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mrmcp1

SSTO won't break mach barrier

Question

I am attempting to build a relatively low thrust to weight ratio SSTO (about 15.5 tonnes to one Rapier) but I just can't get it to break the mach barrier. The SSTO can get to about 315m/s and about 185kn of thrust but it will go no faster. At takeoff I pitch up to 5 degrees and keep it at about the same angle untill about 5000m but then my velocity starts to drop. I have tried @Rune's White Dart which is heavier than mine but has no trouble breaking the mach barrier.  The only thing I can think it might be is that it only has 2 of the 0.625m air intakes. 

Here is a download link if you need a closer look: http://kerbalx.com/mrmcp1/21

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I'd guess and say drag and weight is your issue. You have a number of wing segments and those tanks. Plus you are a bit heavy for one Rapier.

I'd lose that heavy probe core, reduce the fuel load and reduce the wing area and number of parts.

This modified version makes orbit OK...

vTgMMNn.jpg 

Craft file: https://www.dropbox.com/s/e38y5wh7qgv8crx/2_1%203.craft?dl=0

Edited by Foxster

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@Foxster Thanks, I know how to make an SSTO with a higher TWR reach orbit but I was attempting to make a low TWR SSTO like Rune's White Dart (which is 3 tonnes heavier than mine). Thanks though, those your modified version manage the payload as well?

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My understanding of the rapier engine is it's poor low speed performance.  Now I could be wrong as I haven't unlocked or used it.  But I believe that is what I have seen around the forums. It's real power comes after mach 1.  Have you tried dropping the nose to get over the hump then return to climbing?

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1 hour ago, mrmcp1 said:

I am attempting to build a relatively low thrust to weight ratio SSTO (about 15.5 tonnes to one Rapier) but I just can't get it to break the mach barrier. The SSTO can get to about 315m/s and about 185kn of thrust but it will go no faster. At takeoff I pitch up to 5 degrees and keep it at about the same angle untill about 5000m but then my velocity starts to drop. I have tried @Rune's White Dart which is heavier than mine but has no trouble breaking the mach barrier.  The only thing I can think it might be is that it only has 2 of the 0.625m air intakes. 

Here is a download link if you need a closer look: http://kerbalx.com/mrmcp1/21

Yup, I do believe the problem is an air-starved engine here. They throttle down automatically when they run out, and those intakes don't give much. I'm afraid the White Dart misleaad you, because in that bird I "cheated" a shock cone intake on the rear node of the RAPIER, meaning I had both much more air than I needed, and less drag on account of no open node on the back (they are really draggy). Still, if the whole thing ends up with you knowing a bit more about the game, and you enjoyed the WD, I will still count it a win.

The wing is fine, BTW. It might be a bit much, but wings are light and the more you have of them, the shallower you have you AoA be and still fly level, meaning they actually decrease drag. Don't be afraid to prettify your plane with wings, as long as they are not angled to the airstream and the CoL .

 

Rune. I like the bird, BTW. :)

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22 minutes ago, mrmcp1 said:

@Foxster Thanks, I know how to make an SSTO with a higher TWR reach orbit but I was attempting to make a low TWR SSTO like Rune's White Dart (which is 3 tonnes heavier than mine). Thanks though, those your modified version manage the payload as well?

Ah, apologies. I got into fix-it mode without reading the spec properly. 

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@Foxster Thanks anyway 

@Rune I tried clipping a shock cone into the back of the rapier but it still topped out at 320m/s. Thanks for saying you like it. 

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2 hours ago, mrmcp1 said:

I tried clipping a shock cone into the back of the rapier but it still topped out at 320m/s. Thanks for saying you like it. 

Oh, I forgot to mention, you have to turn it into the airstream too. The angle it sees the velocity with matters, an intake 180º (backwards) won't work even close to how it works when facing forward. And if that isn't what is happening here, I would be very surprised indeed... to the point of asking you for the file and seeing for myself how to fix it! :)

 

Rune. Always glad to be of help.

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mrmcp1,

 I have had success with the RAPIER at weight loadings over 40t/ engine. I'm sure the weight isn't a problem.

