Basic jet engine vs. R.A.P.I.E.R jet engine

[Themohawkninja]

There is an obviously huge advantage to the RAPIER. Sure, two turbojets and an aerospike out performs a RAPIER, but two turbojets and an aerospike ways a lot more than a RAPIER, and each only fulfills one role, whereas the RAPIER fills both, but with lower performance as a balancing act.

[RadHazard]

Seriously? How is this better than a mainsail? This has less ISP than a lv 909. It's just a non radial version of the 24-77.

It actually has higher ISP than the 24-77. If you put 50 48-7S engines on a rocket, it has the same thrust as a mainsail, less weight (5t instead of 6t), and a higher ISP (300s - 350s instead of 280s - 330s). The only disadvantage is part count (which admittedly is a somewhat major disadvantage).

Again, on topic, based on the numbers people are posting, I can see the RAPIER is at a disadvantage compared to turbojet/aerospike, but I can't see what the devs could easily do to fix it. If they upped thrust and ISP to match turbojets and aerospikes, there'd be very, very little reason to use either of those engines. They could up the mass of all three engines so that the mass savings the RAPIER gives is more significant, but that'd nerf the other two engines. It's a fine line to walk, I think.

On another note, has anyone had issues with asymmetric thrust with the RAPIERS? I built a test SSTO with them and as I started getting into the thin atmosphere, my left engine started throttling down (I assume from lack of air). My plane started spinning to the left pretty badly before the automatic mode switch kicked in and they went to rocket mode.

Edited December 18, 2013 by RadHazard

[Eric S]

Seriously? How is this better than a mainsail? This has less ISP than a lv 909. It's just a non radial version of the 24-77.

Actually, its ISP is much better than the radial version, and while the 909 has a better ISP, it has five times the mass for twice the thrust, and for low-g environments, I never use all the thrust anyway. If I take the single 48-7S off of my Minmus/Mun lander and replace it with a 909, the total delta-v of the craft goes down because the mass went up.

There is an obviously huge advantage to the RAPIER. Sure, two turbojets and an aerospike out performs a RAPIER, but two turbojets and an aerospike ways a lot more than a RAPIER, and each only fulfills one role, whereas the RAPIER fills both, but with lower performance as a balancing act.

I've got a spaceplane I designed in 0.22 that uses a turbojet and two 48-7S engines. That craft gets to orbit with more fuel remaining than a similar craft with a single RAPIER, and the craft has less mass (from engines) as well. The biggest advantage of the RAPIER is that you only have to find space for a single engine, though only having to set up fuel lines for a single engine is also a solid advantage.

Edited December 18, 2013 by Eric S

[Themohawkninja]

I've got a spaceplane I designed in 0.22 that uses a turbojet and two 48-7S engines. That craft gets to orbit with more fuel remaining than a similar craft with a single RAPIER, and the craft has less mass (from engines) as well. The biggest advantage of the RAPIER is that you only have to find space for a single engine, though only having to set up fuel lines for a single engine is also a solid advantage.

Well... impressive. I couldn't make a single SSTO until the RAPIER, so it has it's own personal advantage to me, along with the more objective space saving ones.

I do wish they wouldn't overheat so easily though.

[Murph]

I do wish they wouldn't overheat so easily though.

Remember that overheat should generally be ignored unless it's getting close to 100%. It will depend just how you have attached them, but I didn't see the overheat being problematic on them (although all of the testing I've done has been with small craft to LKO, so fairly short burns, possibly insufficient to know for sure).

[Nao]

Thanks RadHazard and Eric S for explanation on 48-7S.

I would add that even the old 20kN thrust version of 48-7S was already making LV-909 obsolete. I did a small chart about it some time ago: - there is no flight configuration that make LV-909 better choice than either LV-N or 48-7S.

Back to the topic, i don't hear anybody complaining about Skipper! Like RAPIER, it's not only inferior in relative performance to its counterparts (Mainsail and LV-T30), but also can be easily replaced by 3x LVT-30 for the same thrust (645 to original 650) for better Isp and less mass even with stack tri-adapter. It just happens that since it filled the large gap in thrust between LV-T30 and Mainsail, nobody had any complaints for it.

And RAPIER fills the need of dual propulsion engine. Yes it's not as efficient as some alternatives, but it fills the gap and that is in my opinion the most important thing.

Remember guys this is a Early Access game. RAPIER isn't supposed to be a "new shiny toy" to play with but a first engine for a noob to use on his first SSTO when the game is finally out.

[WhiteWeasel]

Why is it different? There is no changes between the lv1's and it's radial counterpart, what makes the rockomax so special?

[sgt_flyer]

After playing a bit with the rapiers, i would say that the rapier's downside is that they have not enough thrust at sea level - they would feel more like 'ascent' engines if they could have a sea level thrust / isp somewhere inbetween the jets and turbojets.

[NikkyD]

The skipper has gimbal, which can be important for launch.

