VTOL longevity challenge

Reusables · June 11, 2017

1 hour ago, goduranus said:

Also, can parts be separated and reattached later? Some helicopter designs on KerbalX will keep the fuselage stable, but undock the helicopter rotor, which is confined in a fairing, and redock it after landing.

Yes, it's fine.

sevenperforce · June 12, 2017

Random -- so why is the Laythe modifier struck out? There should at least be a 3x boost for it, or something.

Joseph Kerman · June 12, 2017

One needs 4400 d/v at least to get an SSTO to Laythe and back without ISRU. Squeezing that much into a small VTOL SSTO is hard. So far, only Stratzenblitz 75 did it and even that uses ISRU.

sevenperforce · June 12, 2017

Just because it's hard doesn't mean it shouldn't be included as a possibility.

Also, this. With a few assists, you can get an SSTO to Laythe and back, in one piece, on a single Rapier, without ISRU. So it shouldn't be at all impossible to make a one-way trip with a VTOL SSTO, even if that merely means throwing a few Twitch engines on to help get the Rapier off the ground.

sevenperforce · June 12, 2017

Also, question -- what if I am able to circumnavigate Kerbin while remaining below 5 km, but that is not my longest timed run?

For example, let's say I can stay aloft at 4500 meters at 200 m/s for long enough to circumnavigate, but I can stay aloft at 100 meters at 70 m/s for much longer? In the former case, I can circumnavigate; in the latter, I can't. Can I count the time from the lower-speed run even though I didn't circumnavigate on that particular trip?

Reusables · June 13, 2017

14 hours ago, sevenperforce said:

Just because it's hard doesn't mean it shouldn't be included as a possibility.

Also, this. With a few assists, you can get an SSTO to Laythe and back, in one piece, on a single Rapier, without ISRU. So it shouldn't be at all impossible to make a one-way trip with a VTOL SSTO, even if that merely means throwing a few Twitch engines on to help get the Rapier off the ground.

Oooh..

Actually the part was a joke for the debunked VTOL SSTO video.(Thus strikeout)

I thought additional vtol engine would be too much dry mass preventing the Laythe capability. Also there's drag issues. For the craft in the picture, over 320kN of upward thrust is needed. It means over 3 thuds or 2 darts are needed. But now I see that the Nerv could make it possible.

So, I'm going to adjust the multiplier to x5 when it's done.

12 hours ago, sevenperforce said:

Also, question -- what if I am able to circumnavigate Kerbin while remaining below 5 km, but that is not my longest timed run?

For example, let's say I can stay aloft at 4500 meters at 200 m/s for long enough to circumnavigate, but I can stay aloft at 100 meters at 70 m/s for much longer? In the former case, I can circumnavigate; in the latter, I can't. Can I count the time from the lower-speed run even though I didn't circumnavigate on that particular trip?

Great point. I'll change it so that circumnavigation run is separate from the main mission, since it's more about capabilities. Then the lower speed one will count.

sevenperforce · June 13, 2017

All right, @Abastro, here's my entry. Not sure if I can manage the Laythe run, but I got the Duna run down.

Four Rapiers and two nukes; the Rapiers use the node-attach-and-rotate trick to negate parasitic drag. The placement of the wing strakes helps keep CoM and CoL stable across a wide range of airspeeds and fuel loads. There are four bipropellant tanks; everything else is liquid tankage.

The Rapiers are set to manual cycle-switching. I programmed action groups to switch the mode of the rapiers, turn the nukes on and off, and toggle the airbrakes. The canards on the nose rotate with the airbrakes to produce a shuttlecock for passive stabilization during high-speed descent.

The ship takes off VTOL and immediately translates to forward flight for efficiency. At full fuel load, it needs to run the Rapiers in closed-cycle to get enough thrust to get off the ground. For maximum efficiency, it can lift off with only liquid fuel on board by using an assist from the nukes.

The Verniers assist with stability during hovering flight.

