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From: Henry Spencer <>
Subject: Re: Re-entry attitude control (was Apollo 13 re-entry)
Date: Wed, 27 Mar 1996 18:04:39 GMT

In article <4iu2ev$> writes:
>A general question about Apollo re-entry (or even any vehicle).
>Given that there is only a finite amount of thruster fuel on board
>how stable is the spacecraft once in the re-entry attitude? Are
>the thrusters firing continuosly to hold the orientation?

In general, no.  The spacecraft is balanced to be stable at the desired
angle to the airflow.  (Holding a different angle by thruster firings
would indeed be very expensive in fuel.)  Typically this angle is not zero,
because you want a bit of lift for trajectory control.  You can also use
lift to give a gentler reentry, by "lofting" the trajectory, holding it
in the thinner high-altitude air as long as possible.  Even a blunt body,
reentering at an angle, generates enough lift for this sort of thing.

The thrusters are there mostly to roll the spacecraft around its direction
of motion, so that the lift vector can be pointed up, down, or sideways as
required.  If you don't happen to want lift at the moment, a slow
continuous roll is done, so that the average effect of the lift is zero. 
(A spacecraft which wants a purely-ballistic reentry, with no lift, will
also use a slow roll, to cancel out any accidental lift arising from minor
errors of balance.)
Americans proved to be more bureaucratic           |       Henry Spencer
than I ever thought.  --Valery Ryumin, RKK Energia |

From: Henry Spencer <>
Date: Mon, 24 Mar 1997 18:11:59 GMT

In article <>,  <> wrote:
>...What I don't understand is why that same
>atmosphere does not cause the same friction and heat when the shuttle,
>or other spacecraft, is launched and is ascending to space? Why can
>something go up unscathed but be destroyed coming back down through the
>same atmosphere? Is it to do with speeds, trajectories or none of the

Speeds and trajectories.  On the way up, the first thing that a rocket
(including the shuttle) does is to climb almost vertically to get clear
of the atmosphere.  Nearly all the real accelerating is done at extremely
high altitude in what is essentially vacuum.

To a first approximation, the shuttle SRBs serve mostly to throw the rest
of the shuttle up clear of the atmosphere, tossing it upward hard enough
that it can accelerate to orbital velocity before it comes down again.
Acceleration to orbital velocity is the job of the orbiter's main engines.
(In practice the situation is more complicated and the roles are not so
clearly separated, but as a first approximation this is roughly right.)

>2.	Furthermore, what if the shuttle or other object were to re-enter the
>atmosphere really slowly instead of at orbital velocity - hypothetically
>speaking, would it still need thermal protection were that to be

No, it wouldn't, but that would be hard to arrange.  Even if you somehow
stopped the shuttle in orbit, it would pick up considerable velocity just
falling from there into the atmosphere.  In fact, it might not survive
such a reentry, because the descent would be too rapid.  You want to do
most of your decelerating at high altitude, where the air is thin and
deceleration happens relatively slowly and you don't get too hot.  So you
want to enter the atmosphere at a shallow angle, and use aerodynamic lift
to stay up in the thin stuff until most of your velocity is gone.  This
isn't practical if you come down vertically...
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

From: (Andrew Higgins)
Subject: Re: Source of Atmospheric Entry Heating
Date: Mon, 29 Jun 1998 20:25:22 +0200

In article <>, (JamesOberg) wrote:

> I'm completing an article for a major space magazine about atmospheric entry
> and what it does to spacecraft. Part of what I hope will be eye-opening is my
> explication that "friction" has nothing to do with entry heating. The heat
> comes from atmospheric compression in the hypersonic shock front and then
> radiation and convection and even some conduction into the spacecraft
> structure, where the actual laminar air flow across the skin is subsonic at
> all phases of entry.

I'm not sure what the "eye-opener" is here.  The sources of aerodynamic
heating in re-entry are completely understood, and the individual
contributions of radiation, convection, etc. are readily quantified.

Most heat transfer in hypersonic flow is convective.  As you mentioned,
the re-entering vehicle drives a very strong shock wave in front of it,
and the gas that goes through this shock wave is brought to very high
temperatures due to adiabatic compression.  As this hot gas blows over the
vehicle, heat is transferred from the hot gas to the cooler vehicle.  This
is convection.

Now, this is not "friction" as in "rubbing your hands together", but you
must include frictional effects, such as viscosity and mass diffusion, to
properly predict the convective heat transfer to the vehicle.  In fluid
dynamics terminology, this means using the Navier-Stokes equations, as
opposed the inviscid or "frictionless" Euler equations, in order to
correctly model hypersonic heating.

As for the "laminar air flow", the flow may well *not* be laminar.  In
fact, transition from laminar to turbulent flow is a major factor (perhaps
*the* major factor) in determining the heat transfer to a vehicle, since
the turbulent vortices are much more efficient at transferring heat from
the hot air to the surface of the vehicle.  Laminar flow forms a better
thermal boundary layer to protect the vehicle.  The ability to predict the
transition from laminar to turbulent flow is currently the major challenge
in predicting hypersonic heating.

Radiative heat transfer is not terribly significant for re-entry from
LEO.  But for Apollo re-entry, about 30% of the total heating of the
vehicle was radiative.  When the Galileo probe smacked Jupiter at 50 km/s
a few years ago, nearly all of the heating was radiative.

For an introduction to this topic, Anderson's "Hypersonic and High
Temperature Gas Dynamics" is an excellent source:

     Anderson, J.D., "Hypersonic and High Temperature Gas Dynamics,"
     McGraw-Hill, 1989, ISBN 0-07-001671-2.

