Single Stage to Laythe and Back

[CallisTrOn Entertainment]

Hey,

did anyone of you design a spaceplane which is able to go to laythe, land there, and then return to kerbin? I set myself the rule to build as realistic as possible, so i don't spam wings, control surfaces, SAS, and especially intakes.

I would like to know if it is possible at all, with these restrictions. For now my best spaceplane is able to escape Laythe.

[Tank Buddy]

https://www.google.com/webhp?sourceid=chrome-instant&rlz=1C1ASRM_enUS556US556&ion=1&espv=2&ie=UTF-8#q=single%20stage%20to%20laythe

[CallisTrOn Entertainment]

Well, I nearly just find designs which are using cheats, which I mentioned before. The "Joovian VIII" is quite good. But I still would like to see a more realistic one. And no ion engines. They are cheaty too.

[Red Iron Crown]

How are ions cheaty? Especially in comparison to turbojets?

[CallisTrOn Entertainment]

Because imo they are way more overpowered compared to reality than, the jetengines. The SABRE engine is quite powerfull in reality too.

[Linear]

Because imo they are way more overpowered compared to reality than, the jetengines. The SABRE engine is quite powerfull in reality too.

Lets not bother playing the damn game, can't cheat then.

[Red Iron Crown]

I'm of the opposite opinion. Ions are much higher thrust than in reality, but that's more of a gameplay concession because multi-hour burns are extremely tedious. Jets, OTOH, consume fuel at a ridiculously low rate and provide useful thrust at much faster speeds than is possible IRL.

Skylon engines produce 0kN of thrust in reality. It's a paper engine, there are no functional examples as of yet.

[CallisTrOn Entertainment]

Well, like i heard they tested their enginge succesfully already. And with ion engines you add the unrealistic amount of energy you get out of the solar panels. But let's say i don't just want to take a normal SSTO and put some ion engines and solar panels on. That's just no challenge.

[Red Iron Crown]

AFAIK only the SABRE precooler has been tested, though I haven't read much on it yet.

What you're trying to do should be possible, even without ions. A largish SSTO with a nuclear rocket engine would be the way I would pursue it, I think with chemical rocket engines you'll need too much rocket fuel. A lot depends on you definition of intake spam, for it to work you'll need to get your speed up to almost orbital on turbojets which can be a challenge with too few intakes.

[KerikBalm]

Ion engine ISP: 4200 -> within the range of real engines

Maximum turbojet ISP: 40,000 -> TROLOLOLOLOL

The ion engine TWR is orders of magnitude too high, but because of engine limits, it makes sense. Nobody wants to do 500 perahps kicks, and then sit at their computer for days without time warp.

IMO, they only become cheaty when use on low-ish gravity worlds, where their TWR leads to unrealistic craft... like ion probes landing on minmus/Ike/Dres

In comparison, there's no way in heck that the Dawn probe could set down on Ceres using its ion thruster.

There's also no way in heck you could get to orbital speed with a turbojet... yet you can in KSP (and the turbojet gets 40,000 ISP at times, above 10,000 ISP almost the whole flight).

Laythe, with its lower orbital velocity... really lets you get into high orbits with just your 40,000 ISP turbojets....

But yea... 4200 ISP ion engines are cheating.....

[CallisTrOn Entertainment]

Ofc turbojets are overpowered too, but if you stop using them, it gets impossible anyways. And on big crafts (>100tons) ion engines need still awesome much burntime or many engines and even more solar panels.

I really just wanted to know if there are well known designs in the forum I should check out. But by now, the most helpfull answer was indeed the link to google ;-)

[Jouni]

I haven't built spaceplanes for ages, but I think that single stage to Laythe and back is possible without airhogging.

Let's guess that we're going to build a 20-tonne plane. The usual guideline is 1:1 lift to mass, so we'll need 1 tonne of wings. Add another tonne for the Mk1 Inline Cockpit, and we have 2 tonnes of so far. One jet engine (turbojet + shock cone intake + Mk1 fuselage) weights 2.127 tonnes, and we'll need two of them, making the total mass so far 6.254 tonnes.

