chutes ... none on the stranded craft, none on the part of the rescue ship that's decoupled with the extra storage (that they're obviously supposed to ride DOWN!) and chutes and decoupler are in the same stage...

Oh, and it gets better ... I managed to splash down with the whole rocket, full thrust, and still lost the three to be rescued, I should have tried for land instead.

Edited July 23, 2015 by Kryxal