Orbiter 2010 - [Part 4] Venus Slingshot - Launch Do-over
8This video was recorded Sunday, April 28, 2013
Playlist: http://www.youtube.com/playlist?feature=edit_ok&list=PLPu_SHPaJzs49WZ9RmRvL-Vl03liE0Mcx
This is not a tutorial. This is just me doing things and thinking out loud while doing it. If you're watching with the intent of learning something, then hopefully you can pick up some tips by watching me.
If you want to try this mission for yourself, you can download my scenario pack at the dropbox link below. The zip file includes all my save points during this flight as well as the associated XR2 Expert configuration file.
http://db.tt/Z6fwytEr
Required add-ons:
XR2 Ravenstar: http://www.alteaaerospace.com/index-3.html
SpaceX: http://orbithangar.com/searchid.php?ID=5442
NOTE: TransX has an option where you can inherit a plan from another vessel. It's very easy to use the function IF you have the right version of TransX. Unfortunately the version of TransX I am using is missing the EXE button so I am not able to inherit a plan from another vessel. The reason I am using a non-stock version of TransX is because TransX has a known bug in the stock version that causes it to draw graphics improperly. But apparently, for some strange reason, the version of TransX that fixes the graphics bug is missing the EXE button.
After recording this video, I talked to Dimitris and he pointed me to a version of TransX that has the graphics bug fixed, and has the EXE button. So I am now using this as my main version of TransX.
http://orbiter-forum.com/showthread.php?p=237068&postcount=1
NOTE: When I did the calculation for how much fuel I would need to complete the ejection burn back in part two, I simply took the total dV that TransX reported in stage 1 (which was 11.57k), and I subtracted the approximate orbital velocity for a 200x200 low earth orbit (which is 7.8k).
When you do that simple calculation, you come up with 3.77k, or 3770 m/s.
Dimitris (dgatsoulis) pointed out to me that this calculation is a good rough calculation, but when your dV budget is very tight, it's not quite good enough.
A more accurate calculation (provided to me by Dimitris) is as follows:
1. Open Orbit MFD and make a note of the Vel value and convert it to km/s (divide with 1000)
2. Add it to the Delta V in TransX Stage 1
3. Square the result and subtract 3.83
4. Take the square root of the result and subtract 7.79
The result is the ΔV for the injection burn from a 200x200 km orbit.
On step 3 the 3.83 number comes from the difference of the escape velocity on the surface (Vesc0) and the escape velocity at a 200km orbit (Vesc1). The relationship is this: (Vesc0)² - (Vesc1)²
On step 4 the 7.79 number is the orbital velocity at a 200x200 km orbit.
* The result has an error of +/- 10 m/s due to roundoffs.
So using our flight here as an example, here's what we come up with using this new formula.
1. OrbitMFD Vel = 408 m/s / 1000 = 0.408 km/s
2. 11.57 + 0.408 = 11.978
3. 11.978² = 143.472. Then we subtract 3.83 from that. 143.472 - 3.83 = 139.642
4. sqrt(139.642) = 11.817. Then we subtract 7.79 from that. 11.817 - 7.79 = 4.027
So from the ground, the eject ΔV from a 200 km orbit that we SHOULD HAVE come up with was 4.027k, or 4027 m/s.
Notice that this is only 4 m/s different than what TransX is reporting now that we are in orbit.
(When your budget is tight, a discrepancy of 250 m/s difference is quite substantial and can easily turn a good plan into a failure).
So thanks again to Dimitris for helping me become a better Orbinaut!
***
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Common questions I get: What screen recording software do you use? What joystick do you use? What add-ons do you use? Where can I find the XR2 Ravenstar? Where do I download Orbiter? And much, much more. It's all answered in the FAQ I set up at this link (the link is a google document):
http://goo.gl/BDoub
If you prefer the longer link:
http://docs.google.com/document/d/1HCX5lv1lxcxvS6xQBQ_V2h0Iff00nu5h7CITciYFs94/edit
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