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nozzle pitch

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nozzle pitch

Postby bmb » Fri Mar 02, 2007 3:17 pm

I'm trying to model a vertical takeoff rocket.
As a first quick hack to test things, I took the X-15 and moved the engine and nozzle near the center of gravity (it's near 300) and changed their pitch to 90, but it still flies straight. (I also made the rocket engine more powerful and the vehicle lighter to achieve good T/W.)

Did I do something wrong?

I even moved the nozzle to the forward part but it doesn't cause any pitch tendencies, when in reality it should pitch up or down since the nozzle is pitched 90 degrees (dunno if it's up or down). I enabled the variables display to monitor thrust and in ~level glide with idle stick I can throttle up and down without any changes in pitch.

Original X-15:

Code: Select all
 <engine file="XLR99">
            <location unit="IN">
                <x> 600 </x>
                <y> 0 </y>
                <z> 0 </z>
            </location>
            <orient unit="DEG">
                <roll> 0.0 </roll>
                <pitch> 0 </pitch>
                <yaw> 0 </yaw>
            </orient>
            <feed>0</feed>
            <feed>1</feed>
            <thruster file="xlr99_nozzle">
                <location unit="IN">
                    <x> 600 </x>
                    <y> 0 </y>
                    <z> 0 </z>
                </location>
                <orient unit="DEG">
                    <roll> 0.0 </roll>
                    <pitch> 0.0 </pitch>
                    <yaw> 0.0 </yaw>
                </orient>
            </thruster>
        </engine>


After my modifications:
Code: Select all
<propulsion>
        <engine file="XLR199">
            <location unit="IN">
          <x> 100 </x>
                <y> 0 </y>
                <z> 0 </z>
            </location>
            <orient unit="DEG">
                <roll> 0.0 </roll>
                <pitch> 90 </pitch>
                <yaw> 0 </yaw>
            </orient>
            <feed>0</feed>
            <feed>1</feed>
            <thruster file="xlr199_nozzle">
                <location unit="IN">
                    <x> 100 </x>
                    <y> 0 </y>
                    <z> 0 </z>
                </location>
                <orient unit="DEG">
                    <roll> 0.0 </roll>
                    <pitch> 90.0 </pitch>
                    <yaw> 0.0 </yaw>
                </orient>
            </thruster>
        </engine>
...etc
bmb
 
Posts: 3
Joined: Fri Mar 02, 2007 3:05 pm

Postby grtux » Fri Mar 02, 2007 6:19 pm

I am not sure my answer could help you.

I had in the past experimented a VTOL aircraft with an older version of JSBSim,
I did this

<AC_ENGINE FILE="GR5_spe">
XLOC -0.85
YLOC 0
ZLOC 0
PITCH 0
YAW 0
FEED 0
FEED 1
FEED 3
<AC_THRUSTER FILE="direct">
XLOC 0.15
YLOC 0
ZLOC -13.5
PITCH -89.50
YAW 0
</AC_THRUSTER>
</AC_ENGINE>

You can see i did not modify the engine pitch, only Thruster pitch.

It was working perfectly, try it with your JSBSim data

Regards
g.robin
LFMO
User avatar
grtux
 
Posts: 432
Joined: Thu Dec 14, 2006 4:19 pm
Location: Provence France

Postby bmb » Fri Mar 02, 2007 6:37 pm

Well, I did experiment with the X-15 now and change only the thruster angle and nothing else but it still didn't work. Even changing the thruster position in Y coordinate, which should give it a big yaw lever arm, didn't do anything:

Code: Select all
   <thruster file="xlr99_nozzle">
                <location unit="IN">
                    <x> 600 </x>
                    <y> 300 </y> <-- !!!! -->
                    <z> 0 </z>
                </location>
                <orient unit="DEG">
                    <roll> 0.0 </roll>
                    <pitch> 0.0 </pitch>
                    <yaw> 0.0 </yaw>
                </orient>
            </thruster>
bmb
 
Posts: 3
Joined: Fri Mar 02, 2007 3:05 pm

Postby grtux » Fri Mar 02, 2007 11:08 pm

your change shows thruster pitch 0.0

You must try engine pitch 0.0 and thruster pitch -90.0 (minus 90)
it is what i did

regards
g.robin
LFMO
User avatar
grtux
 
Posts: 432
Joined: Thu Dec 14, 2006 4:19 pm
Location: Provence France

Postby bmb » Sat Mar 03, 2007 12:48 am

No, it doesn't. Look in the first post in the second code block:

Code: Select all
      <pitch> 90.0 </pitch>


You must read what I said in the posts to understand what I mean.
I first tried changing the pitch. I then after that also tried a configuration without the pitch change but a position change to see if that could work. It didn't either.

I'm experimenting with the ogel model now, it's much simpler than X-15...

I made a rocket engine for it, and even got it to fire for a while but now that i made an oxidizer tank, it refuses to start up..
bmb
 
Posts: 3
Joined: Fri Mar 02, 2007 3:05 pm

Postby grtux » Sat Mar 03, 2007 1:30 pm

When i told you i was not sure to be useful, i was right :(

Recently was some addons regarding rocket and nozzle within JSBSim stand alone, i did not test it but others users did it.
These add on are said working fine, unfortunately for you that new version has not yet been implemented into Flightgear.

May be you could subscribe to JSBSim mail list and ask for it.

Regards
g.robin
LFMO
User avatar
grtux
 
Posts: 432
Joined: Thu Dec 14, 2006 4:19 pm
Location: Provence France


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