 Your problems look to be insufficient intake area/ wrong intake type and excessive drag.
 I'd recommend switching to 3 structural intakes to take care of that issue. After that, I'd try to stack those side fuel sponsons into a Mk.2 tank in series and give the wings some static incidence so that your nose is pointed prograde at Mach 1.

Best,
-Slashy

Edited by GoSlash27

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43 minutes ago, GoSlash27 said:

I have had success with the RAPIER at weight loadings over 40t/ engine. I'm sure the weight isn't a problem.

 Your problems look to be insufficient intake area/ wrong intake type and excessive drag.
 I'd recommend switching to 3 structural intakes to take care of that issue. After that, I'd try to stack those side fuel sponsons into a Mk.2 tank in series and give the wings some static incidence so that your nose is pointed prograde at Mach 1.

I agree with what @GoSlash27 said. It's definitely not a weight issue. It is likely to be an intake issue.

@mrmcp1 Your craft looks reasonably low drag, except for the lack of wing incidence, so I don't think it's that either.

And @Rune is right. It is a very pretty craft :D

But it shouldn't be low TWR at 15 t / RAPIER. In my hangar anything below 18 t / RAPIER is high TWR and above 22 t / RAPIER is low TWR.

I will shamelessly plug a couple of my own crafts as examples of low and high TWR. (craft links include ascent profile instructions)

High TWR: SSTO B-1 Polo (16-17.5 t / RAPIER depending on payload) 

qp1mE3g.png

Low TWRSSTO S-4 Chibi Skylon (25-32.5 t / RAPIER depending on payload) 

PPvAIec.png

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@Rune After attempting to place the shock cone intake again I managed to break the mach barrier and was just short of orbit. Thanks 

@GoSlash27 Switching the intakes improved things but did not do the trick. When you say stack the side fuel tanks in series with a MK2 tank what do you mean? Also when you say give the wings some static incidence is that the same as angling them upwards? Thanks for the help. 

@Val I am glad you like it, I will have to look at your craft for inspiration. 

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I believe attaching the shock come to the rear of the rapier allowed you to break the sound barrier from the increased intake air more than from reduced drag.  If you want to make it go faster still, try replacing those mk0 fuselages to reduce drag further. 

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5 hours ago, mrmcp1 said:

Switching the intakes improved things but did not do the trick. When you say stack the side fuel tanks in series with a MK2 tank what do you mean?

mrmcp1,

 I mean take that fuel and put it in a series Mk.2 tank so it's not hanging out in the breeze. Drag is your enemy in spaceplanes, and tucking those tanks in where they currently are doesn't shield them from creating additional drag.

5 hours ago, mrmcp1 said:

Also when you say give the wings some static incidence is that the same as angling them upwards?

Yessir. Angle them up a little bit so your nose is pointed where you're going. That helps with drag tremendously.

Best,

-Slashy

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@GoSlash27 Getting rid of the side tanks and placing a mkII tank in series helped. With 2 of the structural intakes I easily broke the mach barrier. Does that mean that the lower the TWR of the SSTO the more radial parts should be avoided? Thanks for the help, I just need to add more fuel and I will have my first working 1.05 SSTO. 

EDIT:  2oXeRiS.png I managed to make orbit and still had 2/3 of the payload fuel left. Thanks for all the help

Edited by mrmcp1

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24 minutes ago, mrmcp1 said:

Does that mean that the lower the TWR of the SSTO the more radial parts should be avoided?

When designing spaceplanes drag is king.  You always want to reduce the number of radially attached parts as much as possible.  The lower your TWR the more you will need to pay attention to this.

Happy landings!

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7 minutes ago, Starhawk said:

When designing spaceplanes drag is king.  You always want to reduce the number of radially attached parts as much as possible.  The lower your TWR the more you will need to pay attention to this.

Happy landings!

^ Quoted for truthiness. 

Best,
-Slashy

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On 1/25/2016 at 7:11 PM, mrmcp1 said:

I am attempting to build a relatively low thrust to weight ratio SSTO (about 15.5 tonnes to one Rapier) but I just can't get it to break the mach barrier. The SSTO can get to about 315m/s and about 185kn of thrust but it will go no faster. At takeoff I pitch up to 5 degrees and keep it at about the same angle untill about 5000m but then my velocity starts to drop. I have tried @Rune's White Dart which is heavier than mine but has no trouble breaking the mach barrier.  The only thing I can think it might be is that it only has 2 of the 0.625m air intakes. 