But i also feel that the rapier has too low thrust and imho a bad ISP in space. If you plan longer voyages in space, like mun-and-back you would be better off with some other engine. For a mere to-orbit transporter that is recycleable (in the future maybe) the rapier is the only one that allows small crafts.

Never underestimate the importance of only needing one mount-point

[Murph]

After playing a bit with the rapiers, i would say that the rapier's downside is that they have not enough thrust at sea level - they would feel more like 'ascent' engines if they could have a sea level thrust / isp somewhere inbetween the jets and turbojets.

Yes, that would be my #1 issue with them. The performance is really poor from the runway up to maybe 12,500m or 15,000m. Combined with not that great fuel economy, the total fuel burnt really kills them for me, compared to just sticking with the turbojets and aerospike. The thrust and fuel efficiency in rocket mode are not a major issue for low orbits, as there's pretty much too much thrust and ok-ish efficiency there. It's the first half of the ascent that really kills them for me, but add in less efficiency than the aerospike in the second half, and they are basically all round mediocre at best.

[maccollo]

Seriously? How is this better than a mainsail? http://wiki.kerbalspaceprogram.com/w/images/9/9a/Rockomax_48-7S.png This has less ISP than a lv 909. It's just a non radial version of the 24-77.

On-topic I feel the RAPIER engine is like the jack of all trades - master of none vibe.

It's the ideal lander engine. Even in 0.21 the 24-7S replaced the 909 for me as the engine of choice for landers, and then it got a 50% thrust improvement in 0.22.

Sure, the 909 has more ISP, but that's irrelevant without considering the mass fraction performance of your stage. The 24-7S gets you more deltaV than the 909 out the vast majority of stage configurations for a lower starting mass, especially landers, which also means all the stages that come before it will also have improved performance.

Edited December 19, 2013 by maccollo

[Eric S]

Why is it different? There is no changes between the lv1's and it's radial counterpart, what makes the rockomax so special?

That's probably a mistake on the part of whoever created the radial version of the LV-1 (Claire?). At one time one of the devs said that they intended radial engines to be inferior to stack mounted ones because it's easier to find places to stick radial engines. This was explaining why the mark 55 had the same TWR but much lower ISP than the LV-T45.

[WhiteWeasel]

That's probably a mistake on the part of whoever created the radial version of the LV-1 (Claire?). At one time one of the devs said that they intended radial engines to be inferior to stack mounted ones because it's easier to find places to stick radial engines. This was explaining why the mark 55 had the same TWR but much lower ISP than the LV-T45.

In that case, maybe the 909 could be buffed to reduce the gap between it and the rockomax. Perhaps make it 60 kN?

[ssTALONps]

On another note, has anyone had issues with asymmetric thrust with the RAPIERS? I built a test SSTO with them and as I started getting into the thin atmosphere, my left engine started throttling down (I assume from lack of air). My plane started spinning to the left pretty badly before the automatic mode switch kicked in and they went to rocket mode.

I can confirm this with twin engine side-by-side configuration, 1:1 engine to intake ratio.

[Specula]

It's even worse when the R.A.P.I.E.R runs out of air. The switchover from air intake to internal oxidiser is horrendous and very late to happen, only kicking in long after breathable air has dropped past 0.0 and your engines have flamed out. And then when the oxidizer kicks in... It consumes fuel at an abhorrent rate. Once it switched over, it consumed 660 Oxidizer in 2 R.A.P.I.E.R engines in about 20 seconds.

[Nao]

It's even worse when the R.A.P.I.E.R runs out of air. The switchover from air intake to internal oxidiser is horrendous and very late to happen, only kicking in long after breathable air has dropped past 0.0 and your engines have flamed out. And then when the oxidizer kicks in... It consumes fuel at an abhorrent rate. Once it switched over, it consumed 660 Oxidizer in 2 R.A.P.I.E.R engines in about 20 seconds.

The breathable air in resource tab is only the air that is not used, when it drops to around 0 we get flameouts.

Now in 0.23 there is some new logic concerning resources distribution. It looks like as the engines are reduced in thrust and liquid fuel flow accordingly to how much breathable air they can get. One engine gets it all to function properly, and the second one is choking but still producing thrust.

This *could* happen with turbojets in 0.22 patch but it required closed radial air intakes on the ship. (glitch)

There are also some more bizarre things happening as we reduce the thrust a little, both engines could start choking at the same time, or rapidly switching the choking from one engine to other. With these glitches it's possible to ride the RAPIRERS as well as other jet engines well beyond their normal capacity.

As much as this is bad design wise, i have ton of fun getting these stubborn things to do my bidding and surpass what was thought to be impossible, feels like really pushing the boundaries of a new not yet understood technology <3.

[NWM]

Yes, the O2 and H used in real world rocketry is typically liquid in cryogenic tanks.

