Flight Efficency!

[DunaManiac]

Guys, I have been trying to build an aircraft capable of circumnavigating kerbin. Does anyone who have done this before know the most efficient flight possible.

[Wanderfound]

If you want to do it in a reasonable time:

* Use Whiplash or RAPIER engines (open cycle only).

* Build to minimise fuselage drag; no exposed draggy bits, and build in a few degrees of wing incidence.

* Climb to 20,000m.

* Crank it up to 1,700m/s.

* Throttle back as much as you can without losing speed.

* Keep an eye on your heat gauges, climb and decelerate if you're about to blow.

See http://m.imgur.com/a/H3z2y for a demonstration.

You can make it more fuel efficient (but a bit slower) by using a Silbervogel suborbital skip; climb at high speed so as to push your apoapsis above 30,000m, coast through the low-drag high altitude phase, power up again as you descend, rinse and repeat. Try to make the "bounces" bottom out not too far below 20,000m.

https://en.wikipedia.org/wiki/Silbervogel

For an even more fuel efficient but much slower version: use Wheesleys or non-afterburning Panthers. Similar flight profile to the non-bouncing version at lower altitude; get as high and fast as you can while holding level flight. It'll take ages, though.

Edited January 26, 2017 by Wanderfound

[AeroGav]

31 minutes ago, Wanderfound said:

If you want to do it in a reasonable time:

* Use  RAPIER engines (open cycle only).

* Build to minimise fuselage drag; no exposed draggy bits

* Climb

Basically, get as high as possible for low drag, but make sure you've got enough wing area to maintain height with the nose 5 degrees above the horizon or less.  If you're having to crank the nose higher than that, you will suffer excess drag. I recommend building your wing out of big S strakes, because they hold more fuel than a big s wing of the same weight, but are just as heat resistant.

If you are at a significant portion of orbital velocity (possible with rapier) then the need for lift will be less, due to freefall supporting you, so for a given lift:drag ratio the drag will be lower.

Don't throttle back,  altitude is your throttle.  You want to be going so high the engine hardly has enough power to sustain you even though it's wide open.  If you keep porpoising out of the atmosphere as a result, you probably have too much engine.

Last thing - control surfaces - you should be able to sustain this high speed cruise without large deflections on your elevons/canards/tailplanes.   Extreme angles create drag.   If large deflection angles are needed to hold the nose up, fit  a larger control surface that can do the job at a lower deflection angle, or take another look at the weight/balance of your craft.

 

[DunaManiac]

Actually, what I was doing was a version of yours. I was flying up to 10-15 km, and up to 600-850 m/s, and throttling back to maintain speed

 

[AeroGav]

2 minutes ago, KerbalXcer said:

Actually, what I was doing was a version of yours. I was flying up to 10-15 km, and up to 600-850 m/s, and throttling back to maintain speed

 

From about 400 m/s all the way to 2200 m/s lift:drag ratios are about the same - 4:1 on a really slick craft, tops.   

So the only reason to go slow is if you are using non-afterburning panther with isp of 9000.   They top out at about 600m/s though - thrust falls to near zero if you try to go faster.

If you are using an engine with flames coming out the back,  then just try to get as close to orbital velocity as possible.

And fly as high as you can to lessen drag.

[bewing]

And if you are good at handling heat dissipation, build your plane out of MK1 parts. They have a third of the drag of any other kind. So they need 1/3 as much engine to go the same speed. Which reduces weight, which reduces fuel needs, which increases range, which reduces fuel needs ....

 

[Wanderfound]

Demonstration with some passengers:

 

Twenty passengers, 44 and a half minutes, 1165 units of fuel.

Album at http://imgur.com/a/ee8KQ

Edited January 27, 2017 by Wanderfound

[DunaManiac]

Thank You all for your advice, I was able to circumnavigate kerbin in 1hr 5 mins! (pictures later)!!!!!!!!!

[ForScience6686]

https://kerbalx.com/ForScience6686/Gannet

It's not too difficult.  This is my newest ultra long distance, water capable craft.  Best to stay as high as you can maintain.  Or parabolic flights that take you up above air breathing range and back down work well too.