Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: X-38 channels Tex Johnston
Date: Sat, 4 Nov 2000 23:00:51 GMT

In article <8u1h24$i2b$1@gw.retro.com>,
George William Herbert <gherbert@gw.retro.com> wrote:
>I don't think anyone's seriously proposing using all-rocket
>descent decelleration; parachutes are just too mass efficient
>for that...

Remember, you don't need to stick with rockets.  There were serious
proposals to land Apollo-class vehicles with turbojet lift engines.  The
hardware was relatively heavy... but no heavier than a lifting body with
go-around capability and off-runway capability, and it was rather more
flexible.  (It did use a modest parachute for initial stabilization and
reduction of descent rate.)

However, a capsule with a gliding parachute turned out to have twice the
payload of either.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)

Date: Sun, 05 Nov 2000 16:54:41 -0800
From: Doug Jones <random@qnet.com>
Newsgroups: sci.space.policy
Subject: Re: X-38 channels Tex Johnston

George William Herbert wrote:
>
> The key here is vertical velocity, horizontal being of secondary importance
> (you don't want to roll over on touchdown, but breaking your back is of
> more immediate disasterous consequence).

Damn right- I survived a few feet-knees-forehead landings just fine
(except for that nervous tic ;) but the one landing I made with a large
vertical component ended my skydiving days.  Knocked an inch off my
height, too.

> Gliding parachutes have the disadvantage of being bulkier but advantages

Huh? Ram-sir canopies are *much* smaller and lighter (typically less
than half) than round caopies for the same payload- unless you're
comparing them to a small round canopy with high descent velocity and a
retro rocket.

X-38's designers should have bitten the bullet and spec'd a "lead sled"
device half the wing area or less, with much higher airspeed. It would
be more crisply controllable, but the landing flare would be a lot more
exciting.  The large yaw angles are due to the great disparity in the
span of the canopy and the riser separation- there's not enough torque
vs the vehicle's yaw moment of inertia to give good damping.

Any device with L/D greater than 3, and a 40% lift reserve to stall, can
do a zero-sink flared landing.  The timing is critical, but that's what
radio altimeters are for.

--
Doug Jones
Rocket Plumber, XCOR Aerospace
http://www.xcor-aerospace.com