Are Sabre Engines More Efficient?

[Blondai]

I set out to build a new (Larger; hence MKII) space plane, with foldaway dock, and cargo storage, after several semi effective models, featuring two ram jets and two/one Spikes, or smaller engines, i decided to give in and put a single Sabre to replace my rocket stage, and Immediately had more success. I also saw a video of a fellow Kerballer who made a MKII using only two sabre engines, and to my surprise it was far more efficient.

Was I wrong in assuming Sabres were less efficient, I heard this a while ago, and believed it.

[temporalExile]

S.A.B.R.E.s aren't in stock KSP. If you're reffering to the R.A.P.I.E.R. engines, then yes, they perform poorly as a pure rocket or jet engine

Edited February 24, 2015 by temporalExile

[Blondai]

Yes Rapier.

[Vanamonde]

Thread moved to the subforum for how-to kinds of questions.

[ArmchairGravy]

While the Rapier engines may be less efficient, you save the weight of needing both jet and rocket engines. I have better luck using Rapiers in Mk. 2 designs versus jets and rocket engines, though YMMV.

[Wabbit]

They aren't quite as effective (or efficient) as either of their individual engines, but they do offer decreased weight. Also, they look cooler! XD

[Scoundrel]

Rapiers are actually better rockets than aerospikes as they have a slightly higher specific power (258 vs 223), and coupled with their dual mode cycle, you'll find that Rapiers are the most efficient engine for getting things to orbit. That being said, the KR-2L's specific power (717!!!!) makes it the best engine in the game for getting anything into orbit, followed by the KS-25x4, the KR-1x2 and the 48-7S with their 500+ specific power.

[EtherDragon]

So, the RAPIER is not more efficient (by itself). But SSTOs using the RAPIER are more efficient because they can get rid of considerable dry mass by using them in place of multiple engines of varying types.

[Pecan]

If you fly a spaceplane properly the efficiency of turbojets more than pays for their mass as you only need a couple (for balance) of 48-7Ss for a tiny circularisation burn.

[FleshJeb]

I don't use Rapiers. I clip an LV-N, 48-7S, or an Aerospike inside my turbojets. Leaving the turbos running (even at reduced throttle) while your rockets are on is a huge fuel savings. I switch over to pure rocket mode when my total thrust from throttled Turbos + Rockets = 100% Rocket power.

[GoSlash27]

Rapiers are definitely not as effective or efficient as turbojet/ rocket combos. Yes, you can save a couple parts and a tiny amount of dry mass, but you have to make up 400 m/sec when using them in air breathing mode compared to turbojets and they also have less thrust. This ensures that you'll need more fuel and associated structure, which winds up making them heavier for the same payload.

Turbojet/ OMS rocket combos are so overpowered that it makes no sense to launch any other way if you have the tech.

Best,

-Slashy

[Blondai]

This is what i was thinking, all this time trying for efficiency, I've lost it. Thanks for confirming my suspicions. Points to you good sir.

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Sorry, OMS?

[GoSlash27]

Blondai,

Sorry, "Orbital Maneuvering System"; engines intended to circularize and adjust orbits, but not intended to be part of the launch itself.

Best,

-Slashy

[Cirocco]

Turbojet/rocket combo is more efficient. Slashy pretty much summarized it for you, but there's two things I'd like to add:

1) the reason why you suddenly had more success with RAPIERS is probably due to the fact that even though they are slightly less efficient than the combo, they are a lot more user friendly. You can be a lot more efficient with the combo, but you need to design and use it properly,using all the tips and tricks to squeeze every single bit of thrust out of those turbojets. If you don't do it right, you'll end up losing more efficiency than you would potentially gain due to aerodynamic losses, gravity losses, etc. RAPIERs are simple and user friendly. They might not be as efficient, but you probably won't end up losing a metric ton of delta-v due to inefficient flight profiles or sub-optimal design.

2) one other reason is the thrust: airbreathers don't produce that much thrust in comparison to rocket engines of similar size. So you'll need a LOT of them on heavy planes and often pack 2 to 3 times as much airbreathing engines than rocket engines. That means you need a LOT of 1,25m hardpoints to put all those engines, but only a couple of rocket engines. If you replace all of those engines with RAPIERs, suddenly you've got double or even triple the thrust in your rocket phase, but with slightly more weight and less engine efficiency. That means you'll suddenly have a lot less gravity losses than you might have had with the turbojet/rocket combo, but you don't want to be using that craft for long burns.

So in short: the Turbojet/rocket engine combo is potentially much more efficient, but less user friendly and forgiving: if you don't design and fly it correctly, you're likely to accrue more losses than gains. RAPIERs are easy to use, especially to get to LKO.

Good example is interplanetary travel: if you can get a spaceplane in orbit with turbojets and one or two LVN nuclear engines, you can get it to another planet easy-peasy. But doing that requires some pretty advanced design and flying. Trying to go interplanetary with a spaceplane that uses only RAPIERS is incredibly inefficient in comparison, but building and flying a plane that can get to orbit with RAPIERs is lot easier.

