Why may my Minmus capture burn require too much Δv?

[sebjf]

Hi,

I am on my first Minmus mission. The delta-v maps say that I should require ~180 m/s^2 to get into orbit. I can get onto an intercept trajectory with the suggested delta-v of about 930, but when I place a manoeuvre node at the periapsis it takes over 400 m/s^2 to capture. I have tried launching a number of times but always the result is the same. My trajectory isn't being perturbed by Mun. I am getting to within 65-100 km of Minmus. 

I was wondering if that estimate expects a free-return trajectory, but none of the tutorials say that?

What about my trajectory can change the capture speed? 

Sj

[Starman4308]

Short version: Minmus injection burns do not actually require 930 m/sec of delta-V. Those dV maps are more for planning out how to build your ship, and so include a small overestimate for safety.

If it takes that much to capture, that's a clear indicator that you're going into Minmus's SOI with excessive velocity. Dial back your Minmus injection burn until you have the bare minimum of delta-V expended, and attempt to capture when traveling approximately parallel with Minmus.

[FancyMouse]

1. Make sure you're doing a Hohmann transfer (i.e. arriving in about 10 days). Any other time will increase capture delta V required. Not all 930 m/s maneuvers are the same.

2. Make sure your Minmus periapsis is within 20km. That's when burn is most effective (Oberth effect).

If there's still problem, better post a picture for better investigation.

Edited December 4, 2017 by FancyMouse

[Human Person]

First of all, the unit of delta v is m/s. m/s^2 is a Unit of acceleration. 

How much your capture burn costs depends on the kind of transfer you chose. Delta v maps assume Hohman Transfers, wich require you to intercept minmus at your Apogee. If you hit Minmus before your apogee (a fast transfer), or after your apogee (a slow transfer), you will need a bit more to capture as you are traveling at a higher relative velocity. 

[Guest]

4 hours ago, sebjf said:

The delta-v maps say that I should require ~180 m/s^2 to get into orbit.

Getting into the bare minimum elliptical orbit (capturing) is different than circularizing at a low orbit, what orbit was specified on the map and what were you trying for?

[Plusck]

The key figure you want is "46,000,000 m". Or close to that number.

That is the altitude that your Ap should be at when you finish your 900+ m/s burn out from LKO. You may need a normal/antinormal burn halfway out to Minimus to actually capture. If your Ap is much higher than that, you will have to spend significantly more than the usual 200 m/s or so dv to capture.

 

The easy way to do it: just like for the Mun, place a node at the point that Minmus just appears above the horizon in front of you. Drag the prograde vector until you get an Ap of 46,000km. Drop a second node halfway there and add/subtract normal until you get a hit. Right-click the Pe at Minmus and then gently drag and/or adjust the two nodes to get it down to 20km or so. That should give you approximately 200 m/s to capture.

Edited December 4, 2017 by Plusck

[Snark]

4 hours ago, sebjf said:

What about my trajectory can change the capture speed?

Lots of good advice already in this thread.  One thing that can help "focus" the advice is if you could post a screenshot  of your transfer orbit, in the map view.  That is, after you do your LKO burn to head for Minmus.  There are lots of potential reasons why your arrival may be going too fast, but having a screenshot will help us know what the actual situation is.

With that in hand, you can get an actual "here's what you're doing wrong and here's how to fix it", rather than our having to just give you a laundry list of "here are all the potential problems that  might be going wrong."

1 hour ago, FancyMouse said:

Make sure your Minmus periapsis is within 20km. That's when burn is most effective (Oberth effect).

You are technically correct... but it's worth noting that this is probably irrelevant in the case of Minmus.  Minmus' mass is so tiny that Oberth effects are fairly minimal there.  The difference between orbital velocity and escape velocity at low altitude over Minmus is only 70 m/s.

Whatever the OP's problem might be ... it's almost certainly some issue with the transfer orbit, and likely not an issue with "Minmus Pe too high so missing Oberth benefit".  An Oberth problem at Minmus would be a matter of a few dozen m/s at most, not the hundreds of m/s that the OP is reporting.

[Aegolius13]

15 hours ago, sebjf said:

I was wondering if that estimate expects a free-return trajectory, but none of the tutorials say that?