It's not explicitly defined in KSP, as far as I'm aware, but I'd guess our liquid fuel is mostly meant to be kerosine, as it's the same used for both jets and rockets (which both use kerosine in the real world, just different grades of it and different additives).

And the other hand, if you count the destiny of fuel it is obvious 1U liquidfuel ~ 5L ~ 5kg - it is not H2! The main problem of the H2-O2 propulsions the low density - the huge tanks kills the advantage of the higher Isp. Apart from this I would pleased if there were H2-O2 engines with water->H2-O2 generators. It would be an ideal local propulsions on distant world with water.

And at the other hands - SCABRE is not a turbojet-rocket hybrid - but a ramjet-rocket one - and its efficiency-graff should show that. In breathing mode should need high speed and low air density...

Edited December 19, 2013 by NWM

[NWM]

I think RAPIER would be pleasent device if it was in his correct location

Edited December 19, 2013 by NWM
mess of forum software

[NikkyD]

What happened to me is that i had a plane with 2 rapier and 2 ram intakes, i ALT-clicked on both rams to show me the air. Once flight my left ram had exactly 0.2 air less (or was it 0.02... smallest digit). It was way below what would be needed to power the rapier and because of fuel lines it would have been impossible for the rams to exchange air. Shortly before the air ran out both ram suddenly showed the same amount.

In another flight i had the issue with the left rapier giving about 2 kN more thrust than the other and that made the plane fly to the right.

[Dispatcher]

0.23 came with the rapier jet engine, a hybrid engine of a jet engine and a rocket engine.

I tested the new rapier on one of my aircraft (project Hawk). The results were not as spectacular as i'd hope. ... At the end of the runway, this version reached a speed of 130m/s, a thrust of 138.6kN and a specific impulse of 1995.8s.

the version with rapier engines only reached a speed of 87.7m/s, a thrust of 97kN and a specific impulse of 832.5s.

in both instances the breaks were off and both dropped their tail from the runway.

though Jet-Hawk preformed much better then Rapier-Hawk, who nearly crashed because of a stall.

My question:

Why is the specific impulse of a rapier jet engine so low?

I don't know "why" Squad spec'd the engine the way it is, but its an interesting part. Right after the update I built an SSTO with 3 RAPIERs in a short amount of time and the spaceplane made it to orbit OK (most SSTOs take much more time for me to develop). I updated my engine chart to include the RAPIER in each mode. I also replaced the "part size" column with "electric charge" generation. You can see how the jet engines (and others) compare to the RAPIER modes; both in specs and in some tests. If you're interested, just use the chart link in my sig line.

[Magma]

I don't know "why" Squad spec'd the engine the way it is, but its an interesting part. Right after the update I built an SSTO with 3 RAPIERs in a short amount of time and the spaceplane made it to orbit OK (most SSTOs take much more time for me to develop). I updated my engine chart to include the RAPIER in each mode. I also replaced the "part size" column with "electric charge" generation. You can see how the jet engines (and others) compare to the RAPIER modes; both in specs and in some tests. If you're interested, just use the chart link in my sig line.

hmm... Rapier only has a Thrust/Weight ratio of ~3?

talking about underpowered '

[Magma]

oops, double post

Edited December 20, 2013 by Magma
accedental double post

[Geoff-AU]

if it is a poor ascent engine, poor jet engine and a poor orbital engine, what is it really good for?

I haven't use a rapier yet. Just read the discussions. Have you ever heard the expression "jack of all trades, master of none"? Sounds like this engine is it. It will do a bit of everything, but of course that's a compromise. If you want a fantastic jet engine, use a jet. If you want a fantastic rocket engine, use a rocket. If you only have the weight budget for one engine to do both, use a rapier. It's a case of pick your compromises and it sounds like Squad have got it right if they are creating this sort of debate!!

[Nao]

hmm... Rapier only has a Thrust/Weight ratio of ~3?

talking about underpowered '

Except when it has 10+ 175 /9,81/1,75 = 10,2

I agree that it has less power for more mass than the Turbojet, but frankly its the Turbojet that's overpowered as it was the "one engine for all" thing and now as we add more parts there is no room for variety without power creep.

It's rocket TWR is 11 to aerospikes 11,9 big difference here .

Also while its jet twr at launch is only 5, but we should not forget that its actually two engines not one, numbers wise its twr is fine, its the balance between jet and rocket power that is "lacking".

[antbin]

but frankly its the Turbojet that's overpowered

Absolutely agree. But how to nerf it?

And bigger question, is there a more fun way to tweak atmospheric ISP / thrust with altitude or speed? Right now, the ISP varies with altitude and the thrust with speed, and that's it.

It's a good idea, to prevent atmospheric engines from being used on rockets (need to gain speed on the runway in order to have enough thrust to climb), but it's a little frustrating to have a 'dead zone' between 7,000 and 14,000m where the basic jet loses thrust and the turbojet hasn't gained enough speed to get enough thrust. Seems like the RAPIER as it is doesn't help that problem.