Friday, November 18, 2011

Mars Science Laboratory 2

The first thing to do of course is research. Maybe someone else, like the guys at JPL, have already published a trajectory. Poking through the files at JPL NAIF, I found a couple of kernels for MSL. One is labeled cruise, and one is labeled EDL. Now since there is no target set yet, these are obviously preliminary. I have read somewhere that this target is near the point that is easiest to retarget to any of the four candidate landing sites. These kernels are still interesting for just seeing what EDL is like. this is the comment section of the EDL kernel:

MSL Sample EDL Trajectory SPK File (Central Landing Site (0.0 N, 45.0 E)
==========================================================================

Created by Fernando Abilleira, MSL MD/NAV. Comments added by
Boris Semenov, NAIF/JPL on Tue Feb 23 16:34:24 PST 2010.


Objects in the Ephemeris
--------------------------------------------------------

This file contains sample MSL (NAIF ID -76) EDL trajectory, from
atmospheric entry interface point to landing, for Type 1B, open of
launch period (11/25/2011), optimal launch time, central landing
site (0.0 N, 45.0 E), landing time 06-AUG-2012 11:35:46.7 UTC.

THIS FILE SHOULD BE USED FOR TESTING PURPOSES ONLY.


Approximate Time Coverage
--------------------------------------------------------

This file coverage is:

COVERAGE BEGIN TIME (TDB) COVERAGE END TIME (TDB)
------------------------- ------------------------
2012 AUG 06 11:30:58.537 2012 AUG 06 11:36:52.837


Pedigree
--------------------------------------------------------

This file was provided by Fernando Abilleira, MSL MD/NAV on
February 23, 2010. The original name of this file was
'day1_0.edlTraj.bsp'.


Contacts
--------------------------------------------------------

If you have any questions regarding this data contact

Boris V. Semenov, NAIF/JPL,
(818)-354-8136,
Boris.Semenov@jpl.nasa.gov


This particular kernel is just a list of positions and velocities at specific times, typically 0.1s apart. The attached CSV is a dump of the kernel at those times, plus some bonus stuff you can read about in the file.

And here it is:


A couple of comments:
12 earth g's on entry! Phoenix peaked at 8.
It flies for a long time, starting to gain altitude at about 110s and not opening the chute until 250s

However, closer examination of this kernel yielded only disappointment.

Excursion funnels are part of an investigation into how well test subjects can solve problems when traveling through a churning funnel of liquid asbestos. Results so far have been highly informative: They cannot.

It doesn't look like this spice kernel can be used for the kind of aerodynamics model I am looking for. It just isn't of sufficient fidelity. For instance, the vehicle never has any significant side lift during the fight portion of the kernel, where we would expect the spacecraft to bank side-to-side. It just pitches further and further down, generating more and more lift, beyond any amount I have seen documented. Also, there is no orientation data associated with this kernel. I can fake it from the lift/drag ratio, but there is nothing authoritative.

However, there are several spacecraft events visible, including the jettison of the six entry balance masses, and I think the heat shield jettison. Data before about 40 seconds is unreliable because there is no significant air, and data after the parachute deploy at around ~250s is unreliable since my code doesn't take into account the new area of the parachute. The PDV separation at ~340s is visible, but the spice kernel ends at about 90m altitude above ground level and 2.5m/s verical speed.



So, what I am doing instead is using the Phoenix aerodynamic model. Apparently, entry capsule aerodynamics is so dominated by the heat shield that it hardly matters what the shape of the backshell is, and with proper scaling, the Phoenix coefficients can be applied to MSL directly. One of the interesting things is that the lift/drag ratio is an almost linear function of angle of attack and not a strong function of speed. So, by looking at the L/D from this plot, I can back out the angle of attack.

Further research reveals the no side-lift thing is intentional. See here .

Movie is in progress, but because of various glitches in the kernel, I'm not sure it will ever be worth publishing. What I would like to do is get the Orbiter model up and running, write a realistic entry guidance autopilot for it, and then collect data and make the movie with that.

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