Are Sabre Engines More Efficient?

[Blondai]

Yep. That's it. I'm levelling out. But not raising pitch at rocket stage. So I get to about 29000 and have a flat trajectory. But don't raise when I switch. Will try this later this week. Thank you.

[panzer1b]

Ive found rapiers to be great on most of my high end SSTOs, or should i say SSTLAB (single stage to laythe and back), theyre less mass then two separate systems, they double up as nice emergency boosters when the nuke cant brake (high gravity areas), and they look so much better (+ they have that cool looking fire effect that makes spacefighters so much more badass looking).

Also, from an efficiency standpoint, in certain designs they are superior to any other combo. If you like small craft (never was much into that myself), the rapiers are probably one of the best engines, as you both cut down on part count, and you end up having less total mass in your ship (despite meh ISP in rocket mode, the rapiers are pretty bloody efficient). Ive found a single rapier to actually beat a jet+48-7s on super light craft, especially in space (provided the slightly less jet thrust doesnt make it underpowered.

All in all, ive really grown accustomed to them, especially after the mass buff (used to be as heavy as aerospike), now they are both competitive and superior for certain things to jets. really the only downside if that their speed is hard capped at 2200m/s vs 2400m/s for the turbojets, so it makes it impossible to obtain a AP of 200-300km on jet mode alone (regardless of the fact that jets are OP, actually blasting to such an orbit is kinda pointless imo for most situations anyways).

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.

My profile for TWR above 1 os 90 degrees up to 10km, then start to level until flat at 30km, then max speed to 1900-2200 depending on engine/craft configuration, then punch out at 30 degrees once ive attained max speed. This is ofc for semi airhoggy designs (such as my HK-133), and most convcentional craft will need to go rocket below 30km at least somewhat, but personally i never cared about 100% realism, and the jets are about getting to orbit as efficienty as i can. Also, i generally follow rocket like trajectories with SSTOs with teh exception that i intentionally stay around 30km to max speed and get as much out of jets. Ive found the shallow angle to waste alot of fuel on low altitude drag (unless you use FAR/NEAR which is another story). Stock aerodynamics (subject to change in 1.0) kinda makes any flight below 10km a massive dV eater, so i do whatever i can (vertical flight seems most efficient way to do it) to get above the soup and use as little dV below it.

Edited March 8, 2015 by panzer1b

[GoSlash27]

Also, from an efficiency standpoint, in certain designs they are superior to any other combo. If you like small craft (never was much into that myself), the rapiers are probably one of the best engines, as you both cut down on part count, and you end up having less total mass in your ship (despite meh ISP in rocket mode, the rapiers are pretty bloody efficient). Ive found a single rapier to actually beat a jet+48-7s on super light craft, especially in space (provided the slightly less jet thrust doesnt make it underpowered.

Demonstrably false, at least in stock.

For any vehicle at any scale, you will wind up with a lower total mass and far less total fuel expenditure by using a turbojet/ rocket combo instead of Rapiers. Rapiers will theoretically give you a lower part count... assuming you don't add batteries to compensate for the Rapier's inability to generate electricity.... but the added weight of fuel required to achieve orbit with RAPIERS means you'll need more structure and thus more wings.

By way of demonstration:

http://wikisend.com/download/479400/turbojet1.craft

The objective here is to place a Mk.1 can in orbit and return it to KSC with minimum overall mass and fuel expenditure.

You cannot hope to get anywhere near this level of efficiency using RAPIERs, but you're welcome to try.

Best,

-Slashy

Edited March 8, 2015 by GoSlash27

[Jouni]

For any vehicle at any scale, you will wind up with a lower total mass and far less total fuel expenditure by using a turbojet/ rocket combo instead of Rapiers. Rapiers will theoretically give you a lower part count... assuming you don't add batteries to compensate for the Rapier's inability to generate electricity.... but the added weight of fuel required to achieve orbit with RAPIERS means you'll need more structure and thus more wings.

There are basically two differences between RAPIERs (with manual switching) and turbojets:

1. RAPIERs have a lower TWR in airbreathing mode than turbojets. If you build large ships, turbojets are better, because you need less engines.
   
2. RAPIERs lose thrust quicker after 2000 m/s than turbojets. If you practice airhogging, turbojets are better.
   

If you build small non-airhogging craft, RAPIERs can be better than turbojets+rockets, because you need less engine mass.

[GoSlash27]

^ Update to the above:

Here's a best case RAPIER implementation to compare to the TJ/ OMS combo above.

