Discussion:
JSBSim rocks!
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Thorsten Renk
2017-07-02 17:43:36 UTC
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I'm somewhat excited that I managed to roughly implement the no yaw-jet
control method for the Shuttle entry and did some roll reversals as a demo
- which is something that lives completely off cross-coupling modes and
uses the elevons in a way that's highly non-intuitive.

This is really a juicy tid-bit for anyone who enjoys aerodynamics, so here
it goes:

During entry and down to Mach 5 or so, the Shuttle has no rudder authority
and is in fact yaw unstable, thus normally the RCS yaw jets instead of the
rudder are used to zero beta, and the rudder is only used for control
below Mach 2.5.

Nominally the elevons can roll the Shuttle, this introduces some sideslip
as well - which the jets have to null quickly to avoid the instability.

The NO Y JET mode controls yaw and roll without having jets available
(it's the fallback mode for damaged jets or no propellant left) and lives
purely off control cross-couplings. The control sequence goes as follows:

* Initiating a roll using the elevons causes both a rolling and a yawing
moment. The rolling moment is ignored and the yawing moment is used to
move the Shuttle to a given sideslip angle and keep it there. This
maneuver causes a small roll opposite to the desired direction.

* There is then in addition a rolling moment due to sideslip, i.e. the
larger beta, the larger the rolling moment - thus using elevons to
maintain a finite beta value causes the Shuttle to reverse the rolling
direction within a few seconds and to roll into the desired direction

* There is also a rolling moment due to roll rate, so one has to be
extremely careful to not feed this mode too much by rolling at a high
rate, otherwise loss of control results

* There is also a yawing moment due to roll rate which needs to be
constantly canceled out by the elevons (which are supposed to control beta
to some value), causing some oscillations around the desired roll rate.

* There's also a yawing moment due to sideslip which also needs to be
canceled, otherwise beta grows too large for the elevons to control.

The resulting roll control is extremely ungainly and slow compared to the
variant where you use elevons to roll and jets to yaw - but it actually
works!

There's been no fudging of any kind to make this happen - all the various
moments are straight NASA data from wind-tunnel and in-situ measurements,
all I did was write a controller that converts roll commands into beta
targets - and it really all falls into place and works like described in
the manual.

Admittedly you have to know a lot about the basic Shuttle aerodynamics to
appreciate why being able to slowly roll the orbiter without yaw jets is
such a big deal - but it actually is huge.

So - another big 'Well done!!!' to the JSBSim coders here!

* Thorsten

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