Need help with low-tech SSTO

[Bullshark187]

Hi, i'm building an SSTO using 2x whiplash engine (for low-alt) and 2x swievel engine (for everything else). It has 2 mk1 jet fuel tanks on each side and 4 rocket fuel tanks in the middle. Looks a bit like an SR-71. It flies stable but i can't really get above 13k meters, at 13k the nose pitches down, and i can't keep it from loosing alt (or speed). Any hints on what i'm doing wrong? Many thanks

[Chibbity]

Whiplashes start running out of air at 13k. You'll want to be doing Mach3+ at that point and starting your ascent to orbital height. Getting spaceplanes to orbit is more about flying them right than anything else. (Whiplashes for instance get their best power around 10kish, so that's where you'll want to level off a bit and get your Mach speed up. Right click the engine and watch the output closely.)

If you have KER keep an eye on your TWR, when it starts dropping fast, light the vacuum engines. If you don't have KER, get it. https://forum.kerbalspaceprogram.com/index.php?/topic/17833-130-kerbal-engineer-redux-1130-2017-05-28/

Also, Reliant>Swivel in almost every way, especially on a spaceplane that already has control surfaces and so doesn't need gimbal. Anyways a screenshot of your plane would help.

[Bullshark187]

You mean the spaceplane is ok, i'm just flying it wrong? That could explain it, i thought the problem was the whiplash engines being too weak or the plane too heavy...
Ok so i will replace the swivels with reliants and try getting to mach3+ before 13k and then start the ascend. Thanks, i'll give KER a try.

[Chibbity]

Well, I'm not "not" saying that as I haven't even seen the plane.

Whiplashes are capable of SSTO, but obviously the RAPIER is better. Most Whiplash SSTO's aren't terribly practical; they don't really bring a lot of payload to orbit. However they can be useful for crew transfers and stuff like that.

A good rule of thumb is the Mach speed, if you can't get up to Mach 3+ in a flat run at optimum height, then yeah; the plane is too heavy, or needs more engines, or more wing for lift, or whatever. Best of luck!

Also, not to throw a ton of mod suggestions at you, but get CorrectCOL too. Not only fixes all the errors with the stock COL marker (Like how it doesn't even account for body lift), but adds a nice graph that can compute your wet vs. dry stability. https://forum.kerbalspaceprogram.com/index.php?/topic/160231-13-correctcol-stock-aerodynamics-design-aid-continued/

[Gromozeka]

Jet engine thrust depends much on speed and altitude. Whiplash thrust is max at speeds near 3mach. All jet engines thrust drops significantly at altitudes above 10km. So it's important to not ascend above 10km until 3M speed is reached.

I am successfully using such low tech SSTO. It even capable to put Munalnders on LKO. Hardest part is to get proper initial trajectory and speed for a rocket engines burn. Mk2 fuselage has very high side drag, so if you are not burning 100% prograde you are losing much of deltaV.
https://steamuserimages-a.akamaihd.net/ugc/903401710736358639/A68C4F4F6E32FECA071BC890B514E0B3FD69B817/

[RoofCat]

I don't agree on whiplashes running out of juice at 13km, Chibbity. You must have been doing something wrong there. Wrong intakes, or too much velocity.

Launching at ~60° double Whiplash compact plane can actually shortly reach vacuum on a suborbital trajectory. Not very useful, but may be used for science eventually.

Don't use Swivel engines in KSP. EVER! Period. Just don't!

Use Whiplash for up to 25km, start 2 Terriers (depends on your plane size) at 13+km up until orbital. I may create one eventually for fun. You need just ~1 jet fuel tank for each Whiplash. Except you plan to spend a lot during landing too. You can reach ~1400m/s with Whiplash under those circumstances. Make it count.

[Chibbity]

Originally posted by RoofCat:

I don't agree on whiplashes running out of juice at 13km, Chibbity. You must have been doing something wrong there. Wrong intakes, or too much velocity.

"Start" running out of juice. What I meant more so was after 13k+ you aren't going to be able to get your speed up to Mach 3+ if you haven't already, as the engine output is gonna start dropping.

You are right though; they won't actually "shut off" till 25kish.

[RoofCat]

here is one rather simple Whiplash SSTO. Passenger version
http://steamcommunity.com/sharedfiles/filedetails/?id=1250427463
http://steamcommunity.com/sharedfiles/filedetails/?id=1250428567
There is not too much rocket fuel left on orbit. If that's not enough for you, I had a version where crew cabin was replaced with another fuel tank and that one had much more on orbit. Or just rearrange fuel types for this one.

Be aware though this design is nowhere perfect. Optimal would be to have ~1 Whiplash for 12..15t which is correct here, but then also just one Terrier. The problem is, you can't have both Whiplash and Terrier pushing through center of mass. So we would have serious balance issues both for thrust as for mass in space. So because Terrier is so much lighter than Whiplash I used double Terrier here. It also simplifies circularization stage a lot. With a single Terrier you have to keep pitch at 15°..20° even as prograde starts to dive above 25km to reach 70km Ap in the end.

Just launch with ~8°, let velocity and planetary curvature do the rest. You should reach 1350+m/s with Whiplash alone. Add Terrier once your acceleration starts to flatten out - somewhere between 15..20km. Should be quite easy except you forget to retract landing gear.