Same exact job, but the TJ/ OMS combo works out lighter, more efficient, and cheaper (both to build and operate).

In stock, RAPIERS never outperform turbojets/ OMS rockets in an SSTO. I would challenge anyone who disagrees to prove it as I have done here.

Best,

-Slashy

Edited March 8, 2015 by GoSlash27

[Jouni]

In stock, RAPIERS never outperform turbojets/ OMS rockets in an SSTO. I would challenge anyone who disagrees to prove it as I have done here.

Your demonstration vehicles resort to heavy airhogging. Two radial intakes in a 2.5-tonne craft is roughly the same as 7 ram intakes in a 15-tonne craft with a single turbojet. Try using 1 or 2 ram intakes in a 10-15-tonne craft instead, and see how the relative performance of the RAPIER and the turbojet changes.

Edited March 8, 2015 by Jouni

[Starhawk]

It seems to me that the number of intakes you need is dictated by the number of air-breathing engines you have. I don't believe that mass comes into it.

It's not like you can put 1/4 of a turbojet on the craft.

I'm quite sure that '1 or 2 ram intakes on a 10-15 tonne craft' (I would use two) works very well for a single turbojet. I haven't built anything with a RAPIER myself in a long time, but I'm pretty sure that Slash is correct on this.

[GoSlash27]

Your demonstration vehicles resort to heavy airhogging. Two radial intakes in a 2.5-tonne craft is roughly the same as 7 ram intakes in a 15-tonne craft with a single turbojet. Try using 1 or 2 ram intakes in a 10-15-tonne craft instead, and see how the relative performance of the RAPIER and the turbojet changes.

Tell ya what:

I will build a fully functional SSTO crew shuttle spaceplane(docking port, RCS, solar arrays) within your stated guidelines using a TJ/ OMS system.

You do the same with a RAPIER and we'll compare results.

Are you game?

Best,

-Slashy

[Jouni]

It seems to me that the number of intakes you need is dictated by the number of air-breathing engines you have. I don't believe that mass comes into it.

This is what you can reasonably expect to achieve with 1 ram intake/turbojet:

In practice, intakes/mass is more relevant than intakes/engine. At high altitudes and near-orbital speeds, all you need is that the engines can provide enough thrust to overcome drag (which is a function of mass). Unless you resort to extreme airhogging, the total thrust of all engines is a function of intake area. For a given intake area, you can either run n engines at X% thrust or 2n engines at (X/2)% thrust, but the total thrust is still the same. Therefore, as a first approximation, intakes/mass is what defines the performance of a jet.

Tell ya what:

I will build a fully functional SSTO crew shuttle spaceplane(docking port, RCS, solar arrays) within your stated guidelines using a TJ/ OMS system.

You do the same with a RAPIER and we'll compare results.

I haven't built stock spaceplanes for ages, so we may have to wait until the next weekend before I have enough time to build one.

I'm also not sure what the exact requirements should be. Probably something like the following:

• The mass of the spaceplane should be in a range, where the reasonable number of turbojets is the same as the reasonable number of RAPIERs.
  
• Total intake area should be low enough that the switch to rockets happens before 1950 m/s (surface speed).
  
• The spaceplane should have enough fuel to rendezvous with something at a 120 km orbit, and deorbit afterwards.
  
• The performance of the spaceplane is measured by its payload fraction. Payload is anything that could be replaced with inert structural parts, and the spaceplane would still be able to perform the mission.
  

[GoSlash27]

I haven't built stock spaceplanes for ages, so we may have to wait until the next weekend before I have enough time to build one.

I'm also not sure what the exact requirements should be. Probably something like the following:

• The mass of the spaceplane should be in a range, where the reasonable number of turbojets is the same as the reasonable number of RAPIERs.
  
• Total intake area should be low enough that the switch to rockets happens before 1950 m/s (surface speed).
  
• The spaceplane should have enough fuel to rendezvous with something at a 120 km orbit, and deorbit afterwards.
  
• The performance of the spaceplane is measured by its payload fraction. Payload is anything that could be replaced with inert structural parts, and the spaceplane would still be able to perform the mission.
  

I certainly hope you can find the time to build one.

As for your bullet points...

#1 You said 10-15 tonnes. That's what I went with. 13.3 tonnes fully loaded and fueled.

#2 To keep it even, you are restricted to 2 XM-G50s, same as I have used. This is comparable to 1 ram intake. The transition speed is whatever it is (a large part of why turbojets are so much more efficient).

