SSTO...how??

[DracoSilverpath]

Hey there! I've been fiddling with a low-tech SSTO design and ran into a small problem that I'm just not sure how to make work, maybe someone can give me a tip or 2 on how this all works?

As shown int he picture, craft has 2 jet engines, and 2 rocket engines. The tank segment directly ahead of each jet engine is only liquid fuel, while all the rest on the craft are LF/O, yet, in my staging, it shows that the rocket engines have 0m/s in total due to they're being no fuel left for them...

My guess, is this has something to do with crossfeeding...but I really haven't got a clue on how to set that up, could someone point me in the right direction on how i might set this up correctly?

[Palaceviking]

1 hour ago, DracoSilverpath said:

Hey there! I've been fiddling with a low-tech SSTO design and ran into a small problem that I'm just not sure how to make work, maybe someone can give me a tip or 2 on how this all works?

As shown int he picture, craft has 2 jet engines, and 2 rocket engines. The tank segment directly ahead of each jet engine is only liquid fuel, while all the rest on the craft are LF/O, yet, in my staging, it shows that the rocket engines have 0m/s in total due to they're being no fuel left for them...

My guess, is this has something to do with crossfeeding...but I really haven't got a clue on how to set that up, could someone point me in the right direction on how i might set this up correctly?

Mechjeb often gives me funny numbers with ssto's but they still work fine so just give it a whirl and see how it does. 

Does it fly ok?

[DracoSilverpath]

10 minutes ago, Palaceviking said:

Mechjeb often gives me funny numbers with ssto's but they still work fine so just give it a whirl and see how it does. 

Does it fly ok?

Thanks for the info  It does indeed fly more or less fine, the problem is that...well..it doens't get to orbit, not even close  I was vaguely following a design I saw in a youtube video that had low-tech SSTO's for stock, but seems this particular design just does not work with FAR, so I got no idea how to build one at this tech level then :/ Any ideas?

[Palaceviking]

Can you do it on one swivel only? And bring the jets into the body thereby reducing your forward facing profile? 

Would reduce drag a lot.

Actually scratch that, you'll need hypersonic jet engines for that kind of ssto anyways, just realised they're wheasley's. 

You could try a pure rocket ssto but I haven't seen one of those since 1.0.4.

For future reference you need to be doing 1100m/s at 17-22km Before switching to rockets** if going with a jet rocket combo.

 

** all quoted numbers have a 20% "ish" factor. 

[DracoSilverpath]

5 minutes ago, Palaceviking said:

Can you do it on one swivel only? And bring the jets into the body thereby reducing your forward facing profile? 

Would reduce drag a lot.

Actually scratch that, you'll need hypersonic jet engines for that kind of ssto anyways, just realised they're wheasley's. 

You could try a pure rocket ssto but I haven't seen one of those since 1.0.4.

For future reference you need to be doing 1100m/s at 17-22km Before switching to rockets** if going with a jet rocket combo.

 

** all quoted numbers have a 20% "ish" factor. 

Thanks for the info! I kinda suspected it might just be the engines I got being the issue, good to know  Guess I'll have to somehow dredge up enough science points to unlock that node...

[Streetwind]

2 hours ago, DracoSilverpath said:

yet, in my staging, it shows that the rocket engines have 0m/s in total due to they're being no fuel left for them...

The way KER works: An engine will be treated as consuming all available propellant before it is staged. After all, why would you drop a stage that still has propellant available? This makes the algorithm fundamentally unable to properly deal with spaceplanes. There is no way the algorithm can know ahead of time when exactly you plan to manually shut off your jets.

Therefore a jet engine low down in the staging order will be treated as consuming all LF available to it before ceasing operations. There is no decoupler, but KER notices an engine running out of fuel, so it inserts a virtual stage into the list (which is why you have two stages listed for your single stage spaceplane). It does this so it can correctly describe the performance of other engines that are still running at the time of the shutdown of the jets.

