# Calculation of V1 (orbital speed) during ascent

I am studying instrumentation of NASA's Orion spacecraft, and there is a readout for V1 (I am assuming this is the "first orbital velocity", for circular orbit), and it seems to be in small decimals. I am assuming this is a fraction of the current velocity to the needed velocity for the orbital speed. When a circular orbit is reached, this value will read 1.0.

In order to calculate this, I am assuming the V1 is calculated using the value of the current apogee (which is constantly increasing during the launch) and as the rocket trajectory changes, rocket catches up with apogee, rocket's speed reaches V1, and the value closes to 1.0.

The NASA video Orion Cockpit Feature shows the Orion instrumentation. The readout in question is at 01:10, upper left corner, a small box showing a value of 0.03 with the label "VI" above it.

Are my assumptions correct?

• Can you link the resources you got your information from? Commented Mar 25, 2019 at 20:00
• Sure thing, I just added a link in my question. Commented Mar 25, 2019 at 20:13

No, your assumptions are not correct.

That's Vi, inertial velocity. NASA doesn't use the V1/"first orbital velocity" terminology, or at least didn't in Shuttle.

If you zoom in on the image you can see the second character is an "i" not a "1".

I don't know if Orion uses English or metric units, but the units are either kilofeet/sec (like Shuttle) or km/sec.

Here's the analogous display from the Shuttle. The M/VI (Mach/Inertial Velocity) readout is indicated by the red arrow.The Shuttle display has a lot of stuff Orion doesn't need because of Shuttle's airplane-like entry flight, and also was designed to be a visual match for the original "steam gauge" cockpit to make the transition to the glass cockpit easier.

It is interesting to see how much of the display derives from Shuttle though.

From here

• Wow - thank you, sir! You are correct (I went through the relevant portions of the shuttle manual). The "Inertial Velocity" is, from what I understand, simply a velocity in Geocentric reference frame (as opposed to Topocentric reference frame, which accounts for Earth's rotation, and which is also displayed when Shuttle switches from re-entry to atmospheric flight). Since this (Vi) is an absolute value and thus gets into thousands or tens of thousands (depending on units), I was confused by the fractional value in the screenshot. Commented Apr 10, 2019 at 18:25
• At least for shuttle, it was the velocity in Earth-centered Inertial frame. So it had a value even sitting on the pad. Commented Apr 10, 2019 at 18:25
• Well, even sitting on the pad, the Vi value would be in hundreds of meters per second, so the fractional value is still confusing... Commented Apr 10, 2019 at 18:34
• For shuttle it's in *kilofeet*/sec so orbital speed is ~25 on the meter. This was chosen because Mach 1 is ~ 1000 ft/sec and the same "tape" is also the Machmeter. Commented Apr 10, 2019 at 18:35
• On reflection, I'm not sure about the non-zero value on the pad. It may be reading Mach at that point. Pretty sure the rest is correct though. Commented Apr 10, 2019 at 18:48