#3 Sounds fine. I'll just have to put something up in a 120x120 orbit to demonstrate. It should also deorbit and land safely back at KSC.

#4 Let's simplify it. The payload is kerbals. As many as you can get up there (each crew cabin is 2 tonnes) . Since we're talkin' efficiency, the benchmark is vehicle mass per kerbal and fuel/ oxidizer expenditure per kerbal.

Additional:

#5 As I said, it should have RCS, solar panels, and a docking port.

I'd encourage anyone who thinks that RAPIERS are competitive with turbojets rocket combos to try their hand at this exercise...

Best,

-Slashy

[Jouni]

I'd encourage anyone who thinks that RAPIERS are competitive with turbojets rocket combos to try their hand at this exercise...

As a first attempt, change the turbojet in your plane to a RAPIER. At what speed and altitude you switched from the turbojet to rockets, when the RAPIER flamed out in airbreathing mode, and how much engine mass you saved by using a RAPIER?

Edit: Remember that we're trying to determine whether there exists a niche, where RAPIERs are more efficient than turbojets + rockets. That means that we're going to change the rules, until we can find a niche for the RAPIER, or until we have covered all reasonable niches. There are no turbojet advocates or RAPIER advocates in this discussion – only people trying to prove that RAPIERs are better than turbojets in some cases.

Edited March 8, 2015 by Jouni

[Starhawk]

In practice, intakes/mass is more relevant than intakes/engine.

I must concede this point. I hadn't been considering that drag is currently a function of mass, and was working with a simplified mental model.

This is what you can reasonably expect to achieve with 1 ram intake/turbojet

I'm not quite sure what you are getting at. The orbital specs? How much mass you can get to a specific orbit?

[Jouni]

I'm not quite sure what you are getting at. The orbital specs? How much mass you can get to a specific orbit?

It was just my old demonstration that if we assume that intakes/engine is a useful metric, then 1 ram intake/turbojet is heavy airhogging, because it can get you to a 171x36 km orbit on jets alone.

[Starhawk]

It was just my old demonstration that if we assume that intakes/engine is a useful metric, then 1 ram intake/turbojet is heavy airhogging, because it can get you to a 171x36 km orbit on jets alone.

Oh. I tend to think of it as 'turbojets are massively overpowered' rather than 'one ram intake per turbojet is heavy airhogging', but each to his own.

[GoSlash27]

As a first attempt, change the turbojet in your plane to a RAPIER. At what speed and altitude you switched from the turbojet to rockets, when the RAPIER flamed out in airbreathing mode, and how much engine mass you saved by using a RAPIER?

Edit: Remember that we're trying to determine whether there exists a niche, where RAPIERs are more efficient than turbojets + rockets. That means that we're going to change the rules, until we can find a niche for the RAPIER, or until we have covered all reasonable niches. There are no turbojet advocates or RAPIER advocates in this discussion – only people trying to prove that RAPIERs are better than turbojets in some cases.

I'd argue that none of this matters.

The only niche where a RAPIER would outperform a turbojet (all else being equal) would be where the mass of rockets attached to a TJ is less than the mass of additional fuel/ oxidizer/ structure required by the RAPIER.

Since a TJ/OMS system can get by with a miniscule OMS system (or in some cases none at all), no such regime exists.

There's no way you can ever make up for 200 m/sec with 200 kg of fuel and oxidizer. Especially in the 27-32km altitude range where lift to drag is at it's least efficient.

Rather than arguing this as a hypothetical, it's a simple matter to prove it conclusively in practice.

If you think you can make a RAPIER space plane that's more efficient than a TJ/ OMS, then do it. I say it can't be done.

While skewing the rules to favor the RAPIER is a sound approach, it will lead to a false conclusion if the TJ is not allowed to utilize of it's advantages over the RAPIER. TJs transition at a higher velocity than RAPIERs in all cases where the payload and equipment are identical. They also produce more thrust. This is what makes them superior, and taking that away invalidates the results.

Best,

-Slashy

Edited March 9, 2015 by GoSlash27

[panzer1b]

It seems to me that the number of intakes you need is dictated by the number of air-breathing engines you have. I don't believe that mass comes into it.

It's not like you can put 1/4 of a turbojet on the craft.

I'm quite sure that '1 or 2 ram intakes on a 10-15 tonne craft' (I would use two) works very well for a single turbojet. I haven't built anything with a RAPIER myself in a long time, but I'm pretty sure that Slash is correct on this.