Several figures from Anderson's book illustrate some of the points I
discussed above.  Fig. 6.34 and 6.35 show the location of transition from
laminar to turbulent flow on the Space Shuttle at Mach 9, and the dramatic
effect this has on the heat loads to the vehicle.  This is *real* data
from STS-2, not just some modeling.

Also, Fig. 18.10 (Anderson) is a good comparison of convective and
radiative heat transfer, showing under which regimes one dominates over
the other.

Also, Fig. 7.12 (Anderson) shows a classic photo of what can happen to the
X-15 on a really bad day :-).

Some other books which have appeared in the last few years are:

     Rasmussen, M. "Hypersonic Flow," John Wiley & Sons,  1994,
     ISBN 0-471-51102-1.

     Bertin, J.J., "Hypersonic Aerothermodynamics," American Institute
     of Aeronautics & Astronautics, 1994, ISBN 1-56347-036-5.

But I don't like these books as much as Anderson's, and they certainly
aren't as "conversation".  John Anderson writes really good text books.
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University    Montreal, Quebec

From: (Henry Spencer)
Subject: Re: Gemini capsule reentry parameters
Date: Mon, 19 Jul 1999 23:46:44 GMT

In article <bruce-1907990207260001@bruce.bgh>,
Bruce Hoult <> wrote:
>> ...If L/D is one or more, reentry gees barely reach 1.1, if that.
>So what you're implying is basically that at any given speed they go only
>deep enough into the atmosphere that they can generate enough lift to
>prevent going any deeper into the atmosphere...?

Basically correct.  You want to stay out in the thin stuff while you
lose speed, until you no longer can.

>> Pure ballistic hits 8-10 gees of course.
>What stops it being infinite?

With the right/wrong shape and trajectory, it can be -- modern ICBM
warheads keep much of their velocity until quite low, experiencing hideous
heating rates and decelerations in the thick lower atmosphere.  For a
spacecraft, you use a near-horizontal trajectory where much of the
deceleration happens at relatively high altitude in relatively thin air.
It's not as good as a lifting reentry because you do lose altitude faster
than you'd like, getting into somewhat thicker air.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: (Henry Spencer)
Subject: Re: NOx generation by reentry vehicles
Date: Thu, 23 Mar 2000 03:02:06 GMT

In article <AEUB4.1$>,
Jeff Greason <> wrote:
>   Does anyone out there have good references
>discussing the generation of nitrogen oxides (NOx)
>by reentry vehicles or vehicles in hypersonic
>atmospheric flight? ...

There's some discussion on this in a paper on shuttle environmental
effects in the March/April 1982 Journal of Spacecraft and Rockets.
(It says a shuttle-like craft produces about 25% of its weight in NOx
on each reentry, spread between 55km and 100km with a peak around 70km.)
Don't think I've run into anything more recent...
Computer disaster in February?  Oh, you |  Henry Spencer
must mean the release of Windows 2000.  |      (aka

From: "Jeff Greason" <>
Subject: Re: NOx generation by reentry vehicles
Date: Fri, 31 Mar 2000 18:16:41 GMT

Henry Spencer <> wrote in message
> I wrote:
> >There's some discussion on this in a paper on shuttle environmental
> >effects in the March/April 1982 Journal of Spacecraft and Rockets...
The only paper I found you may have been referring to was "Environmental
Effects of Space Systems, in the Jan/Feb 1982 issue -- which does cover
SPS environmental effects primarily, though transportation effects are
a subset.

But I did find the paper(s) I wanted.  For future reference, if anyone
wants info on NOx generation by reentry vehicles, I suggest:

"Equivalent-Cone Calculation of Nitric Oxide Production Rate
 During Space Shuttle Re-entry", C. Park & J. Rakich,
_Atmospheric Environment_, v. 14, pp 971-972, 1980.


"Estimates of Nitric Oxide Production for Lifting Spacecraft
 Reentry", C. Park, _Atmospheric Environment_, v. 10, pp 309-313,

These have some great one or two equation estimation methods for
NOx production for a variety of conditions.  Because of uncertainties
in the reaction rate, I'd put a +/- 3x error bar on these, but they're
simple and they can quickly estimate the right ball park.  My favorite
of these comes from the first paper -- the least accurate, but simplest
for a ROM number:

Chemical Energy of NOx formed =
     "NOx efficiency" * Kinetic Energy Dissipated by Vehicle

Where, for the Shuttle, the NOx efficiency is roughly 1.7%

I like this because it's so strongly connected to the physics of the
process that there's a limit to how screwed-up the estimate can be
(a nice feature in a rule of thumb).

For other vehicles with lower energy (which was my interest), it's
a bit more work but the data is still in there.  NOx efficiency is
unfortunately sensitive to flight path details.

"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      President & Eng. Mgr.
thinking in quite the same way." --L. Yau  XCOR Aerospace
   <>                <>

Newsgroups: alt.war.nuclear,sci.physics,
From: (Henry Spencer)
Subject: Re: Scientists Oppose Missile Shield: by The Associated Press
Date: Thu, 6 Jul 2000 01:46:15 GMT

In article <1pO85.7041$>,
Ian Stirling  <> wrote:
>To slightly redirect, what's the terminal velocity of the RV, after the R?

For ICBMs, there is no "after the R".  Warheads are designed to keep their
velocity, as much as possible, all the way down to ground level.  It makes
them harder to intercept.  They do end up decelerating pretty hard in the
troposphere, because of all that thick air, but they're still moving
pretty fast when they hit.  That is, they'll have a plasma sheath around
them all the way down.

>Combining widespread civillian satellite imagary, a camera on the RV,
>and some manoeverability, might create more relatively easy accuracy.

Making a camera work through a plasma sheath is, um, challenging.  (Some
ideas exist on how it might be done, but they're sketchy and unproven, at
least as best one can judge from unclassified information.)
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

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