Thomas Frieling in Spaceflight

In article , Henry Spencer wrote:

"Pinpoint" may be a bit of a stretch. I've never seen a diagram of
the landing eclipse vs landing location for the Apollo missions.

Numbers for Apollo landing errors are not hard to find (see, e.g., "Apollo
By The Numbers") and they're not large, typically a kilometer or three.
In itself that is not pinpoint, but note that I was talking about adding a
gliding parachute to that -- those errors are small enough that a bit of
late aerodynamic maneuverability ought to suffice to take them out and
permit landing exactly at a chosen point.

Okay, here's a question. If you had a gliding parachute, how pinpoint
would pinpoint be? With late-descent manouevering, I assume Apollo could
never land in a duckpond, but could you reasonably expect to hit, say, a
small lake? An Olympic-sized swimming pool? A pre-selected coastal bay?

Just an intriguing sidenote...

--
-Andrew Gray

Henry Spencer wrote:

In article ,
Derek Lyons wrote:
Handwaving away the shock problems inherent in a land touch down, no
they are not a particular challenge. Modifying the Apollo capsule so
that it can tolerate the shock is a very different matter.

This is actually quite straightforward to solve, if you are willing to
postulate a new design which exploits the greater capsule size possible.
On close examination, the hard part of a touchdown without some sort of
terminal velocity reduction (braking rockets, parafoil flare maneuver,
whatever) is trying to reduce the velocity to zero with a very short
shock-absorber stroke.

But there is *no reason* why the stroke has to be so short! Make the
outer (aerodynamic) hull substantially larger than the inner (pressure)
hull, so there is room for a long-stroke shock-absorber system in between.
If you have the extra size -- quite feasible, for an EELV launch -- then
you just need a chief designer who is willing to dig in his heels and
insist that the empty space within the outer hull will *remain* empty, and
will *not* fill up with equipment/cargo/etc. just because it's there.

And split the problem into two: stroke in landing legs, followed by stroke in the
couches. A combined three or four meters will make landings feel almost cushy
:_

In article ,
Andrew Gray wrote:
In itself that is not pinpoint, but note that I was talking about adding a
gliding parachute to that -- those errors are small enough that a bit of
late aerodynamic maneuverability ought to suffice to take them out and
permit landing exactly at a chosen point.

Okay, here's a question. If you had a gliding parachute, how pinpoint
would pinpoint be?

Depends on the piloting, more than anything else. My understanding is
that given good visibility and reasonable flying conditions, sport
parachutists consider a couple of meters to be a sizable error. But a
capsule is going to have more limited maneuverability, and the pilot's
view of the ground isn't going to be very good, although GPS guidance
might well help.

My first guess (with a caution that it's not an area where I'm very well
calibrated) would be a 95% chance of landing within 50m of the target.

That may be too pessimistic; X-38, which landed under a parafoil rather
than as a lifting body, was supposed to be able to do a fully automatic
runway landing, which implies a crossrange error of at most a few meters.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Henry Spencer wrote:
That may be too pessimistic; X-38, which landed under a parafoil rather
than as a lifting body, was supposed to be able to do a fully automatic
runway landing, which implies a crossrange error of at most a few meters.

This suggests an entirely different way of dealing with the landing
stroke requirement. The only problem I see is convincing a bunch of
hot-shot test pilots that they have to land on a giant feather mattress...

- Marshall

On or about Wed, 27 Aug 2003 03:12:52 GMT, Greg D. Moore (Strider)
made the sensational claim that:
Umm, I'm not sure, but I think putting the bathroom outside the pressure
hull might not go over well. :-)

Bah. I've had to go worse places.
--
This is a siggy | To E-mail, do note | This space is for rent
It's properly formatted | who you mean to reply-to | Inquire within if you
No person, none, care | and it will reach me | Would like your ad here

On Sat, 23 Aug 2003 18:08:43 GMT, (Henry Spencer)
wrote:

In article ,
Derek Lyons wrote:
Wings, or a lifting body, or Roton's blades, all allow for greater
crossrange and more predictable and precise targeting of the landing point.