The rocket stage probably needs around 5000 m/s of delta-v: 500 m/s to orbit, 2000 m/s to Laythe, 500 m/s back to orbit, 1100 m/s to Kerbin, and 900 m/s for course corrections and margin of error. One nuclear engine should be enough, making the total mass 8.504 tonnes without fuel tanks. By adding 2000 units of LFO (e.g. five FL-T400 fuel tanks), the total mass becomes 19.754 tonnes, while the rocket stage has 5544 m/s of delta-v.

It definitely sounds possible, but the payload fraction will be only around 5%.

Edited February 7, 2015 by Jouni

[Slam_Jones]

Ion engine ISP: 4200 -> within the range of real engines

Maximum turbojet ISP: 40,000 -> TROLOLOLOLOL -snip-

Really? How the dirty heck are you getting an ISP of 40,000?

[Jouni]

Really? How the dirty heck are you getting an ISP of 40,000?

There is a bug in the game that makes all airbreathing engines burn 16x less fuel than they should. This makes the effective Isp 16x higher than the listed values.

The peak Isp of the turbojet is nominally 2500 s, while the peak thrust is 225 kN. Under these ideal conditions, a single turbojet should burn 225 kN / g0 ≈ 22.9 tonnes of fuel in 2500 seconds, or around 1.83 units of fuel per second. A single Mk1 fuselage (150 units of fuel) should last for around 82 seconds.

[Slam_Jones]

There is a bug in the game that makes all airbreathing engines burn 16x less fuel than they should. This makes the effective Isp 16x higher than the listed values.

The peak Isp of the turbojet is nominally 2500 s, while the peak thrust is 225 kN. Under these ideal conditions, a single turbojet should burn 225 kN / g0 ≈ 22.9 tonnes of fuel in 2500 seconds, or around 1.83 units of fuel per second. A single Mk1 fuselage (150 units of fuel) should last for around 82 seconds.

Wow, I never knew that. Has this always been the case?

[numerobis]

I'm not sure I'd call it a bug exactly, since it works exactly as designed. You are burning that much mass. 1/16th of it is liquid fuel, 15/16th is intake air.

The effective Isp of the turbojet (taking into account the liquid fuel used but not the air, and using the actual thrust produced) is 7-20ks in most operating conditions. You can't really get to 40ks since that would require you to be going 1km/s at 5500m. You could do it, it just isn't a practical thing to do because of the drag.

That said, 7-20ks is much higher than what real jets achieve, except at the low end. Also, real jets only get to about 1km/s, or 1/8th of orbital velocity. The basic jet is a much closer analogue of existing modern jets, though it too gets rather higher effective Isp than is realistic.

[Warmblood]

Single stage to Laythe and back (w/o refuelling) is quite possible using stock parts. It might require some unusual designs and abuse of the atomic engine, but they work.

Here's my craft:

Edited February 7, 2015 by Warmblood

[numerobis]

Wow that's tiny.

[Cupcake...]

There were some drop tanks that went AWOL however I'm pretty confident I can do the mission without shedding tanks now.

Cupcake...

[zekes]

Hey,

did anyone of you design a spaceplane which is able to go to laythe, land there, and then return to kerbin? I set myself the rule to build as realistic as possible, so i don't spam wings, control surfaces, SAS, and especially intakes.

I would like to know if it is possible at all, with these restrictions. For now my best spaceplane is able to escape Laythe.

I worked a long time trying to figure this out - it's not easy. You need to have a perfect balance of TWR and D/V. I should try again sometime, but so far my efforts of building large SSTOs have been failures.

[PB666]

AFAIK only the SABRE precooler has been tested, though I haven't read much on it yet.

What you're trying to do should be possible, even without ions. A largish SSTO with a nuclear rocket engine would be the way I would pursue it, I think with chemical rocket engines you'll need too much rocket fuel. A lot depends on you definition of intake spam, for it to work you'll need to get your speed up to almost orbital on turbojets which can be a challenge with too few intakes.

