Can't get my ssto past much more than 23km.

[dra321]

I've been trying ot design this SSTO and i cannot for the life of my figure out why it just won't get into orbit.

 

It has more than enough delta-v i believe.

 

 

This is the SSTO

Can upload the craft file to an external site if needed.

 

It uses four rapier engines and two whiplash engines, And one Nuclear engine i was intending on using for interplanetary travel. (If i ever get to that point haha) and again, i just don't understand why it refuses to get into orbit.

[Serenity]

2 hours ago, dra321 said:

I've been trying ot design this SSTO and i cannot for the life of my figure out why it just won't get into orbit.

First of all nice design, i dig it .

Whats your ascent profile?, Try this: 

1)Take off and pitch up about 10-20 degrees

2)Have your Rapier engines to Manual Switch

3)Switch to rocket Mode when you are at 23-25k altitude.

After that wait until you run out of fuel and mark down your speed/altitude/apoapsis/periapsis.

If you can post them here, it will help.

Also take a note which and by how much of a difference runs out first, oxidizer or liquid fuel.

Edited May 2, 2018 by Boyster

[bewing]

Your main problem is aerodynamic drag. You might also have a secondary problem with those ramp air intakes running out of air at that altitude.

First, you are in MK2 hell. MK2 parts look very pretty, but have very high drag. This is very nice on descent, but very bad on ascent.

Second, MK2 cargo bays have a drag bug. They have less drag when they are open than when they are closed. I'm betting that you are running them closed.

Third, too many tailfins make too much drag.

Fourth, do you really need the RCS? RCS makes drag.

Fifth, do you really need those struts? Struts make drag.

Sixth, airbreathing engines are deadweight to an SSTO, so you want to minimize their number. To the best I can tell, you have six. Can you get it down to two or one?

Seventh, if you have a cargo bay, then why the heck are your solar panels not in the cargo bay? When they are radially attached, they generate drag.

Eighth, since you are already playing games clipping your rapiers, you should have your whiplashes attached to the backs of your rapiers and then clipped. This will eliminate one entire set of nacelles, and nacelles cause a lot of drag.

Ninth, you should be using the shock cones and not the ramp intakes. Shock cones have much lower drag, and intake a lot more air at high altitudes and speeds.

Tenth, are you manually switching modes on your rapiers when your acceleration stops? It really sounds like you aren't.

And it almost looks like you have shock cones clipped inside your ramp intakes? Is that right? I'm not sure how you would have it all attached if you did, but no matter how you would accomplish that sort of thing, it would make a lot of drag.

And I'd suggest going for the small landing gear on the nose. It masses less and has less drag.

 

Edited May 2, 2018 by bewing

[OHara]

7 hours ago, dra321 said:

It uses four rapier engines and two whiplash engines

I built something really similar with the parts I could see.  It got into orbit nicest if I flew about 10° above the horizon, picking up speed to 1200m/s  below 10km, which might be faster than you have tried.  All the suggestions above are good, but I saw no really big problems with your craft as I saw it ....

but I saw only two rapiers, so...

1 hour ago, bewing said:

And it almost looks like you have shock cones clipped inside your ramp intakes? Is that right? I'm not sure how you would have it all attached if you did, but no matter how you would accomplish that sort of thing, it would make a lot of drag.

So some clipping is going on, and KSP estimates drag as if there were no clipping (which is nice for some aspects of KSP as a game, so that clipping is a purely cosmetic choice) so  depending on what is clipped your plane might have the effect of exposed flat plates.   This aspect of drag in KSP is not obvious at first, so if you remain frustrated you might put the craft on a share somewhere post a link to it.  

 

2 hours ago, bewing said:

MK2 cargo bays have a drag bug. They have less drag when they are open than when they are closed. I'm betting that you are running them closed.

..and you would be correct in flying with the cargo bays closed, so that their contents don't create drag.  The cargo bays themselves can under some high-speed conditions show less drag when open than when closed (and some QA guys just can't let it go) but not enough counter the benefit of avoiding drag of their contents.

