HI Guys can you all please watch what you are saying here
"I am having a problem setting up my Asp2."
No - your problem is not with Asp2 at all. Any one reading this will think "Oh another problem with the unit". IF you have been following the forum and have downloaded any of the Asp model files and installed them onto your radio. You will have found that the complex mixing involved for what you want to do has already been done. No one else anywhere has done this for you.
The Asp model files all contain a mix for CH8 such that you can control an elevation servo on Ch8 by simply plugging the servo into ch8 which is then controlled by the stick on CH3. The background and how to do this yourself is also here on the forum in this post.
viewtopic.php?f=148&t=971Which is cryptically labelled "standard servos for elevation and rotation"
Sirvoe WAS orginally designed for exactly this purpose and in fact if anyone does order a Sirvoe without any additional notes - it comes setup to work as an elevation servo control.
However with this mix for er9x then you don't need a sirvoe board to do elevation and hence many now use sirvoe for non-elevation roles. This mix is fantastic as it allows you to have far greater control over your elevation servo. Far far greater than ANY onboard solution could possibly offer.
Ok so onto the problem at hand. You have a few options but all will involve you getting the elevation servo mix onto your 9x.
Easy Option 1.
Download and put onto your 9X the Asp model file for 6 channel control. In 6 ch mode there are the special servo mixes on BOTH CH7 and CH8. They are setup for controlling a elevation servo and turret servo direct from ch7 & ch8 using the sticks for control. Keep the elevation channel and clear the turret mix and set up to control your hatch on that channel. The DOWNSIDE for this is that your guns are now back on CH3 so you will lose all the extra gun functions and only have 1 main cannon and 1 machine gun.
Option 2 (requires smarts).
Get the Asp2 model mix (standard 7 channel operation) and move your elevation servo control onto CH8 as it should be. Since you are not using the elevation motor and I suspect not using the airsoft motor either - put Asp into Advanced mode which frees up the ele and air switches. These are ground return switches which can be used to trigger stuff. Tell me this is what you want to do and I can program a custom sirvoe board for you such that when you trigger the ele or air switch the hatch opens and closes.
The DOWNSIDE - you have to know what is going on and I have to custom program the sirvoe board guessing how much movement is required for the open/close action.
Option 3 (everything is solved by $$$)
I don't know what radio module you are using but I can only recommend using the FrSky XJT module which will allow you up to 16 channels for operation. Again get regular ASp2 model file for 7 channel operation and move elevation servo to ch8. Get either a second X series receiver or a s-bus converter. Plug your hatch servo into CH9.
Finally...
"I have tried plugging the servo directly into channel 3 ( elevation) but this stops the tank from starting up via 100% on channel 5. Not sure why ?

"
This is because Asp needs to see valid channel info on CH2, CH3 and CH4. Otherwise it assumes you have a receiver problem and waits for you to fix it. When you unplug CH3 -no signal - no workies.
Cheers
KG
PS. I've just thought of a 4th solution. You could TRY using a "Y" cable on CH3 and sending one signal to ASp and then the other to a standard sirvoe unit for your elevation. I do not generally suggest "y" cables but this might also work and becomes the Easiest solution should it work.