This is completely untrue, its based on mass instead of engines. The reason people tend to say x intakes per engine, is that in general, you have x engines/mass. I have a 20t SSTO with 3 jets on it (overkill TWR i know but its a military combat vessel and i need it), and it uses if i remember correctly 12-14 intakes, this is a little bit below 1 intake area/1 ton mass, what id consider optimal. i just wanted to save mass/part count, and went below what id consider perfect airhogging.

And btw, the only reason i like rapiers is that they have their benefits in certain situations i use my SSTOs for (combat, landing in high gravity, ect). My 20t SSTO also has over 5000dV fully fueled, so i dont consider that a bad craft at all (even if it suffers from overengineering and too high part count (90 with missiles loaded, 70ish without). If you actually have a need for the 175 thrust (and a nuke or a 48-7s wont cut it), then the rapier is better for that particular build.

But i do agree, if you do not intend to land said SSTO on another planet, and are going beyond pure LKO, the jets make more sense. For pure SSTO to orbit, rapiers are easy to use, dont require as much finesse in flight (as you get TWR when you need in rocket mode), and are adequate for most situations. Still, although i like their looks, almost all my high efficiency engines use jets (with effect mods, cant stand their lack of afterburner effects in stock game). Also, aside from dropships/VTOLs, ive also found 0 useage for teh regular jet, its less efficient for anything but running around KSC at super low altitudes (the turbojets are 100% better for cruising, high altitude, speeds, and are atill plenty useable at low altitudes provided you dont stick around ground level for too long.

[GoSlash27]

Oh. I tend to think of it as 'turbojets are massively overpowered' rather than 'one ram intake per turbojet is heavy airhogging', but each to his own.

Starhawk,

My position is that "airhogging" doesn't factor into it, regardless of how it's defined.

The TJ/OMS will outperform the RAPIER in any stock SSTO regardless of whether they're using a thousand ram intakes or a single structural intake. 200 m/sec advantage in flameout velocity, superior t/w, and the ability to run air breathing and rocket simultaneously is impossible for the RAPIER to overcome with it's tiny mass advantage.

Best,

-Slashy

[Starhawk]

The TJ/OMS will outperform the RAPIER in any stock SSTO regardless of whether they're using a thousand ram intakes or a single structural intake. 200 m/sec advantage in flameout velocity, superior t/w, and the ability to run air breathing and rocket simultaneously is impossible for the RAPIER to overcome with it's tiny mass advantage.

I agree completely. My comment about turbojets being overpowered was in response to Jouni's assertion that '1 ram intake/turbojet is heavy airhogging, because it can get you to a 171x36 km orbit on jets alone'.

[Jouni]

The only niche where a RAPIER would outperform a turbojet (all else being equal) would be where the mass of rockets attached to a TJ is less than the mass of additional fuel/ oxidizer expended by the RAPIER.

Since a TJ/OMS system can get by with a miniscule OMS system (or in some cases none at all), no such regime exists.

There's no way you can ever make up for 200 m/sec with 200 kg of fuel and oxidizer. Especially in the 27-32km altitude range where lift to drag is at it's least efficient.

My entire point has been to find a niche, where the RAPIER-powered plane burns less fuel, because it carries less engine mass.

Based on some experiments I did, such niche could be found somewhere in the 5-10 tonne range, if we limit the number of intakes. The hypothesis is that if thrust is limited by intake air during the critical part of the ascent, the higher thrust of the turbojet becomes irrelevant. And if the number of intakes is low enough, the plane needs a nontrivial rocket stage, which makes the turbojet/rocket combo less efficient. Ideally the turbojet/rocket spaceplane would have to switch to rockets at 1700-1800 m/s, and the RAPIER-powered plane would be able to reach similars speeds in airbreathing mode. (Those speeds were quite typical for the non-airhogging spaceplanes I built around a year ago.)

[GoSlash27]

I agree completely. My comment about turbojets being overpowered was in response to Jouni's assertion that '1 ram intake/turbojet is heavy airhogging, because it can get you to a 171x36 km orbit on jets alone'.

Oh, I'm with ya...

He's actually got a point about the drag being proportional to mass, but what he's missing is that "airhogging" (however defined) actually masks the inadequacies of the RAPIER. If the intake area/ mass is reduced for both, the advantage of the TJ over the RAPIER actually *increases*.

Best,

-Slashy

- - - Updated - - -

My entire point has been to find a niche, where the RAPIER-powered plane burns less fuel, because it carries less engine mass.