Edited February 25, 2015 by Cirocco

[Blondai]

Thanks, so like, any small engine, or LV model

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Yeah, i am moderately experienced at space planes, I'm not scott manley, but I en too bad, i have had like 4 models that have got into orbit, I even took one off, docked to a massive mother ship, equipped with small landers, went to Jool, landed on laythe with plane, and got back to ship, checked out bop Pol and then to dres, and then home, landing my plane plus escape pod back at space centre... so like i say, not total noob, it is just a bit of teething troubles getting a MkII that is actually useful for refueling and satellite delivery, though i got one now, it is an instability upon return, which i think i can resolve with an extra pair of tail wings

But main point is still just getting to orbit, with maximum fuel leftover

Proof of moderate competance

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Best model so far

[GoSlash27]

Turbojet/rocket combo is more efficient. Slashy pretty much summarized it for you, but there's two things I'd like to add:

1) the reason why you suddenly had more success with RAPIERS is probably due to the fact that even though they are slightly less efficient than the combo, they are a lot more user friendly. You can be a lot more efficient with the combo, but you need to design and use it properly,using all the tips and tricks to squeeze every single bit of thrust out of those turbojets. If you don't do it right, you'll end up losing more efficiency than you would potentially gain due to aerodynamic losses, gravity losses, etc. RAPIERs are simple and user friendly. They might not be as efficient, but you probably won't end up losing a metric ton of delta-v due to inefficient flight profiles or sub-optimal design.

2) one other reason is the thrust: airbreathers don't produce that much thrust in comparison to rocket engines of similar size. So you'll need a LOT of them on heavy planes and often pack 2 to 3 times as much airbreathing engines than rocket engines. That means you need a LOT of 1,25m hardpoints to put all those engines, but only a couple of rocket engines. If you replace all of those engines with RAPIERs, suddenly you've got double or even triple the thrust in your rocket phase, but with slightly more weight and less engine efficiency. That means you'll suddenly have a lot less gravity losses than you might have had with the turbojet/rocket combo, but you don't want to be using that craft for long burns.

So in short: the Turbojet/rocket engine combo is potentially much more efficient, but less user friendly and forgiving: if you don't design and fly it correctly, you're likely to accrue more losses than gains. RAPIERs are easy to use, especially to get to LKO.

Good example is interplanetary travel: if you can get a spaceplane in orbit with turbojets and one or two LVN nuclear engines, you can get it to another planet easy-peasy. But doing that requires some pretty advanced design and flying. Trying to go interplanetary with a spaceplane that uses only RAPIERS is incredibly inefficient in comparison, but building and flying a plane that can get to orbit with RAPIERs is lot easier.

Cirocco,

I'd disagree with point #2. A properly designed and flown turbojet/ rocket spaceplane needs very little rocket thrust to maintain apoapsis and circularize on arrival. More rocket thrust (even if it's essentially "free") doesn't do you any favors.

But as you point out, they *do* need to be flown correctly to achieve their potential. If you find yourself needing more rocket thrust, you're doing something wrong. I've seen turbojet spaceplanes circularize on RCS thrusters.

Personally, I'm hoping that turbojets get beaten unconscious with a nerf- bat in 1.0. They're just plain too good and they wreck the balance of the game.

Best,

-Slashy

[Branjoman]

The MK4 Spaceplane parts has the SCIMITAR engine, which is a beefed up version of the RAPIER - is quite useful!

[Blondai]

I assume thats a MOD, was just trying stock until 1.0

[FlyingPete]

I tend to go with jet engines plus a pair of radial engines- either 48-7S or the O-10 monopropellant engine. With the right launch profile you only need a small push to get into orbit, and for orbital maneuvering you don't need a huge TWR. I find you can keep a dedicated jet engine going longer and higher than the RAPIER in air-breathing mode, which makes a big difference to the amount of rocket fuel you need.

[LethalDose]

Turbojet/rocket combo is more efficient. Slashy pretty much summarized it for you, but there's two things I'd like to add:

1) the reason why you suddenly had more success with RAPIERS is probably due to the fact that even though they are slightly less efficient than the combo, they are a lot more user friendly. You can be a lot more efficient with the combo, but you need to design and use it properly,using all the tips and tricks to squeeze every single bit of thrust out of those turbojets. If you don't do it right, you'll end up losing more efficiency than you would potentially gain due to aerodynamic losses, gravity losses, etc. RAPIERs are simple and user friendly. They might not be as efficient, but you probably won't end up losing a metric ton of delta-v due to inefficient flight profiles or sub-optimal design.

2) one other reason is the thrust: airbreathers don't produce that much thrust in comparison to rocket engines of similar size. So you'll need a LOT of them on heavy planes and often pack 2 to 3 times as much airbreathing engines than rocket engines. That means you need a LOT of 1,25m hardpoints to put all those engines, but only a couple of rocket engines. If you replace all of those engines with RAPIERs, suddenly you've got double or even triple the thrust in your rocket phase, but with slightly more weight and less engine efficiency. That means you'll suddenly have a lot less gravity losses than you might have had with the turbojet/rocket combo, but you don't want to be using that craft for long burns.

