Retro-fire Attitude

Why is it that spacecraft tend to adopt nose-down attitudes for
de-orbit burns?

If the acceleration vector were instead precisely anti-parallel to the
velocity vector, then 1) a smaller impulse would suffice to reduce the
pericenter by a given amount, and 2) the re-entry trajectory would be
shallower, which should give lower g-loads and perhaps lower heating
loads as well.

If one wanted to de-orbit quickly, an inwardly directed acceleration
vector would make sense. But for a typical civilian mission, there's
no big hurry.

A slightly inwardly directed acceleration might make sense just to be
sure that control errors do not accidentally result in an outwardly
oriented acceleration, but it seems to me that this does not explain
30 or 40 degrees of nose-down pitch.

"Proponent" wrote in message
m...
Why is it that spacecraft tend to adopt nose-down attitudes for
de-orbit burns?

If the acceleration vector were instead precisely anti-parallel to the
velocity vector, then 1) a smaller impulse would suffice to reduce the
pericenter by a given amount, and 2) the re-entry trajectory would be
shallower, which should give lower g-loads and perhaps lower heating
loads as well.

I assume you're talking about the Shuttle. This attitude puts the thrust
vector anti-parallel. Remember, the thrust goes through the center of mass,
not along the axis of the Orbiter. The trajectory won't be shallower.

If one wanted to de-orbit quickly, an inwardly directed acceleration
vector would make sense. But for a typical civilian mission, there's
no big hurry.

Nope. Any thrust away from the velocity vector is mostly wasted.
--
James Summers
IBM-ret, "old space guy".
Apollo 201, 202, 203, 204, 1, & 9 RTCC Support. Apollo 13 "back room".
email to: UseNet1 ayt mcsummation dotting com

(Proponent) wrote in
m:

Why is it that spacecraft tend to adopt nose-down attitudes for
de-orbit burns?

Which spacecraft are you referring to? The space shuttle typically deorbits
with the tail (mostly) pointed into the velocity vector, producing the
anti-parallel acceleration you describe.

If the acceleration vector were instead precisely anti-parallel to the
velocity vector, then 1) a smaller impulse would suffice to reduce the
pericenter by a given amount, and 2) the re-entry trajectory would be
shallower, which should give lower g-loads and perhaps lower heating
loads as well.

1) is correct, but 2) is not necessarily true; much depends on details.

Space shuttle deorbit burns are *mostly* retrograde. Usually, the PEG4
targeting algorithm adds a small radial component to shift the line of
apsides over the proper point of the Earth, and sometimes an out-of-plane
component is added to waste OMS propellant to put the orbiter CG in the
proper place. But those components are usually small compared to the
retrograde component.

If one wanted to de-orbit quickly, an inwardly directed acceleration
vector would make sense. But for a typical civilian mission, there's
no big hurry.

A slightly inwardly directed acceleration might make sense just to be
sure that control errors do not accidentally result in an outwardly
oriented acceleration, but it seems to me that this does not explain
30 or 40 degrees of nose-down pitch.

Where are you getting this 30-40 degrees of pitch? The shuttle pitches 10
degrees nose-down, but that's because the OMS engines are canted to point
through the CG of the orbiter, so the orbiter must pitch to put the thrust
line anti-parallel to the velocity vector.


--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

Where are you getting this 30-40 degrees of pitch? The shuttle pitches 10
degrees nose-down, but that's because the OMS engines are canted to point
through the CG of the orbiter, so the orbiter must pitch to put the thrust
line anti-parallel to the velocity vector.

I think he's referring to earlier generations of ships (especially Gemini
and Mercury). They pitched down about 40 degrees from retrograde for their
deorbit burns.

"James Summers" wrote in message om...
"Proponent" wrote in message
m...
Why is it that spacecraft tend to adopt nose-down attitudes for
de-orbit burns?

If the acceleration vector were instead precisely anti-parallel to the
velocity vector, then 1) a smaller impulse would suffice to reduce the
pericenter by a given amount, and 2) the re-entry trajectory would be
shallower, which should give lower g-loads and perhaps lower heating
loads as well.

I assume you're talking about the Shuttle. This attitude puts the thrust
vector anti-parallel. Remember, the thrust goes through the center of mass,
not along the axis of the Orbiter. The trajectory won't be shallower.

