Why use the arospike

[Wanderfound]

The numbers I give are based on this code, which calculates Isp the way you want it to be calculated: figure out the mass flow, and take just the liquid fuel fraction of it.

The conclusion is that the KSP basic jet is 2-3x the Isp of modern Earth turbofans (my initial comparison had bad numbers for Earth jets). The KSP turbojet is yet more efficient at high speed, whereas Earth turbojets are less efficient than turbofans, so that's totally out of whack.

The KSP Turbojet is also apparently some sort of turbo/ram/scram/magic pony dust thruster. Real turbojets don't work at Mach 6.

[tsotha]

The KSP Turbojet is also apparently some sort of turbo/ram/scram/magic pony dust thruster. Real turbojets don't work at Mach 6.

Which is fine. If they model jet engines accurately then there's no space planes. Just like IRL.

[KerikBalm]

going by this chart:

http://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Specific-impulse-kk-20090105.png/700px-Specific-impulse-kk-20090105.png

RL turbofan ISP: 6,000

KSP turbofan/jet ISP 2,500*16 = 40,000

40/6 = 6.6667... I don't see how that is comparable.

Sure, if you're hovering at 0.3 ATMs, its onl 3.33x the ISP... but that isn't a very good comparison.

And of course, given how fast the turbojets go, we should be comparing them with the arterburning turbofans or ramjets... and comparing the basic jet and the CF6...

the basic jet, at a standstill at sea level, gets 40,000 ISP, and still gets 8,000 ISP at 850 m/s.

[Wanderfound]

Which is fine. If they model jet engines accurately then there's no space planes. Just like IRL.

The guys over at the AJE thread (http://forum.kerbalspaceprogram.com/threads/70008-0-25-Advanced-Jet-Engine-(AJE)-v1-6-2-Oct-20) might have an opinion on that...

[Laie]

RL turbofan ISP: 6,000

KSP turbofan/jet ISP 2,500*16 = 40,000

Don't be fooled by the turbo. KSP "Turbo"Jets actually are modeled after ramjets/scramjets. And although even the nominal ISP is a tad high, the x16 multiplier can't really be held against them: that's a programming error.

Edited October 29, 2014 by Laie

[Wanderfound]

Don't be fooled by the turbo. KSP "Turbo"Jets actually are modeled after ramjets/scramjets. And although even the nominal ISP is a tad high, the x16 multiplier can't really be held against them: that's a programming error.

I'm probably wishing for fairies here, but what I'd actually like (apart from correcting the error and sorting the double-counted air mass thing) is to split the "turbojet" into two engines (ram and scram, both with zero thrust below the speeds at which such things work) and push the basic jet into something that has some use in getting up to speed.

At the moment, once you understand how to construct an airframe, it's a bit too easy to get to orbit (especially in FAR/NEAR realistic aero). Adding a bit of engine variety wouldn't hurt, and the RAPIERs would still be there for folks who didn't want to bother with such things.

[numerobis]

Wanderfound: you could do that pretty easily in your own build, and test it, and report back. Just change the .cfg files for the basic jet and turbojet.

For instance, you could make the basic jet velocity curve have thrust slowly fall until about 340 m/s, when it dives to zero (rather than having hypersonic turbofans). And you could make the turbojet velocity curve start have thrust start at zero at a standstill, start to light at 50 m/s, and only start being effective at high speeds -- and while you're at it, rename it to be a ramjet.

You can also boost the mass (or reduce the thrust) to get more reasonable TWR.

Finally, you play with the atmosphere curve to deal with the Isp.

[juanml82]

The guys over at the AJE thread (http://forum.kerbalspaceprogram.com/threads/70008-0-25-Advanced-Jet-Engine-(AJE)-v1-6-2-Oct-20) might have an opinion on that...

Well, he's talking about real life, so the guys who should have an opinion on that are the engineers working in all the HOTOL ideas for spacecraft, none of which worked so far - and the only one which might work is the Skylon, which is based around an engine represented in the game by the rapier, not turbojets

I'm probably wishing for fairies here, but what I'd actually like (apart from correcting the error and sorting the double-counted air mass thing) is to split the "turbojet" into two engines (ram and scram, both with zero thrust below the speeds at which such things work) and push the basic jet into something that has some use in getting up to speed.

At the moment, once you understand how to construct an airframe, it's a bit too easy to get to orbit (especially in FAR/NEAR realistic aero). Adding a bit of engine variety wouldn't hurt, and the RAPIERs would still be there for folks who didn't want to bother with such things.

That would be an SSTO which uses three different types of engines, two of which are dead weight at any given time... "realistically" a spaceplane using a ramjet/scramjet as an air breathing engine would be either air launched or would use rocket assisted take off. IE, strap some SRBs at the back of the engines, fire those first and once they've emptied their fueled, decouple and turn the ramjets on. And there is no need to take off horizontally in that case.

This whole "turbojets aren't realistic" discussion... well, no, they aren't. But the alternative isn't nerfing them or using a work around that allows the construction of a HOTOL spacecraft which uses air breathing only engines. The realistic alternative is to only use Rapiers for HOTOL spacecraft. Anything other than that is complaining, or looking alternatives, to a non realistic engine in order to achieve a non realistic result anyways.