Taking off in open-cycle (partial fuel load):

Using the action group to switch between open and closed cycle really helps with VTOL landings, since the reaction time of the Rapiers isn't very good in open cycle. I use the efficiency of open-cycle to get into position, and then use the quick throttleability of the closed-cycle engines to hover and land gently.

Flying over to the SPH:

Descending in open-cycle:

Switching to closed-cycle for the fine landing burn:

Landed!

I think I could probably manage all the required landings in a single run.

Taking off from the SPH (open-cycle mode):

Jetting (literally) over to the VAB for another landing.

Approach in open-cycle:

Landing, closed-cycle:

Landed!

Taking off again! I'm almost out of oxidizer so I'll have to try and land on the bridge in open-cycle, which will be...challenging.

Alllllmost...

Made it!

Lifting off again, just to prove I can:

Going to refuel and head off to the island for that landing. This will prove I can transition from forward flight back to a hover.

I nose over almost immediately so it is a lot of fun to just barely skim the ground. Having four Rapiers for a vehicle this small makes it VERY speedy, but since almost all my fuel is in lifting parts, I can also get great stability at low speeds. Lots of fun to fly.

Keeping aero turned on during low-altitude flight helps prevent unintended lithobraking. This jet is almost impossible to stall.

Easily supersonic.

Lined up for my approach, cut thrust to zero. This is where I'd extend landing gear.......oh, wait.

If I switch to closed-cycle temporarily with the throttle set to zero, it cuts the thrust on the Rapiers immediately, which is much better than waiting for the turbofan to spin down.

Then it's just a gradual nose-up until vertical, careful not to stall, and I extend my airbrakes and canards.

The canards really help with stability on descent. Back to open-cycle.

Using closed-cycle to make some adjustments.

Easy...

Almost there!

Made it!

Airbrakes closed, open-cycle Rapiers throttled up, and we're off!

As you can see in this image, the outer ailerons control roll while the inner ones control pitch.

Blowing past Mach 1.25 easily.

Back at KSC. Now, I'm supposed to fly UNDER the bridge, right?

But wait, there's another helipad here!

What's happening? Suddenly I find myself landing on it.

Can't help it.

Now to go fly under that bridge.

Some horizontal translation...

Sizing it up.

Hmmm. I had originally planned to hover under the bridge, but I MIGHT just be a little too tall for that.

Guess I'll have to do it the old-fashioned way!

Taking off, dropping my nose...

Dropping pretty fast here, just a few meters from dragging my canards.

But I punched closed-cycle and made it through!

Back to open-cycle and climbing fast!

So, let's see: I've met conditions 1, 2, 4, 5, and 6. I've landed on the SPH, flown under the R&D bridge, and landed on the R&D bridge. So that's a 1.452x multiplier to my score, right there. Now for the endurance run!

I topped up all my liquid fuel tanks and removed all my oxidizer for maximum efficiency...here goes! Letting the open-cycle Rapiers throttle up:

Kicking on the nukes to help jump off the ground.

Immediately nosed forward. Cut the nukes as soon as I had body lift.

Now, the trick is going to be to see how low my fuel consumption can be in level, stable flight. I don't know if it will be more efficient to fly very low or very high...though 11 meters is probably not safe.

0.45 fuel units per second in a climb while gaining speed...that'll be the value to beat.

The rarefied air at higher altitudes allows lower fuel consumption, though I don't know if I can hold altitude. Plus, as I fly for a bit longer, my fuel load will decrease and I'll be able to get away with lower fuel consumption.

This was my lowest fuel consumption, though I couldn't hold altitude.

Cutting it close over these hills!

Haven't flown this far from KSC before...at least, not this low.

Closest call so far!

Starting to get dark and I can already tell I won't be able to circumnavigate, so I should probably just establish minimum fuel consumption in level flight and finish.

With two thirds of my fuel gone, I'm much lighter, so I can maintain lift at higher altitudes at very low speeds.

And here you have it! Level flight, no loss of speed, at 0.23 fuel units per second.