Thanks to you and JRF for you replies.

I had in mind Mercury and Gemini. I didn't realize that the Shuttle
did a nearly anti-parallel burn, though I suppose it should have
occurred to me that techniques might have changed in 20 years!

If one wanted to de-orbit quickly, an inwardly directed acceleration
vector would make sense. But for a typical civilian mission, there's
no big hurry.

Nope. Any thrust away from the velocity vector is mostly wasted.

I beg to differ. Suppose we start in a circular orbit. If we then
make a brief, completely anti-parallel retro burn, we'll now be in an
elliptical orbit with pericenter 180 degrees around the orbit from the
point of retro fire.

If, on the other hand, we start in the same circular orbit and make a
brief inwardly directed retro burn (I guess you couldn't really call
it a "retro" burn, though), then the pericenter of the new orbit will
be only about 90 degrees along the orbit from where the point of retro
fire.

"Bob Martin" wrote in message
...
Where are you getting this 30-40 degrees of pitch? The shuttle pitches
10
degrees nose-down, but that's because the OMS engines are canted to
point
through the CG of the orbiter, so the orbiter must pitch to put the
thrust
line anti-parallel to the velocity vector.

I think he's referring to earlier generations of ships (especially Gemini
and Mercury). They pitched down about 40 degrees from retrograde for
their
deorbit burns.

In that case, it was, if memory serves me correctly, so the "pilot" could
line up the horizon with marks on the window.
--
James Summers
IBM-ret, "old space guy".
Apollo 201, 202, 203, 204, 1, & 9 RTCC Support. Apollo 13 "back room".
email to: UseNet1 ayt mcsummation dotting com

(Proponent) wrote in
m:

Thanks to you and JRF for you replies.

You're welcome.

"James Summers" wrote in message
om...
"Proponent" wrote in message
m...

If one wanted to de-orbit quickly, an inwardly directed
acceleration vector would make sense. But for a typical civilian
mission, there's no big hurry.

Nope. Any thrust away from the velocity vector is mostly wasted.

I beg to differ. Suppose we start in a circular orbit. If we then
make a brief, completely anti-parallel retro burn, we'll now be in an
elliptical orbit with pericenter 180 degrees around the orbit from the
point of retro fire.

If, on the other hand, we start in the same circular orbit and make a
brief inwardly directed retro burn (I guess you couldn't really call
it a "retro" burn, though),

You're right, you can't. NASA generally calls them "radial-in" burns.

then the pericenter of the new orbit will
be only about 90 degrees along the orbit from where the point of retro
fire.

A radial-in burn may move periapsis *sooner* than a retrograde burn, but
it's less efficient. Rough rule-of-thumb: a 1 m/s retrograde burn in LEO
lowers perigee by about 3.6 km, while a 1 m/s radial-in burn lowers perigee
by only 900 m. So the retrograde burn is four times more efficent.

If you need to deorbit quickly, as in the scenario you postulated,
propellant efficiency is not a concern (as long as you have *enough*, of
course...).

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

"Proponent" wrote in message
m...
Why is it that spacecraft tend to adopt nose-down attitudes for
de-orbit burns?

In the course of some other reading, I happened to run into some info on
this... Mercury (possibly Gemini et al as well) used a 34deg nose-down
pitch angle because this angle minimizes the effects of pitch-angle error.
(Remember that attitude control was briefly a bit iffy during retrofire,
the solid retros being rather more powerful than the little RCS thrusters.)

Although as it turned out, the one Mercury retrofire that had a large
attitude error -- Carpenter's -- was off mostly in yaw.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

"Proponent" wrote in message
m...
Why is it that spacecraft tend to adopt nose-down attitudes for
de-orbit burns?

In the course of some other reading, I happened to run into some info on
this... Mercury (possibly Gemini et al as well) used a 34deg nose-down
pitch angle because this angle minimizes the effects of pitch-angle error.
(Remember that attitude control was briefly a bit iffy during retrofire,
the solid retros being rather more powerful than the little RCS thrusters.)

Although as it turned out, the one Mercury retrofire that had a large
attitude error -- Carpenter's -- was off mostly in yaw.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |