WHy is my SSTO not getting speed

[Patayyyyyto]

I made a very simple SSTO. Wings, fuel tank(with some LQ fuel emptied), air intake in the front, and the RAPIER engine.

So I take off and start maneuvering my SSTO into a 70 degree approach. All of a sudden, my SSTO starts to loose speed. I check the air resistance, and its pretty normal. I tried everything! Other engines other wings. Nothing works! I'm getting frustrated

[Lindy]

The RAPIER will suck down fuel like nobody's business... Ya sure your not running out of gas? At speed you may want a different intake. Try the shock cone, it does better at higher speed and altitude. 

[micr0wave]

the RAPIERs will produce more thrust somewhere past mach 1 (~340m/s).

To use the benefits of an airfed engine and wings you should take up as much speed from the engines as long they've air.

Depending on how it flies and how much thrust it has it will differ, but when i go for that, i pick up enough speed to have the rapiers giving more thrust and at 10km or so i go into a 10-20° pitch and let it run until it has most likely a speed above 1100m/s and is above 22km or so before i switch to rocket mode, mileage may vary depending on craft.

p.s.: will most likely be not the best or economic method but works for me

Edited February 24, 2016 by micr0wave
p.s.

[GoSlash27]

#1 all jet engines will lose thrust with speed and altitude. You will eventually hit a wall where the engine can't accelerate you anymore.

#2 Wrong intake for this job. Different intakes will lose efficiency at different speeds. The basic circular intake is a low- speed intake. The best high speed intakes are the shock cone, the precooler, and the radial "variable ramp".

Best,
-Slashy

[Lindy]

As a follow-up, I did a quickie build of something similar, and actually got sub-orbital at mach 3.5 or so :). Are you shifting modes on the RAPIER? Once you get up so high the air gets too thin for the engine to breath, so you shift it over to rocket mode, and go like your hair was on fire, and your back side catchin' ... Well, til you run out of gas anyway!

BTW... Add some RCS thrusters... Trust me you'll want them!

[Val]

It also looks like you have negative Angle of Incidence (the leading edge of the wings is lower than the trailing edge). That increases drag, though this particular craft should have more than enough to overcome it. It does hurt your efficiency, though.

[Lindy]

6 minutes ago, Val said:

It also looks like you have negative Angle of Incidence (the leading edge of the wings is lower than the trailing edge). That increases drag, though this particular craft should have more than enough to overcome it. It does hurt your efficiency, though.

A couple degrees of dihedrial don't hurt neither...

[Patayyyyyto]

Ok, I made a new module of my SSTO.

But, I never have enough fuel. What ever I do I don't have enough LQ fuel. My most recent flight reached a peak of 1700m/s. But I still did not have enough fuel.

[Raideur Ng]

70 degrees is an insane flight profile. Most SSTOs are between 5 to 15 degrees. You should be gathering as much velocity during your air breathing part of your ascent as possible before they flame out around 18-23 km.

[Patayyyyyto]

1 minute ago, Raideur Ng said:

70 degrees is an insane flight profile. Most SSTOs are between 5 to 15 degrees. You should be gathering as much velocity during your air breathing part of your ascent as possible before they flame out around 18-23 km.

I did that, and still got about 1500m/s. Less speed than my cray cray 70 degrees

[Lindy]

2 hours ago, Raideur Ng said:

70 degrees is an insane flight profile. Most SSTOs are between 5 to 15 degrees.

This little flyer has insane TWR, but yea 70deg. a bit much...

Once I get past the end of the runway I've been pointing her up at 45deg. until I get to 20-25km, then drop the nose to around 20deg. til I run out of air then switch modes and increase my AoA...

Keep after it Patayyyyyto I don't think it's the craft as much as it is your flight profile. I Improved the design a bit (shock cone intake and RCS thrusters), and did a hop that had a 90km Ap. Not enough gas left for a circularization burn, but it did get to space.

 

BTW... I need to work on my re-entry profile... Lost the intake and front gear to over heat, made landing interesting, but I set her down and the pilot actually survived!!

Edited February 24, 2016 by Lindy
Aditional info

[GoSlash27]

Patayyyyyto,

 SSTOs are hard to make because they require so much compromise. You need enough thrust to get up to top speed, but not too many engines (dead weight). You need enough fuel (both jet fuel and rocket fuel) to get you to orbit and back, but not too much or else it'll be too heavy. Enough wing, but not too much. And so on.

 You have a design that's pretty much all engine. You need more fuel.

Best,
-Slashy

[Yakuzi]

15 hours ago, Patayyyyyto said:

I made a very simple SSTO. Wings, fuel tank(with some LQ fuel emptied), air intake in the front, and the RAPIER engine.

So I take off and start maneuvering my SSTO into a 70 degree approach. All of a sudden, my SSTO starts to loose speed. I check the air resistance, and its pretty normal. I tried everything! Other engines other wings. Nothing works! I'm getting frustrated

 

Apart from your ascent profile, there's nothing keeping you from getting into orbit with this craft.