You may want to eventually build 25..30t plane with double Whiplash and double Terrier and even more functionality. I will leave that to yourself though :)

Or just go with that sexy double Sparky option. Wastes a bit of fuel, but saves on engine mass.
http://steamcommunity.com/sharedfiles/filedetails/?id=1250486913
A bit less comfort during late launch though.

[Yuki]

Rapier is later down the techtree.
I also just recently had a whiplash based GTFTS SSTO program. So not only did i use ram jets, but wanted to alter everything to a very high-g hot n fast hypercruise.
Works! BUT!

In difference to Rapiers you would probably want to have a higher emphasis put to rocket propulsion. Overall your hypercruise will not get you as much speed as you can gain from rapiers, mainly though not (only) because of altitude, but especially speed. Rapiers cap out later in relation to their speed.

Rams cap out pretty fast. You can barely breach M4 with whiplashs, and even that requires an insanely well balanced drag oprimized craft. Given that a proper orbit is more in the range of ~M7 (depending on target AP)- Altitude is less of a concern.

Then again the way i did succeed was: High TWR setup with whiplashs bringing me at max speed up to about 30 km, shock cone based btw, also helps.
One shock cone can easily feed 2 whiplashs at any time. Potentially saves on drag too.
Then- at that altitude you can already fire up NERV's. Their ISP is within reasonable limits and they also need liquid fuel only, more easy tank setup.
At that altitude and with their already high ISP you do not need >1 twr. Even 0.3 is already enough.
http://steamcommunity.com/sharedfiles/filedetails/?id=1238031076

And thus i created a new star =^.^=

As for the dropping nose: Watch your weight distribution at low fuel and your center of thrust. A screenshot of your craft would help, but from what you wrote i would assume your LF tanks at the crafts rear(?) get drained so your LFO tanks in front pull you down.
I also assume you aint going for an assymetric profile? That would be...worth half a novel here then ;)
http://steamcommunity.com/sharedfiles/filedetails/?id=1229320921

High TWR however is not necessary, that was me being lazy. My Fox crew SSTO also uses the whiplash nerv combo but with a "proper" hypercruise.
http://steamcommunity.com/sharedfiles/filedetails/?id=1237166392

Also SR-71-ish and pretty easy to fly. Pull up, at about 8km activate sas prograde until you are at about 5-10°. Short before the whiplashs cease to work, fire up nervs, keep nose prograde until your AP is at whatever you want it to be. For stock only exchange the large crew compartments for double the ammount of normal ones (even saves on mass!), and use another nose. Range Mün and beyond.

[Chibbity]

Originally posted by Yuki:

You can barely breach M4 with whiplashs, and even that requires an insanely well balanced drag oprimized craft.

Everything else you said was great, I find this a little odd though. I've definitely had Whiplashes up to almost 5k, and not even trying that hard.

Or were you specifically talking about Whiplashes on a useful orbital ascent angle?

[Yuki]

Im also writing of memory only for not having access to KSP atm, but if i remember right they do hard cap at M4. And i was too lazy to search for those graphs you can get with googles help ;)

Either way, rapiers cap out way later, let that be the lesson :D

Edit: Me being less lazy: Chibbity is right, its 5:
https://wiki.kerbalspaceprogram.com/images/c/c8/J-X4_Whiplash_Turbo_Ramjet_Engine_velocity_curve.png
With rapier going further, as in near orbit velocity:
https://wiki.kerbalspaceprogram.com/images/thumb/3/38/CR-7_R.A.P.I.E.R._Engine_velocity_curve.png/400px-CR-7_R.A.P.I.E.R._Engine_velocity_curve.png

[Chibbity]

Good to know, thanks for checking.

Not just trying to prove you wrong or something, I get worried when someone says something like that and it doesn't jive with what happens in my game because I start to panic a bit that something is wrong with my game or I've messed up one of the settings accidentally lol.

[RoofCat]

4 Whiplash low altitude manned beast
http://steamcommunity.com/sharedfiles/filedetails/?id=1192622035

[Yuki]

Nuh its fine. Not a shame to be proven wrong, less given that we are just talking about a number and not in general ;)
If at all its usually me using dozens of mods. Like the planet pack that also alters the athmospheric model to a minor degree (making it more difficult again).

Originally posted by RoofCat:

4 Whiplash low altitude manned beast
http://steamcommunity.com/sharedfiles/filedetails/?id=1192622035

I take that and present the start velocity of one of my GTFTS attempts ;)
http://steamcommunity.com/sharedfiles/filedetails/?id=1235210358

[RoofCat]

Originally posted by Yuki:

http://steamcommunity.com/sharedfiles/filedetails/?id=1235210358

I have seen people going much faster on surface. Balancing that is really hard though.

[Yuki]

More so on a not fully upgraded runway. Although with proper wheel setup its very well possible. Just keep in mind to deactivate steering on rear gear.
It wasnt about going fast at the surface though, just with its drag and weight optimization- it would simply need that much to take off with full tanks.
Landing velocity while still fast was not even half of that. And no, that one of course didnt make it to orbit, it was a test flight for the special hypercruise i wanted to achieve. High-G High-pitch, so damn lazy me would not even have to hit the "sas to prograde" button :D