Based on the above (and Rule 7), I calculate a total remaining flight time of 4,304 seconds, for a total projected flight duration of 2 hours and 47 minutes.

Now, before I apply multipliers, I need to do one more thing...

...go to Duna!

Full fuel load this time, using the Rapiers in closed-cycle to jump off the ground:

Immediate transition to open-cycle.

Going to nose up as rapidly as I can.

The Rapiers are roaring at this point.

Kicking on the nukes to help maintain acceleration and ram effect so the Rapiers don't lose thrust; trying to level out so I don't get too high too fast. I haven't flown many airbreathing spaceplanes before so I'm not quite sure about the right balance.

Switched to closed cycle at a lower speed than I'd like, but oh well.

Cut the Rapiers to conserve oxidizer and I'm powering up toward apogee.

Orbital node set.

Orbit achieved, with quite a bit of fuel remaining!

Duna transfer node set.

Completing Duna transfer burn.

Some fine adjustments....

31 km should be a good place for an intercept, I think.

Inside Duna's SOI, I adjust to a good aerocapture altitude.

Airbrakes extended and canards feathered.

First re-entry heating.

Heating wasn't nearly as bad as I'd anticipated, actually.

Captured and ready to land!

Kicking the nukes on to start slowing down:

Since the nukes don't gimbal I had to turn the Rapiers back to closed-cycle to help straighten out, which burned through my oxidizer.

Final landing approach!

And I'm down! Didn't stay upright, but at least I managed to stay intact!

So let's see here. I'll add the 1.5x multiplier for reaching orbit and the 3x multiplier for landing on Duna intact, which gives me a total score multiplier of 6.534, for a total score of 65,405 seconds.

Reusables · June 15, 2017

@sevenperforce, that's splendid! Thanks for sharing your great plane & entry.

I didn't seriously thought that long-range VTOL SSTO is actually a thing, with tilting the whole plane. Great job you've done there. Also passing under the R&D bridge SSTO... I just want to give one more karma for this!

By the way, I'm trying my own hovercraft on this challenge. Hopefully this will show how a non-plane VTOL could be more efficient on flight..

gchristopher · June 16, 2017

On 6/13/2017 at 7:59 AM, sevenperforce said:

Four Rapiers and two nukes; the Rapiers use the node-attach-and-rotate trick to negate parasitic drag.

Nice entry!

Is that attachment trick explained anywhere? I'm endlessly frustrated by spaceplanes because it seems like the only way to discover drag is to launch something, watch the numbers in the physics debug screen change inscrutably, try to guess a reason, change something and start over. Any part stats related to drag seem to be completely hidden.

sevenperforce · June 16, 2017

11 hours ago, gchristopher said:

Nice entry!

Is that attachment trick explained anywhere? I'm endlessly frustrated by spaceplanes because it seems like the only way to discover drag is to launch something, watch the numbers in the physics debug screen change inscrutably, try to guess a reason, change something and start over. Any part stats related to drag seem to be completely hidden.

Apart from lift-induced drag, you have two areas of drag to contend with: pressure drag and form drag. There's not much you can do about form drag (basically, the friction over the skin of your vehicle), but you can do a little more about pressure drag. Pressure drag happens because you have high pressure at the front of your craft, pushing you backward, and low pressure behind your craft, pulling you backward.

You can decrease the effects of high-pressure drag on your nose by using streamlined parts, like nose cones and shock intakes, which direct the air around the craft without letting its pressure rise as much. But low pressure at the back is tough, especially with engines like the Rapier that just have a big blunt end.

The trick is to place two Rapiers at the back and then add two more Rapiers, flipped backward, attached to the rear node of the first pair of engines. Then rotate the Rapiers and offset them and build the rest of the craft from the rear forward. This tricks the aerodynamics engine into thinking that there is no low-pressure drag at all on the Rapiers.

If you press F12 while flying, you can see the drag over the vehicle and you'll see that there is no red drag at all on the engines that are attached this way.

Spoiler