[Dafni]

2 hours ago, bewing said:

Second, MK2 cargo bays have a drag bug. They have less drag when they are open than when they are closed. I'm betting that you are running them closed.

 

No, really? I did not know that. Thanks for the info, this is very handy to know. But how annoying is this bug? Lets hope they fix it soon. Or is there already a community fix for this??

[bewing]

27 minutes ago, OHara said:

..and you would be correct in flying with the cargo bays closed, so that their contents don't create drag.

Depends on whether there are any contents or not.

 

[blakemw]

To a large extent this is a matter of poor engine to propellant rations. My rule of thumb:

• each RAPIER should have about 3/4 of a Mk1 Fuel Fuselage and 2-3 FL-T800's worth of LF/Ox
• each whiplash should have about 2/3rds of a Mk1 Fuel Fuselage
• each LV-N should have about 4-6 Mk1 Fuel Fuselages

Using these engine to propellant ratios give a very respectable payload to orbit, or to further destinations. Turning it into a plane that can get into orbit is a matter of wings and streamlining. A good acid test is the plane should be able to break the sound barrier (get up to 500m/s) while flying level across the ocean at low altitude (<1000m), if it can't then you can try: reduce drag by improving streamlining (aerodynamic overlay helps), add wings to increase lift (allows a shallower angle of attack which reduces body drag), reduce weight, or add more jet engines (with an appropriate amount of fuel). 

[dra321]

11 hours ago, bewing said:

Eighth, since you are already playing games clipping your rapiers, you should have your whiplashes attached to the backs of your rapiers and then clipped. This will eliminate one entire set of nacelles, and nacelles cause a lot of drag.

Oh, i see, I had no clue nacelles caused that much drag.

 

As for the clipping yeah, i do tend to clip engines when i get desperate for power, And well, then i came here.

 

9 hours ago, OHara said:

So some clipping is going on, and KSP estimates drag as if there were no clipping (which is nice for some aspects of KSP as a game, so that clipping is a purely cosmetic choice) so  depending on what is clipped your plane might have the effect of exposed flat plates.   This aspect of drag in KSP is not obvious at first, so if you remain frustrated you might put the craft on a share somewhere post a link to it.  

Oh! Well, if clipped parts cause so much drag i'll try reducing the amount of intakes.

I tried modifying the craft a but it still doesn't seem to work for me.

 

Here's the craft file if anyone could take a look? https://www.dropbox.com/s/c4smkoa35xtwumm/IDS.rar?dl=0

[blakemw]

I had a look, here's an aerodynamic overlay and the debug option to display drag values in action menus:

I probably didn't find everything secreted away inside the vessel, but I found a bunch of stuff.

The major cause of drag is the cargo bay nodes are wrongly connected, this is causing roughly 2 RAPIERs worth of drag in the case of the Mk2 LF fuselage, and 1 RAPIER worth of drag in the case of the MK2 RF fuselage. The rule of thumb is to never attach stuff which is outside the cargo bay to the inside nodes of a cargo bay. Attach fuselage only to the appropriate outside nodes.

Also the deployable solar panels are not "inside enough" the cargo bay for the game to consider them shielded from drag. The two of them are causing about half a RAPIER worth of drag (those things in general are draggy pieces of excrements, even when placed edge-on to the wind which is the ideal case, use Gigantors pointy-on to the wind if you can). The Monoprop tanks are also contributing about half a RAPIER worth of drag, I don't think they are even trying to be in the cargo bay. They need to be in order to be shielded from drag. Anything you want to be shielded from drag needs to be convincingly inside a cargo bay, the game errs on the side of making things drag unless it's absolutely sure it's inside a cargo bay.

Fix those issues and you would eliminate about 4 RAPIERs worth of drag, and could remove that many jet engines and still have enough power to get through the sound barrier. (but I'm by no means confident I found everything "sneakily" generating lots of drag, in particular, I'm sure there's something dodgy going on with the LV-N attachment)

 

 

2 RAPIERs can push a fairly sizable spaceplane through the sound barrier in level flight across the ocean, at which point the mach multipler kicks in with a vengeance allowing the plane to do an accelerating ascent. 

