Jump to content

Just how important are ejection angles, anyway?


Khrissetti
 Share

Recommended Posts

I've gotten to Eve multiple times with fuel to spare simply by waiting for a ~60 degree phase angle between the two worlds and starting my burn at midnight. Last night I attempted to put the Budapest 1 into orbit around Duna (any Hungarians get the joke?) and found that even by launching at the precise phase angle at midday, I was still very far ahead of Duna when it came to intercepting the red planet to the extent that I had to burn ahead of Duna then fire retrograde to get recaptured and rely on a hair-raising <20,000m aerobrake to actually get me into orbit.

I'm guessing that my 4,000m/s(ish) delta-v maneuver could have been significantly lessened by launching at the correct ejection angle, but does it really make a huge difference, or is there still usually a need for large correction burns in interplanetary transits?

Link to comment
Share on other sites

The ejection angle defines the direction in which you will be flying. I figure it's more important than the phase angle (which can be off a degree or two). Whatever you do, without super-precise calculations, you will always need a mid-course correction burn. Even NASA does them, and they are usually better at maths than I am ;)

Link to comment
Share on other sites

Well most of that 9000 m/s comes from kerbin's orbit so really the ejection angle only effects the velocity you have just before leaving kerbin's sphere of influence. So ejection angle gets more important the further away from kerbin you want to go, its not so important for Duna or Eve but quite important for Jool

Link to comment
Share on other sites

found that even by launching at the precise phase angle at midday, I was still very far ahead of Duna when it came to intercepting the red planet to the extent that I had to burn ahead of Duna then fire retrograde to get recaptured and rely on a hair-raising <20,000m aerobrake to actually get me into orbit.

If you are starting your burn at mid-day, then you are probably starting your burn too early. The periapsis of your hyperbolic Kerbin escape trajectory will move during the burn because you don't instantaneously accelerate to your escape velocity, but it won't move by that much. I don't know how the various mods like MechJeb or Protractor account for this effect because I've never used them, but for reference I started my Duna transfer burn 20 minutes and 44 seconds after Kerbol rise, as viewed from my 100 km parking orbit.

Below are screenshots that I took during my first (and so far only) attempt to reach Duna. While I did require orbital corrections en route to lower my periapsis at Duna, it wasn't more than a few m/s.

The RCS tank in the spacecraft is empty; the boffins at KSC re-fitted it with sensors to measure Duna's atmospheric properties during descent.

bNAMW.jpg

Transfer trajectory (screenshot taken 6 minutes and 45 seconds after the one above, immediately after the transfer burn and trim were completed.)

JOKbU.png

Also, it is relevant and interesting to point out that NASA intentionally biased Curiosity's initial transfer trajectory so that it wouldn't intercept Mars [Ref. F. Abilleira, 2011 Mars Science Laboratory Mission Design Overview]. They corrected the trajectory only after the spacecraft separated from the Centaur upper stage. This ensured that the Centaur booster would not crash into Mars, which would have violated their planetary protection criteria.

Edited by PakledHostage
Link to comment
Share on other sites

Protractor have an "adjust for TMR" option which gives you a recommend angle for ejection burn depending on the TMR of your craft. You just have to wait till the adjusted ejection angle reaches 0, fire up your engine to max and keep your nose on prograde. By the time you've reached the required speed your angle should be just right so that the sum of your acceleration vector during your burn adds up to be the same as the impulsive burn ejection angle.

27ywv8o.jpg

In my case though I had to start the burn 4 degrees before the adjusted ejection angle because it doesn't understand that my craft's TMR is lower than what it appears to be since I have a big hulking 50 ton lander docked on the nose.

Link to comment
Share on other sites

Yesterday, I used the protrator mod to help me deliver a moderately sized lander to Duna's surface. In the end the mission was a success and my lander reached its target with enough fuel to return to orbit. However, I found that even though I departed Kerbin at the right time according to the information I was provided, I missed Duna by quite a wide margin on my first orbit of Kerbol. This got me to thinking that it might also be important not just to depart at the right moment but to be able to accelerate fast enough that you hit that window at the proper time as well. My interplanetary stage obviously lacked enough thrust, but I'm wondering if it would be possible to calculate or adjust your departure position to account for very slow acceleration and if that's something that a layman could do without too much trouble. Anyway thought I'd through that out there, my experience did make me curious.

Link to comment
Share on other sites

Well, they're both important. I'm not sure which has more tolerance for error, but being dead-on for one will not correct for being way off on the other. If your phase angle is off by 180 degrees, it doesn't matter how precise you were with your ejection angle, you've missed.

Link to comment
Share on other sites

You can be 10° off as long as you watch the map for the intercept when you burn. Phase is actually more critical IMHO especially where intercepts are at PE.

Duna eject burn is more like 8pm so midday is about 130° out...

Edited by boolybooly
Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...