First TransX-guided flight, lunar return

[cr1]

I was following this tutorial an hour ago (http://flytandem.com/orbiter/tutorials/lunar_return/index.htm).

First time I'm touching TransX... while in lunar orbit, adjusted parameters, etc, so that periapsis was at ~500km. Lunar ejection burn done successfully, Delta-V correction was only 0.0009m/s.

Well on my way to Earth. I was a little freaked out when I saw that I've passed into the Sun's sphere of influence according to Orbit MFD (even though I'm technically always in solar orbit). Pressed 't'. Then I got into Earth SOI. Fired to get periapsis to 80km. Checked Orbit MFD again two minutes later, it says PeR ~80km, inside Earth SOI. Everything seemed OK to me.

Then I thought something was wrong when I saw the velocity marker, inside the Earth globe. I switched back to Orbit MFD and it still said PeR ~80km. This time I seemed to remember something, I pressed [DST] and it showed a huge negative number. Oh , maybe something's wrong. Since I thought that to raise periapsis you had to burn prograde, that's what I did. I did not think further. The PeR was indeed rising. But by the time PeA was at ~-5M I was already at 1200km altitude. Pressed 'r' many times. Switched to Surface MFD, it says I'm going at ground speed 17m/s and accelerating, with vertical speed very big.

No prize for those who know what happened afterward.

 

[flytandem]

First time I'm touching TransX... while in lunar orbit, adjusted parameters, etc, so that periapsis was at ~500km. Lunar ejection burn done successfully, Delta-V correction was only 0.0009m/s.

Perhaps you set stage 2 (the trip moon to earth) with a PeD of ~500 km? If so it pretty much did about what you set up, meaning you executed the maneuver very well.:speakcool:
You aimed relative to planet center thinking you were aiming relative to planet surface. You should have set the return PeD to more like 7000 km which is a few hundred km agl since the planet radius is about 6371 km.

 

[Andy44]

I always wind up with a subterranean perigee when using TransX to return from the Moon. Only solution is to aim higher, as far as I know.

 

[HiPotOk1978]

got to remember to watch for perigree measurements, be it radius or altitude. I have a similar problem with "The Fundamentals of Astrodynamics". It is an older book and was written with imperial measurements. Made a few mistakes in some of the knowledge that it gives me such as nauticle miles vs miles. Always sucks to know you did your math right, but screwed up with conversion from X to metric

 

[cr1]

Yea, I did make PeD to be at 500k thinking it was 500km above surface rather than above Earth center... Thanks for your answers! I'll definitely try to remember this in future flights

Would it have been possible to correct this during mid-flight? Like when I was still at about 100M from Earth?
I'd definitely try this again. What is the (exact) Earth surface's distance from the Earth's center in Orbiter?

It was really fun though, watching the stock DG getting fried as it went thru the atmosphere... VS was ~-22000m/s, and vertical acceleration once reached somewhere like +5000m/s^2 (500G's?!)...
If a meteor about the size and mass of a DG plunged into Earth in the way I did, what would have happened? Curious

 

[2552]

In that situation, a more effective way to raise the Pe might be to turn towards 90 degrees on the Orbit HUD (not normal/antinormal; the equivalent of Outward vel in TransX) an burn that way. This will raise the Pe directly without accelerating you towards Earth.

 

[Andy44]

I use the above-mentioned turn 90-degree method when I get to the point where Earth gravity is at about 0.75 influence in the Orbit MFD. Then I do one or more fine tunings of my periapsis altitude as I get closer to Earth.

If you're flying Apollo, you want to hang on to your propulsion module until you are getting close to re-entry so you can make final adjustments, because once you jettison it you are pure ballistic and can't make any more changes. Flying Apollo forced me to become a more efficient and precise pilot in translunar flight, since the Apollo CSM has very limited delta-V compared to the DG, which spoils you with it's lavish fantasy Isp. At some point, you've got to take off the training wheels.

 

[Tommy]

That's one of the reasons I prefer IMFD for simple flights like that. TransX has capabilities that IMFD doesn't, especially for multi-sling trajectories, but for something as (relatively) simple as a lunar return, IMFD works better for me. Even if you want to use TransX for a flight like that, IMFD's Map Program is extremely valuable for it's accuracy. You can see if there's a problem with you trajectory far sooner and more accurately than OrbitMFD will show.