[Wanderfound]

Well, he's talking about real life, so the guys who should have an opinion on that are the engineers working in all the HOTOL ideas for spacecraft, none of which worked so far - and the only one which might work is the Skylon, which is based around an engine represented in the game by the rapier, not turbojets

The claim isn't that a non-SABRE HOTOL spaceplane would work on Earth, it's that an accurately modelled jet engine allows spaceplanes on Kerbin. If AJE meets your standards of "accurately modelled" (and it does for me), then it demonstrably is possible get to orbit on Kerbin in a RAPIER-free spaceplane.

RSS, on the other hand...

That would be an SSTO which uses three different types of engines, two of which are dead weight at any given time... "realistically" a spaceplane using a ramjet/scramjet as an air breathing engine would be either air launched or would use rocket assisted take off. IE, strap some SRBs at the back of the engines, fire those first and once they've emptied their fueled, decouple and turn the ramjets on. And there is no need to take off horizontally in that case.

Yup, that's the whole point. It would motivate more diversity in spaceplane design and allow builders to adjust their level of challenge to taste by deciding on whether or not to use RAPIERs. It would make mothership/parasite and shuttle-style designs more valuable, while still maintaining the Skylon option. And the impetus for horizontal takeoff is still there for the RAPIER and ram/scram designs, as HOTOL permits much lower TWR at launch.

Spaceplanes already carry sometimes redundant gear; it's their defining feature. This is no different from carrying control surfaces and existing turbojets into orbit.

The rocket crew have over a dozen engines to choose from. Why should jets be limited to three, one of which is nearly useless?

Edited October 29, 2014 by Wanderfound

[Wanderfound]

Wanderfound: you could do that pretty easily in your own build, and test it, and report back. Just change the .cfg files for the basic jet and turbojet.

For instance, you could make the basic jet velocity curve have thrust slowly fall until about 340 m/s, when it dives to zero (rather than having hypersonic turbofans). And you could make the turbojet velocity curve start have thrust start at zero at a standstill, start to light at 50 m/s, and only start being effective at high speeds -- and while you're at it, rename it to be a ramjet.

You can also boost the mass (or reduce the thrust) to get more reasonable TWR.

Finally, you play with the atmosphere curve to deal with the Isp.

Anyone know how to do this in a way that would play nicely with FAR's existing tweaks to thrust curves? I could build a demonstration ship.

[juanml82]

Yup, that's the whole point. It would motivate more diversity in spaceplane design and allow builders to adjust their level of challenge to taste by deciding on whether or not to use RAPIERs. It would make mothership/parasite and shuttle-style designs more valuable, while still maintaining the Skylon option. And the impetus for horizontal takeoff is still there for the RAPIER and ram/scram designs, as HOTOL permits much lower TWR at launch.

Spaceplanes already carry sometimes redundant gear; it's their defining feature. This is no different from carrying control surfaces and existing turbojets into orbit.

The rocket crew have over a dozen engines to choose from. Why should jets be limited to three, one of which is nearly useless?

Which is why even Skylon is planned to have short wings, and every functioning spaceship with wings has relatively short wings (at least compared to some KSP contraptions) which are only used during landing and never during take off nor ascend to orbit.

What I think is that the turbojet shouldn't be criticized based on a realistic approach because HOTOL without Rapiers aren't realistic, no matter what. It could be very well criticized based on a gameplay approach - for instance, diversity for people who want to design HOTOL, as you say. But the usual critic isn't that, it's "Oh, real world turbojets don't work at Match 6"

[tsotha]

The guys over at the AJE thread (http://forum.kerbalspaceprogram.com/threads/70008-0-25-Advanced-Jet-Engine-(AJE)-v1-6-2-Oct-20) might have an opinion on that...

Perhaps, but since I'm not going to read 62 pages looking for it, maybe you can be more specific.

[Wanderfound]

Perhaps, but since I'm not going to read 62 pages looking for it, maybe you can be more specific.

The original claim was that if you model jets accurately you can't build spaceplanes. AJE is an attempt to model jets accurately; its users fly spaceplanes.

The reduced size of Kerbin has a large effect on the ease with which you can construct an effective spaceplane. SABREs or staging may be necessary on Earth, but they aren't on Kerbin. Even with realistic jets.

[Hodo]

The original claim was that if you model jets accurately you can't build spaceplanes. AJE is an attempt to model jets accurately; its users fly spaceplanes.

The reduced size of Kerbin has a large effect on the ease with which you can construct an effective spaceplane. SABREs or staging may be necessary on Earth, but they aren't on Kerbin. Even with realistic jets.

I have an alternate RSS install and I use AJE in that install. I can say that the dozen or so air breathing jet engines none of which really help in creating a functional SSTO that uses jets on a real scale Earth. I have achieved SSTO spaceplane status and got above the Karmin line of 100km. But it wasn't able to achieve orbit in "space" of 185km. It was 175m/s short......

I however made a nearly functional SSTO spaceplane using the Linear Aerospike rocket engines. I modeled it on a real life concept. It weighed in at over 375 tons sitting on the runway and it had to cut engines as it lost weight. But that one was 100m/s shy of orbital velocity at 185km altitude.

I love AJE and it does make massive improvements to the jet engines, in a realistic fashion. The only thing is I wont use it for my stock sized kerbin install. The main reason is Kerbin is just over 1/3rd the size of Earth, and even at that scale you can still achieve orbit with realistic jet engines.

Fuel consumption is the biggest difference in the two. A real jet engine will blow through some fuel when using afterburner.