From: "Jeff Greason" <jgreason@hughes.net>
Newsgroups: sci.space.policy
Subject: Re: Interesting RLV article in Orlando Sentinel
Date: Tue, 26 Oct 1999 16:50:50 -0700

Jake McGuire <jamcguir@yahoo.com> wrote in message
news:38163B94.5C314073@yahoo.com...
> Jens Lerch wrote:
> > Re-entering nose-first is generally a bad idea, because the heavy
> > engines at the base of a VTVL make a nose-first attitude
> > aerodynamically unstable, the vehicle has to be stressed for loads
>
>   Aerodynamic instability is not the end of the world, you just need
> an active fly-by-wire system (which isn't a big deal) and a lot of
> control authority (which is a bit bigger of a deal, but still
> solveable).
>

Yes, that's what everyone thinks when they start out -- "we'll
just let the avionics guys solve it".  Then when the price tag, and
the failure modes, and the schedule start to add up, good old
fashioned aerodynamic stability keeps looking like a better bet.
We figured this one out independently from, and about the same
time as, the X-33 team figured it out.

While many launch vehicles are statically unstable aerodynamically
through part of the flight envelope, they are usually only *slightly*
unstable.  Stability isn't an "all or nothing" proposition.  The control
system and steering authority (both in magnitude and in response
time) you'd need to balance a hypersonic vehicle flying with the
C/P in the nose and the C/G in the tail would be *highly*
nontrivial.  It's a different story from ascent instability, which is
usually lower magnitude, and where you have TVC of the main
rocket engine providing high authority and rapid response for the
control system.

----------------------------------------------------------------
"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
   <www.xcor-aerospace.com>                <jgreason@hughes.net>

From: gherbert@crl3.crl.com (George Herbert)
Newsgroups: sci.space.policy
Subject: Re: Interesting RLV article in Orlando Sentinel
Date: 28 Oct 1999 15:14:18 -0700

Jake McGuire  <jamcguir@yahoo.com> wrote:
>Jeff Greason wrote:
>> While many launch vehicles are statically unstable aerodynamically
>> through part of the flight envelope, they are usually only *slightly*
>> unstable.  Stability isn't an "all or nothing" proposition.  The control
>> system and steering authority (both in magnitude and in response
>> time) you'd need to balance a hypersonic vehicle flying with the
>> C/P in the nose and the C/G in the tail would be *highly*
>> nontrivial.
>
>  So take advantage of the C/G in the tail and the C/P in the nose by
>mounting your control surfaces on the nose.  Long moment arm and all
>that; see NASA TM-4726 and TM-4533 for an example.  With the new
>hafnium diboride TPS coming out of NASA Ames you could probably even
>have nose-mounted control surfaces at hypersonic speeds.

It's not a question of control authority, it's a question of magnitude
of instability and therefore timescale of departs due to random fluctuations.

In ships, there's a measure of stability "Metacentric height" which is
the radius arm from what is effectively the dynamic center of bouyancy
to the center of gravity.  If a ship has a very small GM (distance from
CG to M) then it rolls very slowly.  If it's got a very long GM then it
will snap right around.  If a ship unfortunately got a very small negative
GM it would slowly roll over (this is not uncommon for gentle progressive
flooding); if there's some sort of structural failure and you end up with
significant negative GM, it goes over with a snap.

Same thing with aircraft and rockets.  For those, with active stabilization
and control, the magnitude of instability determines how fast the flight
control system has to sense and react to problems.  Ten times less stable
means it has to react ten times as quickly, as it's going to start rotating
ten times faster if it goes out of straight and true.  It's not a question
of the control moment directly; it also has to react much faster.
Reaction time is always the killer in attitude control; it's easy to sense
a motion in realtime (thousandths of a second for modern inertial systems),
but you have to fire a rocket thruster, turn a fin or control surface,
tip a rocket to counter the motion, and those are hard to move all that
quickly.  And trying to do so hurts on mass; to turn a fin ten times as
fast you need ten times the torque, either a much longer moment arm for the
fin's actuator or a much stronger actuator, or both.  That brings actuator
mass into play as a problem area for vehicle design...


-george william herbert
gherbert@crl.com