One of my better spaceplanes is my "Duna Operations Plane", it's around the limits of mass & drag which 2 RAPIERs can get through the sound barrier:

So your plane should be easily able to get by with 2 RAPIERs, if all the unnecessary drag is eliminated.

Edited May 2, 2018 by blakemw

[Lego_Prodigy]

Maybe you can put those advanced canards to normal positions, and then lengthen the plane fuselage with another fuel tank for more range. 

[Lego_Prodigy]

https://imgur.com/a/xC28rmm 

my latest SSTO design, using Twin Rapiers for atmospheric thrust and twin Nerv rockets for spaceflight. Keep in mind that I more intend SSTOs for interplanetary flight rather than as an STS.

The front fairing is very nice and aerodynamic and more heat resistant than the regular MK2 cockpit.

[Serenity]

38 minutes ago, blakemw said:

I had a look, here's an aerodynamic overlay and the debug option to display drag values in action menus:

Nice, very good testing i like this.

[dra321]

51 minutes ago, blakemw said:

So your plane should be easily able to get by with 2 RAPIERs, if all the unnecessary drag is eliminated.

Yeah, i took your advice about the drag and started by making sure nothing in the cargo bay was causing drag and most light things clipped were in the cargo bay to reduce all the drag and i've gotten much higher than anytime before!

 

I'll continuing doing some testing and hopefully it'll all work out from here ^^

[AeroGav]

11 hours ago, blakemw said:

2 RAPIERs can push a fairly sizable spaceplane through the sound barrier in level flight across the ocean, at which point the mach multipler kicks in with a vengeance allowing the plane to do an accelerating ascent. 

https://kerbalx.com/AeroGav/Andromeda
Indeed,  this mk3 cargo plane can put an orange tank in orbit with 2 rapiers and 2 panthers.   It climbs to 5-7km (depending on weight) , then when the air is too thin  for efficient subsonic flight, levels off to accelerate through the sound barrier.   The Panthers cut out at mach 3, but by then the RAPIERs are really going some and bear in mind you can pretty much have two Panth for the weight of one RAPIER.    In level flight at 20km it can still get 1400 airbreathing,  after which I start up all 5 NERVs and off we go

Spoiler

 

I reckon the combo of one Panther and one RAPIER can comfortably get 50-60 tons of well streamlined spaceplane up to hypersonic speed.  Of course, that means either mounting them in pairs or clipping the panther onto the RAPIER to avoid thrust asymmetry problems.  

@dra321

Main thing -   Don't use mk2 parts for fuel storage if you can help it.    A mk1 LF tank holds the same fuel as a mk2 short LF tank, but has less than half the drag.   However, wing parts are better still.     The real champ is the Big S wing strake - 5 of them have the same dry mass and lift rating as a Big S delta Wing,  but hold 500 fuel instead of 300.    Now, wing parts do have more dry mass than LF tanks for a given capacity, but the difference is pretty tiny, and the lower drag means you need less engines , which weigh (literally) tons.

So, build your wings and strakes out of fuel holding parts, and get rid of as many tanks as you can, especially the draggy mk2 sort.   Also, you can construct tail fins out of strakes.

Have a look at this one if you like https://kerbalx.com/AeroGav/MK1-Griffon-Deep-Space-Crew-Shuttle.       Note that the cargo lifter i linked in the first pic uses no oxidizer, and keeps all its liquid fuel in strakes -  the only fuselage tank is the adapter at the front, which you have to have.

 

Minor thing -  it looks like you're not doing rapier cones correctly.  The rear attach node on the RAPIER is 1.25m in diameter, so you need to use a cone with a 1.25m attachment node.  It looks like you're using 0.625m cones.      Obviously , if you put a 1.25m cone on the back of the engine, that blocks the exhaust,  but if you use the offset tool to move it forward, when the game checks the path of the engine's plume it will not see any obstructions.      Sounds cheaty but all it does is reduce engine drag to be the same as an engine without a rear attach node, like a Whiplash.