So in short: the Turbojet/rocket engine combo is potentially much more efficient, but less user friendly and forgiving: if you don't design and fly it correctly, you're likely to accrue more losses than gains. RAPIERs are easy to use, especially to get to LKO.

Good example is interplanetary travel: if you can get a spaceplane in orbit with turbojets and one or two LVN nuclear engines, you can get it to another planet easy-peasy. But doing that requires some pretty advanced design and flying. Trying to go interplanetary with a spaceplane that uses only RAPIERS is incredibly inefficient in comparison, but building and flying a plane that can get to orbit with RAPIERs is lot easier.

I completely agree with points 1 & 2 leading to the "So in short" summary provided here; The RAPIERs pack a great punch, and are why simpler than multi-engine configurations that are pretty darn temperamental. And anyone who tries to tell you extra thrust doesn't do you any good is full of jetwash. Increasing TWR (to a point) has been demonstrated to reduce dV requirements, both in theory and in practice. And the point where the diminishing returns from increased TWR becomes an issue is typically way above where most SSTO spaceplanes' TWR hangout.

I'm personally not a fan of SSTO spaceplanes with nukes, though. It's trying to do to much with a single vessel. IMO, spaceplanes excel at ferrying resources, kerbals, and small payloads between the surface and low orbit of Kerbin (and too a lesser degree Laythe). Beyond that mission profile, you're better off using specialized orbital vessels.

[Empiro]

How much of the advice is Stock vs. FAR? Particularly in the latest version of FAR with the increased drag, you cap out at around 1300-1400 m/s, and adding extra jet engines doesn't get you that much. Because of the extra drag, you also want to climb quickly and get out of the atmosphere fast, which means that you'll still want a decent TWR when you switch to rockets, which means that tiny Rockomax engines might not cut it. I definitely lean toward RAPIERS when I play FAR.

[Cirocco]

Cirocco,

I'd disagree with point #2. A properly designed and flown turbojet/ rocket spaceplane needs very little rocket thrust to maintain apoapsis and circularize on arrival. More rocket thrust (even if it's essentially "free") doesn't do you any favors.

But as you point out, they *do* need to be flown correctly to achieve their potential. If you find yourself needing more rocket thrust, you're doing something wrong. I've seen turbojet spaceplanes circularize on RCS thrusters.

Personally, I'm hoping that turbojets get beaten unconscious with a nerf- bat in 1.0. They're just plain too good and they wreck the balance of the game.

Best,

-Slashy

Oh don't get me wrong, I totally agree that, if your craft is well designed, you don't need much thrust at all to make that final boost to orbit. But that requires you to have something that can climb to about 30km and something near 3 km/s (or more) horizontal velocity without an asymmetric flameout. And that in turn requires knowledge of airflow and in what order to place your engines and intakes.

I personally know how to do this, my point is that people new to building spaceplanes often don't know that and will have their turbojet/LOX combo craft do an asymmetric flameout at about 23km and 2.2-2.3 km/s, forcing them to go to rocket stage before that happens. At that point you *do* need a lot of thrust to get your plane up into orbit. Igniting from that point, your RAPIER's suddenly *seem* to be a lot more efficient because you can overcome the gravity losses. That was what I was trying to explain: why new players often think that RAPIER engines are super-efficient, ever though they're actually not, they're more user-friendly and forgiving.

But again, I wholeheartedly agree that if you avoid an asymmletric flameout, turbojets are vastly superior to RAPIERs. Now, if memory serves me correctly, SQUAD will fix the whole airflow leading to asymmetric flameout thing in 1.0. If that is indeed the case and asymetric flameout will no longer be a thing, then I do agree that Turbojets could do with a serious nerf.

Now in response to the post just above mine: My advice is all stock. FAR has nerfed the turbojet considerably so I don't know how well it performs under that mod.

[GoSlash27]

Cirocco,

Yeah, we're in agreement.

Empiro,

I'm talking about stock.

Best,

-Slashy

[Blondai]

I wonder if this is actually my issue, my space planes certainly follow the rules that most people abide, but I always get higher into orbit when I accidentally fly too steep, I always that that flying flat as possible is the best way to achieve orbit, but I am beginning to wonder if my launch profile might be too shallow?

[Cirocco]

I wonder if this is actually my issue, my space planes certainly follow the rules that most people abide, but I always get higher into orbit when I accidentally fly too steep, I always that that flying flat as possible is the best way to achieve orbit, but I am beginning to wonder if my launch profile might be too shallow?

Dpends. What is your launch profile exactly? From experience and forum-browsing, I think that the best way is to go steep (45 to even 60° if your engines can handle it) up to about 15-20km, then level out and slowly climb to 25-27. Once you pass 27 km (roughly) you level out almost completely and squeeze whatever delta-V you can out of your turbojets. At flameout (around 30-32 km) you switch to rockets and pitch up hard to 45-60° in order to boost yourself out of atmo as quickly as possible. after that just coast to apoaps (giving small boosts to compensate for drag when neccesary) and circularize as usual.