DockingMFD Difference between CVEL and RVel

[markopolo5437]

I'm working through Tex's excellent video tutorial on Launch and Docking, and I noticed that there are different values for the Docking MFD CVEL and the Sync Orbit MFD RVel. Obviously, I'm missing something about what the two values indicate. CVEL is closure velocity, I believe, and RVel is Relative velocity. How are these two values defined differently?

BTW, I feel obligated to commend Tex for his Orbital Tutorial Series for Beginners. They are well-executed, detailed, and explanatory of the various sim functions and the relevant mechanics. In another thread, I disparaged his video tutorials on IMFD. Those are indeed badly out of focus and almost useless, to me.

Thanks in advance.

 

[Zatnikitelman]

It's been a while, but I believe CVEL is the velocity relative to the docking port while RVEL is the velocity relative to the vessels. On a straight in docking, they should be identical, but if your port is rotating, I believe there is a difference. I think that's useful if you're approaching something like a rotating space station, that lets you sync RVEL, but it's been a while since I've done Orbiter so this may not be correct.

 

[Tommy]

CVEL is, as you said, Closing Velocity. It only considers velocity as a function of linear distance from the docking port, and ignores any "sideways" movement not on that direct line.

If your velocity vector is exactly aligned with the port CVEL = RVEL. If your velocity vector is not aligned with the port, the law of Cosines applies, and CVEL = RVEL times the Cosine of the angle of deviation.

In practical application, CVEL could be measured by a laser using the doppler effect, much like Police Speed detection Radar works

 

[markopolo5437]

Docking

Thanks, guys. That helps me to understand what's going on.

Docking is more difficult than I'd thought; I'm unsuccessful after 4 or 5 attempts, but I'll get it eventually. Balancing ROT and LIN thrusters takes a lot of practice.

 

[Tommy]

I recommend learning how to dock with Gravity Gradient Torque turned off, until you get the hang of it. Installing the Absolute Killrot add-on doesn't hurt either.

The other main secret is patience. and start by getting RVEL as close to zero as you can. Then keep the RVEL under 10 m/s.

 

[ADSWNJ]

Thanks, guys. That helps me to understand what's going on.

Docking is more difficult than I'd thought; I'm unsuccessful after 4 or 5 attempts, but I'll get it eventually. Balancing ROT and LIN thrusters takes a lot of practice.

It sure is tricky. You are trying to solve for six independent degrees of freedom all at the same time. If you have *any* residual rotation on the target vessel, then the port will have a wobble on it that makes things really challenging. (Hint: F3, select the target vessel, then engage zero rotation, then F3 and back again to your vessel).

Have a look at RV Orientation on the orbithangar.com ... my MFD to give you precise manual or autopilot control of the approach and docking.