Here is a download link if you need a closer look: http://kerbalx.com/mrmcp1/21

I found the issue with your original craft. It was a case of unfortunate node snapping. The primary issue was that, the rear part of the craft was attached to fuel tank in the cargo bay, instead of being attached to the cargo bay. And that created too much drag to get up to speed.

GrOOhAh.png

I removed the tank in the cargo bay, reattached the rear fuselage correctly, and removed the unnecessary fuel ducts.
 

Spoiler

 

Now I was able to reach Mach 1 in a 5° climb. Then I leveled out and let it build speed to 450 m/s, before climbing at 10°.

0ogaXEF.png

I was able to get it up close to 1000 m/s, before the Intakes became a problem.

X0y8CIH.png

I replaced the small circular intakes, with small nose cones and added a couple of adjustable ramp intakes under the wings. I also gave the wing 1° of incidence, added RCS ports, and replaced the fuel cells with solar panels (for FPS reasons).

z5gys8V.png

And now it can just barely go to orbit without payload, but with enough fuel to deorbit. There's a little excess oxidizer, which could be replaced with a very small payload. Or 2 more small LF tanks could be added.

Craft link, if anyone's interested.

Edit: I added the 2 small LF tanks mentioned above, and that made the fuel balance better. Craft link updated and here's a video.

 

 

 

Edited by Val
Added video

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Yup, you figured it, Val. And a snappy SSTO that you got out of that. I'm still not fully sold on incidence, though. It is hell to make it look good, and at most you save a bit of body drag, which in Mk2 fuselages comes with it's own lift (the thrust portion you waste to cosine losses is truly insignificant). And AoA is very small anyhow... anyway, the thing that I really don't get it this:

On 28/1/2016 at 11:17 PM, Val said:

replaced the fuel cells with solar panels (for FPS reasons).

What? I would get that a static single part should be much easier on the CPU than two moving ones, right? Because FPS means Frames Per Second in this instance? Or is there something weird about fuel cells that I don't get? :confused:

 

Rune. They are becoming my go-to powerplant for everything with oxidizer.

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I feel like people are really sleeping on how little fuel the FCAs use in real-world use (well, real-game).

I'm sure you guys have seen ion-powered probes that use FCAs and a tiny LF/O tank instead of solar arrays, yielding lower overall mass and higher overall dV (not to mention lower part count).

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15 minutes ago, fourfa said:

I feel like people are really sleeping on how little fuel the FCAs use in real-world use (well, real-game).

I'm sure you guys have seen ion-powered probes that use FCAs and a tiny LF/O tank instead of solar arrays, yielding lower overall mass and higher overall dV (not to mention lower part count).

I really don't see the point of using solar panels for ion craft at all. They are big, heavy, weak, they get occluded and they don't work out at the outer planets. 

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You can delete "ion" from your comment too.  RTG for all ships (once researched), FCA as needed, and leave the solar at home.  Haven't had a mission-ending broken solar panel since!

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Agreed. I am even thinking of making a little mod to add an RTG into all capsules (and the larger probe cores and reaction wheels) as I always add one. 

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10 hours ago, Rune said:

Or is there something weird about fuel cells that I don't get? :confused:

I was referring to this:

Edit: Arg. the link won't go to the intended post, because the default sorting of the thread is by rating. The post was originally on page 2, but is now 3rd on page 1, because it was upvoted.

Edited by Val
Link fubar

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Please excuse the ultra-noob question, but what is meant by "incidence" with respect to wings?

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19 minutes ago, CdrFuzz said:

Please excuse the ultra-noob question, but what is meant by "incidence" with respect to wings?

It's when the wings have a built-in angle of attack. Basically you rotate the wings so the if the plane is level, the wings are angled higher in the front and lower in the back. This gives them an angle of attack with which to generate lift, while the rest of the plane is level and making as little drag as possible. 

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