Maximum air speed

[Jarin]

I was toying around with a couple speed challenges, and ran into a weird issue. My plane would slowly creep up over 1750m/s, then at 1751, the engines would shut off for a split second (no visual effect, but thrust dropped to zero on right-click readout), then start spooling up again. They're producing less than 10kn max at this point, so that doesn't take long, but it means the jet consistently bounces between 1751m/s and then down to 1740, rather than holding a consistent speed. I'm using a single precooler per rapier, but I was wondering if there's some kind of maximum-velocity intake cut off that could be causing this. Any ideas?

Edited title to reflect further discovery.

Edited January 20, 2017 by Jarin

[Speeding Mullet]

7 hours ago, Jarin said:

I was toying around with a couple speed challenges, and ran into a weird issue. My plane would slowly creep up over 1750m/s, then at 1751, the engines would shut off for a split second (no visual effect, but thrust dropped to zero on right-click readout), then start spooling up again. They're producing less than 10kn max at this point, so that doesn't take long, but it means the jet consistently bounces between 1751m/s and then down to 1740, rather than holding a consistent speed. I'm using a single precooler per rapier, but I was wondering if there's some kind of maximum-velocity intake cut off that could be causing this. Any ideas?

there's a maximum-velocity cut off for the engines rather than the intakes as far as I am aware.

The Wiki (admittedly old information but I don't see why the basic principle would have changed) states:

"One large difference from the jet engines compared to conventional rocket engines is, that the thrust also depends on the velocity relative to the atmosphere around. Starting with the (nominal) stationary value, thrust starts rising with additional speed........At a certain speed (the design speed of the engine) peak thrust is reached, from where on it decreases with additional velocity. At the maximum speed of the engine, it stops putting out thrust, resulting in a shutdown"

This is almost certainly what you are facing into.  I couldn't find any definitive information on what engine cuts out at what speed I'm afraid!

Edit:  At risk of my post being immediately and succinctly proved wrong, I'm going to pull in the human calculator and font of knowledge @GoSlash27 with a request to impart smarts, very possibly against his will.

SM

Edited January 20, 2017 by Speeding Mullet

[Jarin]

1 hour ago, Speeding Mullet said:

there's a maximum-velocity cut off for the engines rather than the intakes as far as I am aware.

That's what I thought as well, but that's supposed to function through the kn of thrust produced by the engines slowly dropping to nil. I've never run into this weird thrust-bouncing before. I'll try to get some screenshots to better demonstrate, and maybe slashy can drop by and tell me what I'm missing.

Okay, here we go. Images of the odd behavior below:

Spoiler

Approaching cutoff,engines steady at 7.7kN

1751m/s, there's a popping noise and the engines' thrust drops to 0 in an instant.

Speed bottoms out around 1745, then starts climbing again as the engines spool up from 0 back to 7.7.

 

Edited January 20, 2017 by Jarin

[bewing]

1 hour ago, Speeding Mullet said:

 I couldn't find any definitive information on what engine cuts out at what speed I'm afraid!

SM is right. When an airbreathing engine goes below a certain minimum thrust, it shuts down -- it doesn't go all the way to 0 smoothly. And that shutdown number is usually right around 10kn. But the current thrust depends on the speed, and altitude, and several other factors. So there is no exact speed for shutdown. It's different for each engine type. AFAIK, goliaths shutdown at the lowest speed, then wheesleys, then junos, then panthers, then whiplashes, then rapiers.

Edited January 20, 2017 by bewing

[Jarin]

Further testing determined the cutoff is 1751.7m/s. I can cruise at 1751.6 with both engines at 5.6kN indefinitely. I suspect this varies slightly with altitude, but I need sleep and will test more tomorrow.

Edited January 20, 2017 by Jarin

[GoSlash27]

Jarin,

 The engines themselves limit the top speed and their efficiency varies with altitude. The intakes, OTOH, are only limited by minimum airspeed.