Based on some experiments I did, such niche could be found somewhere in the 5-10 tonne range, if we limit the number of intakes. The hypothesis is that if thrust is limited by intake air during the critical part of the ascent, the higher thrust of the turbojet becomes irrelevant. And if the number of intakes is low enough, the plane needs a nontrivial rocket stage, which makes the turbojet/rocket combo less efficient. Ideally the turbojet/rocket spaceplane would have to switch to rockets at 1700-1800 m/s, and the RAPIER-powered plane would be able to reach similars speeds in airbreathing mode. (Those speeds were quite typical for the non-airhogging spaceplanes I built around a year ago.)

Your best bet would to be provide an example. Short of requiring an unequal allocation of parts and resources, the math says that this doesn't work out.

The exception would be where you declare any use of intakes at all "air hogging", in which case air breathing is disallowed and a properly designed rocket beats the RAPIER.

I'm thinking you were comparing an optimized RAPIER to a non- optimized TJ/OMS and misinterpreting the result.

I'm keen to try this for kicks, though. Although why anyone would attempt to build an SSTO with severely limited intake area is beyond me...

Best,

-Slashy

[Jouni]

Your best bet would to be provide an example. Short of requiring an unequal allocation of parts and resources, the math says that this doesn't work out.

I've been trying to narrow it down. If the niche exists, it's narrower than I would have expected. Medium orbits such as 240 km or 600 km might be better for the RAPIER, because they require more from the rocket stage.

Although why anyone would attempt to build an SSTO with severely limited intake area is beyond me...

For the same reason why some people still use actual rocket engines in first stages, when turbojets beat them by a huge margin as rocket engines. Some people find the game more fun, when they're not intentionally abusing things that are broken in every conceivable way.

[Starhawk]

My mistaken assertion that mass doesn't matter was a red herring. The point that RAPIERS are always less efficient is still valid.

[GoSlash27]

I've been trying to narrow it down. If the niche exists, it's narrower than I would have expected. Medium orbits such as 240 km or 600 km might be better for the RAPIER, because they require more from the rocket stage.

For the same reason why some people still use actual rocket engines in first stages, when turbojets beat them by a huge margin as rocket engines. Some people find the game more fun, when they're not intentionally abusing things that are broken in every conceivable way.

Well... I don't think that assigning 1 ram or shock cone intake per engine is "abusing" intakes, and surely any combo with that (or even a whole heckuva lot less) will show the turbojet has an overwhelming performance advantage.

Since nobody's ever going to design an SSTO spaceplane that runs out of steam before it even gets to 2 km/sec, I wonder what the point of this line of reasoning is. Perhaps there is a RAPIER vehicle out there that could come close to it's TJ equivalent... but only if it were intentionally designed to be ridiculously inefficient (which nobody does).

If you're designing a stock SSTO spaceplane and you want to know which engine is more efficient, the answer is "turbojet".

Best,

-Slashy

*edit*

Although I do agree that turbojets are definitely broken.

[Jouni]

Since nobody's ever going to design an SSTO spaceplane that runs out of steam before it even gets to 2 km/sec, I wonder what the point of this line of reasoning is.

Many people build stock spaceplanes with 1 intake/engine. Those planes won't reach 2 km/s on jets, unless their TWR is very high.

If you're designing a stock SSTO spaceplane and you want to know which engine is more efficient, the answer is "turbojet".

I'm starting to believe that you're right in this one. A turbojet/rocket combo beats RAPIERs basically every time, though a turbojet/RAPIER combo can be even better in some cases.

[GoSlash27]

Many people build stock spaceplanes with 1 intake/engine. Those planes won't reach 2 km/s on jets, unless their TWR is very high.

Oh, sure they will. It's just a matter of allowing it to build speed at 32km altitude. The trick is to not climb too fast past 20km, leaving you high and slow.

If you pace yourself and reduce throttle at flameout, you can still achieve orbital velocity with very little intake area.

The example I knocked together for this demo uses 2 XM-G50 intakes, which is comparable to a single ram intake or shock cone and weighs about 13.5 tonnes. It would still work with a single intake or even less, but then real-estate to mount the intakes becomes a problem on my design.

Here's an example with less than 1 standard intake:

I'm starting to believe that you're right in this one. A turbojet/rocket combo beats RAPIERs basically every time, though a turbojet/RAPIER combo can be even better in some cases.

This is true, although as you said upstream the entire thing hinges on the vehicle being properly designed and flown. If it's too heavy, doesn't have enough wings, or is flown like a rocket... then closed cycle thrust becomes non-trivial and the RAPIER can become preferable.

Best,

-Slashy

Edited March 10, 2015 by GoSlash27