A free return trajectory has no effect on delta-v needed to capture around Minmus.  A free return trajectory is a form of gravity assist, where the target body's gravity removes some of your orbital velocity around the original body (or at least does not add any).  However, gravity assists only affect your orbit after you leave a flyby of the target body.  So if you're capturing, it's not applicable.  

Put another way: to perform a free return trajectory around Minmus, you'd need to approach it retrograde vs. its rotation/revolution.  But doing so would not result in a Minmus capture burn that any higher, or lower, than a a symmetrical from the other direction.  

[sebjf]

Thanks for the replies everyone!

Quote

the unit of delta v is m/s. m/s^2 is a Unit of acceleration. 

Right, Δv != Δv/t, D'oh!

Quote

Make sure you're doing a Hohmann transfer (i.e. arriving in about 10 days). Any other time will increase capture delta V required. Not all 930 m/s maneuvers are the same.

OK this is what I am doing wrong!

Below is a screenshot of my trajectory (different PC so didn't have it yesterday). I will try again without shooting right past it...

Edited December 5, 2017 by Snark
fixed broken link for you ;-)

[Plusck]

1 hour ago, sebjf said:

OK this is what I am doing wrong!

Below is a screenshot of my trajectory (different PC so didn't have it yesterday). I will try again without shooting right past it..

 

Yes, you're going way too fast.

If you look at KER's Apopsis numbers at the top left, you have a negative number. That means you are on an escape path out of Kerbin's system.

Also just looking at your path, that is way too straight.

Again, the number you want at "Apoapsis" (sans capture by Minmus) is roughly 46,000km. That means that your vertical velocity relative to Kerbin is about zero when your ship encounters Minmus. The only speed change needed to capture is therefore the difference between your horizontal velocity relative to Kerbin (which will be very low) and Minmus's orbital velocity (which is apparently 274 m/s). Your ship's path should describe a graceful curve until it just clips Minmus's orbital path and exactly the time that Minmus comes to occupy that spot.

[Zhetaan]

1 hour ago, sebjf said:

Below is a screenshot of my trajectory (different PC so didn't have it yesterday). I will try again without shooting right past it...

Three days is an insanely fast transfer.  I'm a little curious to know how you're getting such a trajectory if you're burning only 930 m/s in low Kerbin orbit; such a burn should keep you within Kerbin's sphere of influence, if only just.  Since it does not, there is something else at work, even if it is only that you are burning far more than 930 m/s without realising it.

When you set the node for the Minmus transfer burn, do you do so using only the green prograde/retrograde adjustments, or do you use the magenta normal/antinormal toggles to change your inclination in the same burn?  If so, then that could explain why your capture burn is an extra 400 m/s--it can take about that much to change inclination in low Kerbin orbit.

You are far better off correcting your inclination in a separate burn, either in low Kerbin orbit or as a mid-course burn where it isn't quite so expensive.  Better yet, you can try launching directly into an inclined orbit, but you have to do it at the right time of day.

[Snark]

8 hours ago, sebjf said:

Below is a screenshot of my trajectory

Yeah, that's going way too fast.

Also, it's going the wrong direction.  Notice that your path is going across Minmus orbit, practically at right angles to it.  That means that you have a high Minmus-relative velocity.

For any encounter:  to have a low relative velocity, you need to be going parallel to them.  In other words, you don't want your path to cross the other body's path like an X.  You want it to be tangent, i.e. at the point of intersection, your ship and the body are moving in the same direction.

Basically, what happened here is that you burned way, way, way too hard in Kerbin orbit, streaking across Kerbin's SoI and hitting Minmus going too fast and in the wrong direction.

What you need to do is to burn so that your Ap is only as high as Minmus, so that at the point of intersection, you're traveling in the same direction that Minmus is going.  Minmus is going faster than you, so it catches up to you-- that's the encounter speed.

[Aegolius13]

10 hours ago, Zhetaan said:

  I'm a little curious to know how you're getting such a trajectory if you're burning only 930 m/s in low Kerbin orbit; such a burn should keep you within Kerbin's sphere of influence, if only just.  Since it does not, there is something else at work, even if it is only that you are burning far more than 930 m/s without realising it.

That may be a result of his altitude prior to the burn.  In the pic it's almost 200 km.  If the burn started around that altitude, OP had already invested some delta-v getting out of Kerbin's gravity well, vs. starting from 70km or so.