I made a reconstruction of your design and got it in a 100x100 km orbit with ease and quite a bit of fuel to spare, you can even keep the RAPIER in 'automatic switching' mode:

 

Ascent profile:

1. SAS on, launch and start at a 30° attitude angle (no need to touch the controls after setting the initial angle)
2. When your speed is 1200 m/s, set SAS to 'prograde'
3. As soon as the RAPIER switches to closed cycle, set SAS to 'stability assist'
4. Burn to desired apoapsis height, cut engines and set SAS to 'prograde'
5. Circularise orbit @ ~apoapsis

 

 

Give it a shot and see if you can get to orbit, please let us know if you're successful (or not)!

Edited February 24, 2016 by Yakuzi

[Patayyyyyto]

Yakuzi, I did every step of your procedure. And I never got to 1200m/s at 13k meters. Actually I slowed down somewhere at 5k meters half the time. I got to space and had about 1700m/s and then no more fuel. My highest apoapsis was at 70k meters. Never more.

I was at a 30 degree angle for most of the time. The only way I was able to get to 1200m/s was the automatic switching for the engine.

At apoapsis I was going at about 1400m/s before the circulation burn.

The big big problemo I keep having is that my engine starts to decline power at 5k meters.

[Yakuzi]

28 minutes ago, Patayyyyyto said:

Yakuzi, I did every step of your procedure. And I never got to 1200m/s at 13k meters. Actually I slowed down somewhere at 5k meters half the time. I got to space and had about 1700m/s and then no more fuel. My highest apoapsis was at 70k meters. Never more.

How peculiar... Are you running KSP version 1.0.5(.1028)? Please download my craft and see if that behaves similarly to the ascent profile I posted above:

https://www.dropbox.com/s/fum0bbnvxhhte7e/Solayyyyynum%20tuberosum.craft?dl=0

 

Also, if you could post a link to your craft file I can test it on my end...

Edited February 25, 2016 by Yakuzi

[Patayyyyyto]

Ok, here is my craft:

https://www.dropbox.com/s/fug7qoe7wn320z4/Prototype%20SSTO%20%20MK%204.craft?dl=0

[Yakuzi]

1 hour ago, Patayyyyyto said:

Yakuzi, I did every step of your procedure. And I never got to 1200m/s at 13k meters. Actually I slowed down somewhere at 5k meters half the time.

 

1 hour ago, Patayyyyyto said:

Ok, here is my craft:

https://www.dropbox.com/s/fug7qoe7wn320z4/Prototype%20SSTO%20%20MK%204.craft?dl=0

 

Thanks for the craft file, it definitely explains why you didn't get to orbit... you made significant changes to your previous design (it has 33% more mass a.o.). This obviously changes the effective ascent profile, I tested it at 20° initial pitch angle followed by the same protocol as I mentioned above, and got into a ~80x80km orbit.

N.B.: Let's keep the designs consistent for future reference, or things will get complicated quickly (first rule in science, minimise all variables except for the independent variable, and have a proper control)

 

Your NEW craft:

 

Ascent profile:

1. SAS on, launch and start at a 20° attitude angle* (no need to touch the controls after setting the initial angle)
2. When your speed is 1200 m/s, set SAS to 'prograde'
3. As soon as the RAPIER switches to closed cycle, set SAS to 'stability assist'
4. Burn to apoapsis of ~80 km, cut engines and set SAS to 'prograde'
5. Circularise orbit @ ~apoapsis

*Note, you can probably optimise this quite a bit. Play around with your initial pitch... try as shallow as possible, if you burn up, revert and raise the initial pitch by a few degrees.

Note, you really don't need that huge amount of monoprop for a small craft like that. Also, the liquid fuel in the back fuel tank is locked, you can remove that tank anyway as you can get to orbit easily without it (with more delta-V). Finally, remove the fuel lines as they are superfluous and add extra drag.

 

Did you get your original craft and my copy in orbit according to that craft's specific ascent profile I posted earlier?

Edited February 25, 2016 by Yakuzi
links for profit

[Patayyyyyto]

I'm very thankful for your help Yakuzi! ^-^ I got my SSTO into orbit with lots of fuel to spare! I think this is resolved!

[Yakuzi]

Congratulations mate, hopefully it's the first of many spaceplanes!

Keep in mind that you'll need to tailor your flight profile specifically for individual craft, though typically that just involves changing the initial pitch*. Also try to keep drag to a minimum (i.e. don't use fuel ducts/struts when you dont have to, and ideally put batteries/RTG's/etc in cargo bays, service bays or fairings) and make sure you have enough thrust to punch through the sound barrier (~343 m/s).

Make sure you submit an entry for the K Prize and hope to see you around on the forums!

*My personal preference which is by no means perfect, other players may use different approaches.

Edited February 25, 2016 by Yakuzi