When to do the de-orbit burn?

[Oxyd]

I've re-entred few times already, but always I ended either 2000 km before the Cape  or 2000 km after the Cape... =) . It seems, I'm unable to find the point to when de-orbit... I wonder what method to use to get
as close as possible.

Oxyd



Post Edited ( 07-13-04 18:44 )

[DocHoliday]

That's my next project. Assuming the DG3s descent computer does its thing equally well every time, it should just be
a matter of running a few tests and see the best result..

[Wes]

Hey Oxyd,

Check out the topic "Can't Dock" - The guys have helped me alot with docking, de-orbit burns and re-entering! Maybe
it'll help!

Cheers,
Wes.  

[Arkalius]

The problem with setting up procedures for deorbit burns is that your altitude plays a big part in deciding when to do
the burn. You obviously can't be too high to deorbit, but a difference of 100km in altitude is enough to require a
change in the point of your burn by a few thousand kilometers or more.

[DocHoliday]

Arkalius. I know.. The higher your apoapsis, the higher the reentry angle, and more importantly the reentry speed...
the longer it takes to reduce it...

 I'll run a series of tests, changing just one variable at a time if possible and try to get some meaninful correlation.. I
such at math, so I can't calculate it directly

[Arkalius]

The most annoying part of doing this kind of thing is that you cannot time accelerate the reentry, so it takes quite
awhile to get experimental data...

The best way I think would be to set up a circular orbit at about 200km that passes over the cape, and have the
orbiter start on the opposite side of the planet from the cape (180 degreesish). Mark down the exact degrees
distance when you finish your deorbit burn (to a reentry angle of .70 to start), and then let the thing play out using
the autopilot. Then, once you have slowed to sub-sonic, mark your exact degrees distance from the cape, and
subtract from where you started, and that will give you the degrees distance you covered from deorbit burn to
subsonic, which tells you about how far away in degrees you want to end a deorbit burn around 200km alt with a
reentry angle of .70 (I like that angle, nice and gentle reentry).

Then you can repeat the experiment changing only one variable each time, like increase altitude by 50km, or increase
reentry angle to 0.8

If you want, I can help you out in this experiment...

[Arkalius]

Well I did a few test runs... The distance travelled is the degrees of distance from the end of the deorbit burn to the
point when speed drops below mach 3 (this is the point where I disable the autopilot and take control).

200km alt, 0.70 reentry angle, 133.11 degrees travelled
250km alt, 0.70 reentry angle, 152.03 degrees travelled
300km alt, 0.70 reentry angle, 163.37 degrees travelled

0.70 degrees for reentry angle seems to work quite well, as in each instance the max G's from deceleration was 2.9
which is well within tolerance.

From 200km to 250km alt, the distance travelled jumps nearly 20 degrees (around 2000km) but, from 250km to
300km alt, the distance travelled only jumps around 10 degrees. My guess (and I will test it) is that the distance
travelled for an alt of 350km will be around 168 degrees. Also, if this pattern of halving the extra distance every 50ish
km is correct, then by extrapolation, the distance travelled for an altitude of 225km would be around 146 degrees,
and for an altitude of 275km, it would be around 158 or 159 degrees. I will test these altitudes and see what I come
up with.

[Pierre_le]

use the excellent BaseSync MFD to predict your drop point on the planet. it looks and feel like the OrbitSync instrument

set "cape canaveral" as target and do +/- orbit normal burns to get the distance closest to 0m. you will have to
correct your trajectory later as you lose speed to keep the distance as lower as you can. change bank angle using
keypad 4 & 6 while running reentry 104s40 autopilot.


http://koti.mbnet.fi/jarmonik/Orbiter.html (bottom of page)

[DocHoliday]

Pierre. I'm not sure, does the BaseSync also predict the effect of atmospheric drag? I know it works perfectly in non-
atmospheric landing.

Great work Arkalius. This sort of tables could very well be included in the DG3's manual!! And your approach is exactly
what I wanted to do myself. I will still do it, to test the reliability of the figures.

What kind of a periapsis did you use for deorbit burn???

[Simonpro_2]

Of course you can time accelerate during reentry, i usually run the whole thing at 100x or else i get bored :p
Generally i perform a burn on the west coast of australia, that brings me into the region of the cape. More important
than where you burn is how you handle the ship during reentry. A shallow AoA will get you much further, useful if you
overburn, and a high AoA will bring you up shorter. Useful if you underburn.

