Absurdly high plane delta-v?

[Duke Leto]

OK, so the amount of liquid fuel in Mk1 fuselages has gone from 150 to 400 as of yesterday's update. I'm also using Kerbal Engineer Redux, in case that was not obvious.

Means I have to redo all the ship designs incorporating it, including the planes.

So what I now see is insanely high ranges on the plane models, which are also drastically slower.

Is this intentional behavior or is my install now FUBAR?

Edited June 23, 2015 by Duke Leto

[Streetwind]

dV is a largely meaningless figure for planes. It's useful in space, because any velocity change you make is permanently kept, so you can count velocity changes. However, in an atmosphere, even as you thrust to increase velocity, drag makes you constantly lose velocity. How much you lose depends on your current velocity, on your flight altitude, on the shape of your plane, on the temperature and makeup of the atmosphere, even on your current orientation on the navball...

So yeah, I'm afraid that you shouldn't put much value in that absurdly high number. It only makes sense in space. For your plane, you're better served finding your maximum range, which is a result of finding the cruising altitude at which you can maintain the highest level airspeed while having your throttle the lowest and keeping Isp up as much as possible. It's a complicated problem, unfortunately.

[Evanitis]

From patch notes:

* With lowered drag for sharply-tapered cubes, wing lift and wing drag lowered to match.

(...)

* Drag curves modified to lower transonic hump.

* Wing curves modified to lower change in drag based on deflection.

(...)

* Jet thrusts rebalanced for new drag (thrusts lowered, BJE curves altered). Jet Isp halved due to increased fuel quantity and lower drag.

Can't say I understand every bit, but those seem relevant. So according to this, air-drag is drastically decreased, thus the trust of jet engines got halved. I see that it means an extreme atmo-dV, but who cares about that? I'm more interested in how turbojet SSTOs react to the patch.

Guess I won't update yet so I can check it myself. I'll wait for a few more threads like this so I can see how it worked out for the others.

[Padishar]

The most relevant thing is that the deltaV calculation in the current release of KER is broken for air-breathing engines due to hurried changes to support KSP 1.0. The code is currently undergoing an overhaul to fix this and several other issues. Hopefully a new version should be released quite soon...

[Duke Leto]

OK, thanks for verifying that everything is kosher.

Most of what I'm working on right now is related to using the nuclear engines with these larger capacity Mk 1 LF fuselages.

This produces some truly huge delta-v, and I guess I just thought it seemed too good to be true.

Do have to redesign all my planes now though... (All 3 of them...)

[pincushionman]

As mentioned earlier, dV is a somewhat meaningless value for planes, given that they fly by generating aerodynamic lift and are always subject to drag. The relevant measure for real* planes is range, which is a function of burn time (which can be shown in KER) and cruise speed (which is not).

Spaceplanes be a little different, where you actually want maximum speed at a particular altitude. But since you're creating it through the same means as traditional flight, it's still more like "achieving a velocity" than it is a dV "change in velocity."

* complete flight in-atmosphere

[Padishar]

The issue in the deltaV calculation is more accurately described as an issue with the burn time calculation as the code concerns mass flow rates and time required to drain tanks. The burn times for air-breathing engines are out by the ratio of intake air to fuel (can't remember the exact figures but a quick test will demonstrate this, just create a vessel with a jet and intake and a small amount of fuel, see what KER says for the burn time and then test it on the pad).

[ajburges]

Don't pay too much attention to KER aerobic dV.

The best use for it is a relative range. Two craft given similar profiles will have a corollation between range and listed dV.