So why are the rockets not getting any dV? Because rockets also need LF, in addition to oxidizer. But the jets already consumed all available LF before shutting down, so there is nothing left to run the rockets with. This means two things:
- 1.) You have unwanted fuel crossfeed going on from the rocket fuel tanks towards the jet engines.
- 2.) In all likelyhood, due to the order in which sequential tanks are drained in 1.1.3, this means that the jets draw from the rocket fuel tanks first, instead of their own tanks.

So you fail to get into orbit both because your rockets don't have as much fuel available as you think they do, and because you're carrying a heavier load than you think you do - because the jet engine LF tanks will still be full when you light the rockets! Less fuel for a heavier load --> you're not going to space today.

Edited September 5, 2016 by Streetwind

[mk1980]

1 hour ago, DracoSilverpath said:

Thanks for the info  It does indeed fly more or less fine, the problem is that...well..it doens't get to orbit, not even close  I was vaguely following a design I saw in a youtube video that had low-tech SSTO's for stock, but seems this particular design just does not work with FAR, so I got no idea how to build one at this tech level then :/ Any ideas?

that design wouldn't work in the stock aero model, either. those wheesley engines really don't provide anywhere near enough speed and altitude to make them worthwhile. in the best case, they might get you to maybe 400 m/s at 10 km before they stop working, so you'd still need like 3000 m/s worth of rocket-based deltaV to make orbit. and the dead weight of the jets probably costs you more deltaV than what little you gained while they worked in the low atmosphere.

your best bet is to (at least) unlock the panther jet engines (160 tech node). i've built a few somewhat viable SSTO planes around those engines. still, it will be mostly a rocket that uses jet engines to save maybe 1000 m/s worth of deltaV by using jet engines in the lower atmosphere. not really worth the hassle tbh. the first decent SSTO engine is the whiplash. it's a difference like night and day compared to the lower tech stuff. but it's quite late in the tech tree (550 point node, so it also required fully upgraded R&D facility)

or to forget about SSTO, remove the jets, slap on some disposable thumper or kickback boosters and launch vertically. SRB's are fairly cheap.

[TheEpicSquared]

Hmmm... no advice for flying, but I did notice that the solar panel that you have on your SSTO can't retract. That means it will probably burn up on reentry. Just a heads-up.  

Also, I've found that vertical rocket SSTOs *can* be easier to make, especially since then KER will show the right values. However, reentry heating could be a problem. Landing would be achieved with chutes, I guess.

[foamyesque]

14 hours ago, Palaceviking said:

You could try a pure rocket ssto but I haven't seen one of those since 1.0.4.

Consider it seen.

Edited September 5, 2016 by foamyesque

[Palaceviking]

5 hours ago, foamyesque said:

Consider it seen.

Seen.

Can you land that? 

Edited September 6, 2016 by Palaceviking
Curiosity

[Streetwind]

1 hour ago, Palaceviking said:

Can you land that? 

Landing is not really a requirement for it being an SSTO, now is it Rocket SSTOs get much much simpler like that, even. The simplest one with useful payload (one Kerbal to LKO and back) takes a grand total of six parts. It doesn't recover fully though.

For that you want a spaceplane. As far as rocket-only spaceplanes go... I'm not sure that makes sense from a payoff vs. effort standpoint, but it may be possible. @foamyesque's thingy looks like it might at least glide to an acceptable degree.

Edited September 6, 2016 by Streetwind

[Palaceviking]

Oh, nitpicking :-D 

[TheEpicSquared]

2 hours ago, Streetwind said:

Landing is not really a requirement for it being an SSTO, now is it Rocket SSTOs get much much simpler like that, even. The simplest one with useful payload (one Kerbal to LKO and back) takes a grand total of six parts. It doesn't recover fully though.

For that you want a spaceplane. As far as rocket-only spaceplanes go... I'm not sure that makes sense from a payoff vs. effort standpoint, but it may be possible. @foamyesque's thingy looks like it might at least glide to an acceptable degree.

Well, I've made a rocket SSTO with a Mk1-2 crew cabin on the top that is fully recoverable. Launch, land, refuel, repeat, in the words of Blue Origin (I think). It's easier than you might think, actually.  

