Transition from VTOL to horizontal flight

[gilflo]

OK, but but shock cones not in the air flow does not give a "real aircraft look".....

Try a dozen of structural air intakes, take à 30° angle on mechjeb ascent, let speed increase without overpass terminal speed - find the good angle each 5000m - and you should climb that to 30000m.

When air density decrease I am wondering if increasing speed is a factor for air intake to help swallow more air and so keep air cycle running.

[FlipNascar]

I completely agree that a shock cone placed like that does not look "real". That shock cone was placed purely for CoM reasons, as it was just a quickly thrown together jet that would have been potentially overly sensitive to balance issues, and there was no "realistic" place to add the cone to it without altering the design. It was purely to demonstrate the differences between the two intakes. And, given that orbit is your objective, you want as much speed from the jets as possible. Therefore, you want more intake area, not necessarily more intakes. More intakes = more drag, less speed in atmosphere possible, therefore less dV when you reach orbit because you'll need a longer burn from the closed cycle.

Try it for yourself. One Rapier engine, one shock cone. Then try it again with the exact same jet, except for replacing the shock cone with a structural intake. The advantage of more intake air is only relevant at low level altitudes if you are trying to feed more engines with fewer intakes for a more mass (all relative right?) and drag optimal design.

I see no point in repeating the exercise for you as I'm convinced I am right. Therefore I strongly encourage you to try it too, so that you can make your own conclusions. The experiment was as controlled as it could be, all three flights were flown using the same base craft and the same ascent profile. By changing the ascent you may well change the values (alt/speed etc before flameout), but overall the result will still be the same. A shock cone will outperform the structural intake for getting to orbit.

[gilflo]

OK I'll try it and keep you advised.

[gilflo]

Hi FlipNascar

Can we agree that the higher we go on Rapier air cycle at the higher possible speed before switching on Closed cycle the more fuel and oxidizer we save and the more DV we save once in orbit?

The purpose is to switch on closed cycle as higher as possible

Here is a Rapier Aircraft I built to make the test with structural Air intake versus Shock cone intakes. All test with Mechjeb turn shape 30% - Manual switching for Rapier cycle, I let it run until flame out.

First pic with 10 Strucural air intakes: We are at 20000m

- Total air intakes is 10 - Effective air intake 0.09

- Air Flow is 0.47 per Air intake (totalAir flow 4.7)

- Speed 525m/s Fuel remaining 409/450

Second Pic: 25000m

- Total air intakes is 10 - Effective air Intake 0.04

- Air Flow is 0.20 per Air intake (totalAir flow 2.0)

- Speed 786m/s Fuel remaining 404/450

Third Pic: 29500m just before flame out, RAPIER is still running on air cycle. Thrust is 126.9. It will flame out at 29800m

- Total air intakes is 10 - Effective air Intake 0.02

- We have still Airflow as Rapier is running

- Speed 953m/s Fuel remaining 401/450

4th Pic: Same aircraft but with 4 Shock cones, because with 1 only RAPIER flamed out at 19500m

We are at 20000m like Pic number 1

- You can see speed and fuel remaining are quite similar

- Total air intakes is 0.80 versus 10 with structural AI - Effective air intake 0.05 versus 0.09 for structural AI

- Air Flow is 0.71 per unit versus 0.47 for structural AI and Total is 2.84 (4 unit) versus4.7 with structural AI

5th Pic: Rapier is still running but you can see Thrust is only 117.4 , less than the thrust we have on Pic 3 at 29500m with our 10 structural AI . We are only at 23500m and he will flame out at 24000m

My conclusion is the drag and weight of Air intakes are not a factor.

The factor is the more airflow you get the higher altitude the Rapier will switch on Close cycle.

Last Pic: Same aircraft 16 air intakes

- Same speed and fuel remaining at all altitude than other air intakes configurations

- notice the altitude and speed just before the Rapier flame out

[FlipNascar]

I think you may have used the wrong intakes for your comparison (in the pictures they look like the circular intakes). So in the pursuit of an answer, I have copied your craft and have run the four following tests. Also don't forget that while you are correct in the assumption that the higher you are the better before switching to closed cycle, you also want maximum speed too, this way you are closer to orbital velocity, and therefore need to burn less dV to make orbit. Also, if you are going to do direct comparisons between the intakes you should use an identical number of intakes for both.

Test A (10x Shock Cone) - MJ ascent to match your profile, 80km desired AP, 1km Gravity turn and 30 degree profile - 10 Shock cones. In a bid to remove human error/influence, MJ autopilot is switched on at c. 1km, MJ prevents flameouts, smooth throttle and manages the intakes for me. Take off run, due to shock cone placement - full power and rotates once the ground drops away from it at the end of the runway, 45 degree climb to 1km, then autopilot.

Test B (10x Shock Cone) - as Test A, but autopilot will be switched off at c. 20km and a typical spaceplane launch profile will be used, aim for no more than 100m/s vertical speed, and will switch to closed cycle once the craft stops accelerating. Maximum speed will be noted as accurately as possible. And Orbit circularized.

Test C (10x Structural) - as Test A, bar the change in intakes.

Test D (10 x Structural) - As Test B, bar the intakes being swapped.

In a nutshell, Tests B and D were the most of value, but also the most likely to be influenced by human error. However, the plane in Test B made it to 40,000m before having to switch to closed cycle, whereas Test D, it really was around the 30,000m point, and with a lower speed. The image gallery shows the test results from A to D. Overall to Orbit, and yes these tests should really be repeated to allow averaging, but in this instance the difference upon reaching stable orbit was around 193m/s in favour to the shock cone craft.

Zip file with both test aircraft: https://www.dropbox.com/s/7s9zfn9pds4dujh/IntakeTesters.zip?dl=0

Image Gallery:

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[Claw]

Intakes are pretty straight forward. There is no inherent difference between the types of intakes, only difference in their respective area. So more area = more IntakeAir for a given speed/altitude.

The second thing to consider is the angle of the intake with respect to airflow. Once an intake's "forward" direction is more than about 30 degrees from the air flow, it's efficiency drops off quite a bit. However, even when placed 180 degrees to the airflow, an intake will still produce intake air.

The actual position of the intake does not matter either. The IntakeAir resource flows anywhere on the vessel without regard to crossflow capability or any connecting parts (just like monopro and electricity).

So yes, more intake area is going to be more effective than less intake area, regardless of which intakes are providing that air. And if you orient intakes off of prograde, their efficiency will be reduced. You can place them "anywhere," but no necessarily in "any direction" without having an effect.

Cheers,

~Claw

[gilflo]

The shock cones allow better airflow, so you're right, I should have put as many sick cones as structural air intakes.

The flow is better with shock cones, but they don't seems very realistic.

The result is, whatever are the air intakes, the more airflow you swallow, the higher you climb on air cycle, the better it is and better again if you got the higher possible speed.

[Claw]

Very true. Sometimes it's easy to get caught up in "what is most optimal" when choosing parts or making designs. I also like structural intakes for looks on some designs too, even though they aren't "the best" in terms of mass to intake area.