Working Towards Flight OneMake a list, check it twice...
Made up another list of things I need to do to get this into the air. Surprisingly long
The original M4 metal spacers holding the fiberglass discs of the assembly together were replaced by titanium bike spokes (custom cut) and some CF strut, ID=2.5mm, outer=4mm square. This saves an enormous amount of mass. Each set of spacers went from about 140g down to less than 7g IIRC.
Provision is made to feed the ejection control (TeleMega) from the NC through the VTS and down to this bulkhead. The NC stays attached to the VTS and this means that the whole assembly is treated like an extended nosecone on the test vehicle. This should make preparation for flight straightforward.
A break-wire on the back of the VTS breaks when the assembly is ejected from the airframe and ensures the VTS is put into a neutral, low current-draw state so the servos are not fried or the batteries drained. This thing can draw around 140W of power if it is provoked! The flight battery for the servos is rated for around 3 minutes of operation at those levels.
A piece of 6mm allthread holds the VTS to the coupler and the bulkhead.Verification of Function
Now comes the task of working through each of the functions and how it is intended to work, verifying that my expectations are being met.
- Update schematic diagram. Check
- Test that servos actuate fins as expected. Check
- Verify that there are no integrators winding up in the control system. Check
(PD control loops only)
- Check servos actuate as expected in inverted position. SNAFU
“Why?” you ask. Remember that I am using a commercial autopilot for this. Thinking about a rocket flight there is an enormous acceleration in the upward direction during boost. I have the AP set to a neutral position during this phase as the accelerometers and gyros cannot be believed. At burnout there is actually a net deceleration on the airframe due to air drag, so the sign of the “gravity field” according to the AP swaps. This is equivalent to our fixed-wing aircraft flying inverted. This autopilot is not particularly programmed for this mode of operation. Any slight inclination will cause the fins to slam into the stops in an effort to flip the bird end-on-end. Not good for my Velociraptor R this way. Data from previous flights indicates about 0.5G in the earthward direction, on top of the standard 1G Earth gravity field.
To get around this I propose to mount the AP essentially upside down in the VTS. This will give it the correct orientation for the coast phase steering, and fins will be in the neutral position for the boost phase as before.Launch Safety
Part of this journey involves considering what can go wrong and how to keep the flight safe. This will be flying on a K or L, so significant energy is involved. The master plan is to:
- Double the standard distance to the launchpad as it is a complex rocket
- Keep the fins neutral during boost, so the VTS commences active mode when the somewhere over 3000’ altitude.
- Main deployment is set for 2000’
All those things mean that, whatever the result, it happens in the air and the hardware is under chute for the landing.Next Tasks
So my first priority is to confirm my hypothesis of inverting the AP is acceptable. If that turns out OK then I proceed with integration and hope to fly this in the next month or two, assuming we don’t get scrubbed. Having the AP upside down assaults my sense of what is appropriate, but in the end the engineering definition of being fit for purpose is what matters.
Strength testing of the fins is also high on the priority list now.
Other than that, there are a myriad of minor jobs to get this project soaring.Stay Tuned!