This has a lot to do with reusability and routine operations.

Crossrange has nothing in particular to do with reusability, and makes
only the most minor contribution to routine operations unless flight rates
are very high.

It's true that crossrange (and downrange) and reusability aren't
related for the hardware, but crossrange is very important in the
reusability of the wetware. Being able to land at a predicted
location makes crew recovery effective.

Given the accurate descent navigation demonstrated by Apollo and ARD, a
gliding parachute is almost certainly sufficient to make pinpoint landings
with a capsule.

Er, Apollo, like Gemini, had enough L/D for some crossrange and
downrange control. I think Apollo had about 0.5 and Gemini about 0.3.
There was more to the reentry trajectory than just ballistics and
parachutes. Actual aerodynamics played a part, too.

Mary

--
Mary Shafer Retired aerospace research engineer

In article ,
Mary Shafer wrote:
This has a lot to do with reusability and routine operations.
Crossrange has nothing in particular to do with reusability, and makes
only the most minor contribution to routine operations unless flight rates
are very high.

It's true that crossrange (and downrange) and reusability aren't
related for the hardware, but crossrange is very important in the
reusability of the wetware. Being able to land at a predicted
location makes crew recovery effective.

Some degree of crossrange *control* is important to make semi-precision
landings. But the *amount* of crossrange available determines nothing
except the orbital waiting time and/or maneuvering needed to bring the
orbit track over the desired landing point. This becomes really important
only when the flight rate is so high that a twelve-hour wait puts a real
crimp in the scheduling.

Given the accurate descent navigation demonstrated by Apollo and ARD, a
gliding parachute is almost certainly sufficient to make pinpoint landings
with a capsule.

Er, Apollo, like Gemini, had enough L/D for some crossrange and
downrange control.

But *not* enough for pinpoint landings. (Defined as: anyone who stands
on the exact aim point had better be prepared to move aside fast.) Even
with some modern improvements like GPS (as tested on ARD), the error due
to remaining navigation uncertainties, high-altitude winds, etc. is still
measured in kilometers.

The problem is that most of the aerodynamic maneuvering capability occurs
very early in reentry, while a lot of the error creeps in later, when the
ability to correct for it is very limited. Pinpoint landing requires, or
at least is greatly aided by, some degree of low-speed aerodynamic
maneuverability -- a gliding parachute, a rotor, jet lift, whatever.

I think Apollo had about 0.5 and Gemini about 0.3.

Apollo was about 0.35, Gemini 0.19. Apollo was *meant* to get about 0.5,
flying at an angle of attack of 33deg, with the windward side of the cone
essentially parallel to the airflow... but the crowded interior made it
impossible to offset the center of mass enough to trim it for 33deg.

(Ironically, in the end it would have cost almost nothing in mass to make
the CM a bit bigger, giving a roomier interior and more trim flexibility.
Its diameter was dictated by early ideas about the diameter of Saturn
upper stages, but the rockets kept getting bigger, and in the end there
was no compelling reason why the CM had to be that small. Losses like the
growth in heatshield area would have been largely canceled out by gains
like the lower ballistic coefficient and hence less severe reentry.)
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Mary Shafer writes:

Er, Apollo, like Gemini, had enough L/D for some crossrange and
downrange control. I think Apollo had about 0.5 and Gemini about 0.3.
There was more to the reentry trajectory than just ballistics and
parachutes. Actual aerodynamics played a part, too.

This worked out well for Apollo, allowing it to generate enough lift
to markedly change the reentry profile on lunar missions. A quick
search turns up a reference at my favorite space reference site:

Skip lunar reentry trajectories studied for Apollo
http://www.astronautix.com/details/ski16333.htm

I just found out the JSC Digital Image Collection's text search is
case sensitive! Searching for "apollo reentry" or even "apollo"
results in zero pictures found. However, "Apollo reentry" finds lots
of pics. Go figure. I don't see the picture I remember which shows
what the skip trajectory looked like on a 2D plot. Any good book on
Apollo would have this picture.

Jeff
--
Remove "no" and "spam" from email address to reply.
If it says "This is not spam!", it's surely a lie.