 I've never run an air breather fast enough to replicate what you're seeing, so I'm not sure why your engines are cycling. That usually happens when they're starved for air, not when they cease to produce usable thrust. I'm guessing you need to add intake area and see if the problem goes away.

 There's also a quirk in KSP where the order of assembly affects how engines are fed. You can run into a situation where one engine cuts off and then all intakes feed the other engine. To keep that from happening, you need to not install engines and intakes using symmetry. Instead, do intake 1, engine 1, intake 2, engine 2, and so on.

HTHs,
-Slashy

[Jarin]

7 hours ago, GoSlash27 said:

I'm guessing you need to add intake area and see if the problem goes away.

Two shock cones added, cutoff point is exactly the same. It's gotta be the engines. Current theory is that if thrust would drop to 5.5kN, it instead cuts off the engines. Going to test at different altitudes in a few and report back.

Okay, two discoveries. First is that I apparently stumbled entirely accidentally into a perfect sweet spot of thrust, altitude, and drag. Re-creating that caused some headaches.

Secondly, it does vary with altitude. Here's what I found (note, thrust is per engine, with 3 engines):

19000m = 1749.6m/s, 7.2kN thrust, 21.2kN drag
19500m = 1750.6m/s, 5.8kN thrust, 17.0kN drag
20000m = 1751.3m/s, 6.1kN thrust, 18.3kN drag
20500m = 1751.6m/s, 6.5kN thrust, 19.6kN drag
21000m = 1751.8m/s, 6.8kN thrust, 20.7kN drag
21500m = 1751.4m/s, 7.1kN thrust, 21.5kN drag

Does NOT correlate to exact level of engine thrust. On repeated tests, thrust and drag vary ever so slightly with fuel load, but top speed remains unchanging.

Attempted to test at 22000, but my craft isn't capable of reaching the cutoff speed at that altitude, There is no variance in my testing, though, so that 0.4m/s drop at the end is notable. Top speed increases up to 21000m, then starts going down again. @GoSlash27 would love to hear your thoughts.

Edit: added drag for completeness sake.
 

Edited January 20, 2017 by Jarin

[GoSlash27]

Jarin,

 You're into territory I haven't explored, I'm afraid. I'm used to engines staying lit so long as they have resources even if they happen to be producing *negative* thrust. Something must've changed in the code that I didn't notice because I don't drive that fast on air breathers due to heating. You've eliminated air starvation as the culprit, so the engines must be causing it.
 I'm going to take a page from @Speeding Mullet and summon the Kraken. @NathanKell is the resident expert on the coding regarding engine resource allocation and shutdowns. Perhaps he can shed some light on this weird phenomenon...

Good luck,
-Slashy

Edited January 21, 2017 by GoSlash27

[Jarin]

1 hour ago, GoSlash27 said:

I don't drive that fast on air breathers due to heating.

I'm rather proud of that little speed demon up there. Still working on how to integrate the heat-spike design into something actually career-useful though.

[fourfa]

yeah, more detail on how you're not blowing up?  I've never beaten 1650m/s and now I have a new goal...

[Jarin]

Radiators inside the faring. :3

Plus strategic placement of heat shields (ablator set to 0) to block spread of heat where necessary.

Edit: @fourfa here you go, There's a few other details in there, but it's not too fancy.

Edited January 21, 2017 by Jarin

[fourfa]

@Jarin I finally got around to flying this ship.  Deeply impressive!  Giving me all kinds of ideas (don't worry, I don't usually do challenges)

[Jarin]

Just now, fourfa said:

@Jarin I finally got around to flying this ship.  Deeply impressive!  Giving me all kinds of ideas (don't worry, I don't usually do challenges)

Eh, have fun. I'm looking forward to seeing what people come up with now that we're pushing the edge of what's possible. Just be warned with that craft... it's a (very accidentally) precisely-tuned instrument. Every adjustment I've made to it seems to kill its ability to max out the engine speeds.