[steve_v]

On 5/09/2016 at 9:32 PM, DracoSilverpath said:

I kinda suspected it might just be the engines I got being the issue, good to know  Guess I'll have to somehow dredge up enough science points to unlock that node...

Yeah, you'll need at least panthers for an airbreathing SSTO spaceplane. Insert obligatory plug for Kerbodyne SSTO division, my go-to for spaceplane inspiration.

At lower tech levels, just go for a cheap SRB first stage. SRBs are cheap.

[Streetwind]

3 hours ago, Palaceviking said:

Oh, nitpicking :-D 

Of course. ORDNUNG MUSS SEIN!

[DracoSilverpath]

37 minutes ago, steve_v said:

Yeah, you'll need at least panthers for an airbreathing SSTO spaceplane. Insert obligatory plug for Kerbodyne SSTO division, my go-to for spaceplane inspiration.

At lower tech levels, just go for a cheap SRB first stage. SRBs are cheap.

Well, good news~ I scrounged up enough science from Kerbin to unlock the supersonic science node  bad news is...well, my engines are blowin up! I got these fancy pants B9 turbojet engines that you get from that node, that have a dual mode switch from dry to wet modes. I climb to about 10k and speed up to around ~600ms, then kick on the wet mode which doubles the engine output at the cost of additional fuel, but after a few moments...boom! The engines cease to exist :/ 

Do I need to be super careful on the throttle with that mode or something?

[RoboRay]

On 9/5/2016 at 5:56 AM, Streetwind said:

The way KER works: An engine will be treated as consuming all available propellant before it is staged. After all, why would you drop a stage that still has propellant available? This makes the algorithm fundamentally unable to properly deal with spaceplanes. There is no way the algorithm can know ahead of time when exactly you plan to manually shut off your jets.

You can work around this in the VAB/SPH by switching the staging order around so that the rockets are fired first.  You'll need to undo that to go flying, of course, and so the in-flight dV displays will still be wrong, but at least you can see what you'll get our of your rockets for planning purposes.

And when designing, include fuel lines from the jet tanks to the rocket tanks.  This allows the rockets to scavenge any unused LF from the jet tanks if the jets were stealing from the rocket tanks.

Edited September 6, 2016 by RoboRay

[Streetwind]

32 minutes ago, DracoSilverpath said:

Well, good news~ I scrounged up enough science from Kerbin to unlock the supersonic science node  bad news is...well, my engines are blowin up! I got these fancy pants B9 turbojet engines that you get from that node, that have a dual mode switch from dry to wet modes. I climb to about 10k and speed up to around ~600ms, then kick on the wet mode which doubles the engine output at the cost of additional fuel, but after a few moments...boom! The engines cease to exist :/ 

Do I need to be super careful on the throttle with that mode or something?

Last I heard (which was admittedly over a year ago, since I never use B9 myself), some B9 engines require you to attach them to dedicated precooler parts in order to keep them alive.

This is a question better asked in the B9 mod thread though, tbh.

Meanwhile, stick to Panthers

Edited September 6, 2016 by Streetwind

[steve_v]

27 minutes ago, DracoSilverpath said:

my engines are blowin up! I got these fancy pants B9 turbojet engines

It's been a long time (0.90) since I played with B9 Aerospace, IIRC engines overheating means you're going too fast at too low an altitude. Climb.
Have you tried this with the stock turbojet? I've yet to have one of those explode on me.

[Pecan]

7 hours ago, Palaceviking said:

Oh, nitpicking :-D 

You're just lucky I was bored of explaining what the words Single Stage TO ORBIT mean (see my signature, there's no point arguing as I already know I'm being pedantic).

[KerikBalm]

On 9/5/2016 at 11:19 AM, DracoSilverpath said:

Thanks for the info  It does indeed fly more or less fine, the problem is that...well..it doens't get to orbit, not even close  I was vaguely following a design I saw in a youtube video that had low-tech SSTO's for stock, but seems this particular design just does not work with FAR, so I got no idea how to build one at this tech level then :/ Any ideas?