Just take a poodle and nerf it with 10,000 ISP. Simple solution to a complex problem.

[Pecan]

...a spaceplane which is able to go to laythe, land there, and then return to kerbin? I set myself the rule to build as realistic as possible...

Thank you - that's the most I've laughed in a couple of days.

It's a good enough challenge to set yourself, if you like, but there's really absolutely nothing realistic about it anyway, so you might as well use anything KSP allows. There is no real model for any part of such a mission and no-one in real life would design that way even if they could, because it's such an inefficient way to do things.

[Tank Buddy]

Lets not bother playing the damn game, can't cheat then.

I think Linear has got a good point. If you are going to complain about how something implemented into a singleplayer sandbox game in a fictional universe is "cheaty" or "unrealistic", then you might as well not play the game at all. I'm not one to tell other people how to play said type of game, but you want to create an SSTO that has ridiculous design criteria and you add silly constraints on top of that... so to answer your question, it is impossible to make a Laythe SSTO in the manner you want to.

[KerikBalm]

Ofc turbojets are overpowered too, but if you stop using them, it gets impossible anyways. And on big crafts (>100tons) ion engines need still awesome much burntime or many engines and even more solar panels.

I really just wanted to know if there are well known designs in the forum I should check out. But by now, the most helpfull answer was indeed the link to google ;-)

So you won't use one engine type because its cheaty and makes it too easy, but you will use another engine type because its cheaty and makes it easier.

You can SSTO with just rockets.

I've never understood the obsession with SSTO beyond LKO. 100% reusable-> sure

Question: Do you mind refueling after reaching LKO? Because to me, that implies a 2nd craft (ie, now 2 elements or stages instead of a single one) to supply fuel.

What I do for full reusability:

* SSTOs to deliver (a) the mission payload/interplanetary stage ( Refuel things in LKO

* The laythe SSTO self launches from Kerbin (if it can SSTO from kerbin, it can easily do it on Laythe), and rendevouses (sp?) with the interplanetary stage (fuel is topped off before the SSTO carrier de-orbits)

* A reusable tug to push the payload to just under escape velocity (may or may not do multiple perapsis kicks depending on payload size)- it then aerobrakes to LKO again and refuels

* the mission payload, now only needs to PE kick once more for about 1,000 m/s to aerobrake at laythe.

* The mission payload first goes into a highly eliptical orbit that is nearly at escape velocity, and leaves a fuel tank/small tug there

* The remaining mission payload aerobrakes to Low laythe orbit

* SSTO descends and lands

* orbital rendevous with the fuel/tug in LLO

* orbital rendevous with the fuel/tug in a high eliptical laythe orbit

* ~1,000 m/s ejection to aerobrake back at kerbin

Everything comes back, nothing is discarded, the wings and air breathing engines to lift everything up to LKO aren't carried along as dead weight, the only wings and airbreathers carried are for the laythe lander.

The margins for doing it this way are much higher than trying to do a single stage there and back.

Its a lot more complicated, but to me that's playing the game and part of the fun.

FYI, the best high bypass turbofans on airliners (which are subsonic) have effective ISPs in the range of about 6,500.

Thats not because their exhaust velocity is ~8x higher than a NERVA rocket.... but because it is *effective* ISP and they do not count the mass of the air used as reaction mass.

KSP does count that mass in their ISP equations... in a 15:1 ratio.... which means they should be using the *real* ISP(ie counting all the reaction mass, not just fuel mass), but 2500 as a value for *real* ISP is ridiculous.

[Jouni]

KSP does count that mass in their ISP equations... in a 15:1 ratio.... which means they should be using the *real* ISP(ie counting all the reaction mass, not just fuel mass), but 2500 as a value for *real* ISP is ridiculous.

It's about right for a turbojet (or a turbofan with afterburner on). The J-58 engine used in the SR-71 had a specific impulse of 1900 s at Mach 3.2, while the Olympus 593 engine used in the Concorde had around 3000 s at Mach 2. The basic jet engine should have higher nominal Isp values than it currently has, but the high-performance engines used for supersonic flight are much less efficient.