[fourfa]

I'm pretty happy with 1751m/s and a 39 minute circumnavigation!  Hard to slow down at the end though LOL, and she REALLY does not handle below Mach 4 or so

Edited January 26, 2017 by fourfa

[Jarin]

55 minutes ago, fourfa said:

I'm pretty happy with 1751m/s and a 39 minute circumnavigation!  Hard to slow down at the end though LOL, and she REALLY does not handle below Mach 4 or so

Yeeeahh... that model's not exactly... designed to land again. <.<;

 

[Kerbal Noob]

I know this is an old thread, but I just bust 1910m/s which is some way above anything here, with stock parts, straight and level on air. I used Rapier's with a single pre-cooler, they are fine at sea level. 

The problem is always heat. So pop Heat Shield (10m) on the front. Don't open it. Leave it closed. It makes a nice cone for you to fly behind. Now power is the issue ... 

I don't think busting 2000m/s is out of the question. 

[Gaarst]

1 hour ago, Kerbal Noob said:

I know this is an old thread, but I just bust 1910m/s which is some way above anything here, with stock parts, straight and level on air. I used Rapier's with a single pre-cooler, they are fine at sea level. 

The problem is always heat. So pop Heat Shield (10m) on the front. Don't open it. Leave it closed. It makes a nice cone for you to fly behind. Now power is the issue ... 

I don't think busting 2000m/s is out of the question. 

I'm curious to see how you reached 1910m/s since the cutoff speeds for jet engines haven't been changed recently.

You need to upload your screenshot to an image hosting website such as Imgur (not account required; other hosts work fine) and then paste the image link in your post (the link ending in .png .jpg or whatever) since the forum can't host images itself.

[AeroGav]

20 minutes ago, Gaarst said:

I'm curious to see how you reached 1910m/s since the cutoff speeds for jet engines haven't been changed recently.

Aren't the thrust / velocity curves based on mach number?   At sea level, Mach 6 corresponds to a higher absolute speed, though of course heating. drag and fuel consumption will be insane. 1900 sounds like a lot though.

My 42.7% payload fraction liquid fuel only ship got to 1400 before starting the nukes.   It was still accelerating but very slowly.  We were in level flight and drag was now 2/3 of our thrust,  so i figured i'd be better off adding nukes to the mix rather than stay on jets only.   Jets may be 4x as efficient but if the thrust has become so feeble 2/3 of it is being gobbled by drag, time to add more power.

[DarkOwl57]

On 1/19/2017 at 6:01 PM, Jarin said:

I was toying around with a couple speed challenges, and ran into a weird issue. My plane would slowly creep up over 1750m/s, then at 1751, the engines would shut off for a split second (no visual effect, but thrust dropped to zero on right-click readout), then start spooling up again. They're producing less than 10kn max at this point, so that doesn't take long, but it means the jet consistently bounces between 1751m/s and then down to 1740, rather than holding a consistent speed. I'm using a single precooler per rapier, but I was wondering if there's some kind of maximum-velocity intake cut off that could be causing this. Any ideas?

Edited title to reflect further discovery.

I believe (But not certain) that your problem might be coming from air intake issues

When I was making a hyper jet (4 Whiplash engines strapped, clipped, and otherwise added to the fuselage), I found the shock nose intake to be the best bet for an intake. Of course it could also be that you're too high in the atmosphere

[Jarin]

2 hours ago, DarkOwl57 said:

I believe (But not certain) that your problem might be coming from air intake issues

When I was making a hyper jet (4 Whiplash engines strapped, clipped, and otherwise added to the fuselage), I found the shock nose intake to be the best bet for an intake. Of course it could also be that you're too high in the atmosphere

Nah, been over that. The discussion's around here somewhere (a challenge thread I think?), but put simply, changing the intakes up didn't seem to have an effect on the top speed. Still waiting for a response from @Kerbal Noob up there to clarify the 1900... I suspect shenanigans. 

Edited August 30, 2017 by Jarin