This is FAR? I can't comment on FAR specifically. In stock I doubt this sort of design can work either, you really want at least the panther.

On 9/5/2016 at 9:34 AM, DracoSilverpath said:

Hey there! I've been fiddling with a low-tech SSTO design and ran into a small problem that I'm just not sure how to make work, maybe someone can give me a tip or 2 on how this all works?

As shown int he picture, craft has 2 jet engines, and 2 rocket engines. The tank segment directly ahead of each jet engine is only liquid fuel, while all the rest on the craft are LF/O,

I see some modded parts, but essentially it looks like you've got 2 swivels with an equivalent of an FL-T800 each. That is not nearly enough LFO.

Turbofans are nearly useless for SSTOs, they don't get you high and fast enough to pay for their own mass. The only "turbofan" that works is the panther afterburning turbofan... in afterburning mode. Without the panther, you're better off with a pure rocket SSTO. A swivel can be used for a SSTO rocket. A swivel with a FL-T800 tank won't SSTO as far as I know, at least not with any payload. Those LF tanks for the turbofans are also probably way too full of fuel. The wheesleys won't consume much at all and the rest of the fuel is just excess mass holding you back

You might be able to make a single wheesely design work, where the wheesley will sturggle to keep in in the air and get it very high (because you'll also be haulling more LFO)

Spoiler

in my staging, it shows that the rocket engines have 0m/s in total due to they're being no fuel left for them...

My guess, is this has something to do with crossfeeding...but I really haven't got a clue on how to set that up, could someone point me in the right direction on how i might set this up correctly?

The staging assumes your wheeseleys use all the LF available to them. This leaves no LF available to mix with the oxidizer for the rockets.

All jets in KSP draw fuel from the entire vessel, not just their own stack. I think there is an exception if there is a decoupler inbetween its stack and the next stack... such that you could have airbreathers on outer/lower stages that get discarded without touching the inner/upper stack/stages. I think if you put a decoupler (you don't have to use it) between the wheeseley stack and the swivel stack, it should show you more than 0 m/s for the swivels

[mk1980]

did some testing. at that tech level (before panthers), a pure rocket plane works reasonably well. don't expect great performance, but at least it can get to orbit. 

i made a simple plane with just a cockpit, a 2 seats mk1 cabin and 2 swivel engines:

made it to orbit with ~235 m/s left. not much. just enough for *maybe* a rendezvous with anohter craft on a very close orbit...

 

and then i retrofitted that same plane to a hybrid jet/rocket plane. replaced the 2 swivels with a pair of panthers and a single swivel, replaced 3 of the FLT 800 tanks (1 FTL 400 + 2 engine nacelles providing air intake and storing 150 LF each)

as you can see in the KER readout, it has plenty of deltaV left in orbit. so the panther + rocket hybrid is definitely more potent than a pure rocket plane. i probably could have cut one more fuel tank, but didn't want to redo the whole plane, so i just swapped the easily accessible parts

[foamyesque]

14 hours ago, Palaceviking said:

Seen.

Can you land that? 

 

ln pieces. The outboard boosters land on Kerbin, then the central plane lands on Minmus and Eve, then the nose shuttle lands on Kerbin.

 

The core plane is also capable of SSTOing from, and l believe landing back on, Kerbin, though I have not tested it through a full entry since that wasn't the mission goal.

[Palaceviking]

Can I have the craft file,  would like to try it.

[DracoSilverpath]

22 minutes ago, Palaceviking said:

Can I have the craft file,  would like to try it.

For which? the enw one I made with exploding engines, or the original? I've since scrapped the original design for a more robust mk2 part variant with turbojet engines and such.

If anyone would care to go take a look at this, I'd appreciate any constructive criticism

https://kerbalx.com/DracoSilverpath/HMSC-Lost-Explorer

That is my updated SSTO made with the supersonic tech node parts. It "should" be able to ssto just fine, if I can ever figure out the explodey engine bit with the mode switching.

Edited September 6, 2016 by DracoSilverpath