[DocHoliday]

Agreed Simon.  (what happened to your username?)

But handling the ship during reentry is advanced flying, not suited for newbies. The temp is quite sensitive to quick
jerky changes of attitude, plus I was never able to properly execute the shuttle-style S-turns Also, it's quite hard to
keep yourself from "bouncing off" of the atmosphere if you keep the AoA too shallow, alternativelly, you have to deal
with higher temp and potential excessive altitude loss if you go for a high AoA. As someone put it. It really is more art
than science sometimes

Anyway, the point is that since you're most probably using the reentry autopilot (unless, that is TOO boring, hehe)
this part of the flight path is more or less fixed and as such predictable. So basically the geographic location of
periapsis, angle of reentry then determine when and where you will find yourself at the end. That's my way of
methodically approaching the subject.. Change one variable at a time or as close as you can get to that

Cheers,

[Arkalius]

I didn't record my periapsis, I just used the deorbit screen on the DGIII computer to determine when to stop the
burn, and each time I stopped at an angle of 0.70. I do recall that on the 200km alt, the periapsis was somewhere
around 35km altitude. I time accelerate until I reach about 130km alt, then level out and pitch up to around 40
degrees, put time accel back on and just continue pressing killrot to hold me somewhere near 40, then as I dip below
100km, I kick in the DGIII reentry autopilot with a 40 degree AOA and let it go from there. As soon as my speed drops
under mach 3, I pause and take a reading on angular distance. The orbit I used has its highest lattitude at the cape,
which seems the most "direct" path to me.

The ELEMENTS I use for the orbit are here:

ELEMENTS 6671010.0 0.0 49.0 204.0 0.0 132.5 53024.75
AROT 157.05 51.03 -150.78

If you set the scenario's time to be the same as that elements' epoch, you will be nearly 180 degrees away from the
cape, and will reenter into the atmosphere during midmorning (so there's plenty of light). You can obviously modify
the first value for altitude (that one is set for 300km). I included the AROT value, this will have your ship pointing
prograde at the beginning of the scenario.

[DocHoliday]

Yup, we're getting close here..

- A target angle of reentry of 0.7 deg
- Keep your nose around 40 deg below 100km
- I particularly like Pierre's input that you should keep your vertical velocity around -100m/s, but that would obviously
have to be a little higher if you start from a 200km apoapsis.

After Dan fixed the temp bug, reentry makes a whole lot of difference. I used to burn up almost EVERY time, because
it's kind of hard NOT to exceed 600deg or something like that:)

[AphelionHellion]

Wow, I dunno if this is scary or cool...
Last night I dreamed that I made a perfect re-entry and landing (though I didn't wind up at KSC's runway, it was Dallas-Ft
Worth or something)
When I get back from work I'm gonna see if my subconcious managed to figure out any improvements in my procedure

[Arkalius]

I have made a perfect reentry in the DGIII before. I had did a nice little mission to the moon and back, and when I
got back to Earth and lowered my apoapsis, my orbit happened to be perfect for a landing at KSC. I obviously already
set up the inclination for it beforehand, but the timing was such that it was passing over KSC right as I got to Earth. I
had to do the deorbit burn a second time because I wasn't sure how far away I needed to do it the first time, but
after that, the whole thing went flawlessly and I landed at KSC just about a week after having left.

[Wes]

Hey Guys,

I got the BaseSyncMFD and to open it, it says to press "SHIFT-B", but when I do, Orbiter shuts down!!
It says I can change the key in the "IMFD.cfg", which I found, but I dont know how? Im no programmer!
Please help!  

Cheers,
Wes.  

[Wes]

OK, nevermind - got it to work!! Just had to hit it several times!!  

Cheers,
Wes.

[DocHoliday]

Good, because I had no advice other than get it using the SEL button

[Wes]

LoL - WoW - Its great to know people are looking out for you!!    

Cheers,
Wes.

[Simonpro_2]

I'm not at home and i cant remember my password for the other username, i always use the "remember password"
feature to save my mind

[BigSmartChimp]

Alright... i've tried a bunch of rentrys using the above data table... but my ranges are way off.  I seem to travel almost
10 degrees farther than you did.  Maybe it's a silly point... but does the mass of your ship have anything to do with
it?