Why use the arospike

[KerikBalm]

48-7s clusters have a very low profile as well.... and if one or two hits and is destroyed... well, if its a cluster with many 48-7s, you may still be able to ascend.

And also with stock aero, its easy to mak pancake rockets with low profiles.

I don't find Eve to be any more lumpy than Duna or the Mun, and I land on Duna and the Mun with LV-Ns... so...

Sure, by the time I need rocketry, I'm at 35,000m and Mach 5.5 and a wet sneeze can get me to orbit. But my spaceplanes have enough slack in their performance envelope that I don't need to sacrifice everything to efficiency.

...

but they also function well as atmospheric afterburners when I'm playing around. 4g thrust at treetop height is way fun, and there's something to be said for orbital burns that don't take ten minutes to do.

The 48-7's give better efficiency, but the 'spikes give better performance.

A LV-T30 can also give you that lower atmosphere afterburner effect... for that matter, so can a 48-7s cluster.

Sure, they burn a bit more fuel, but you can get a lot more thrust, because their TWR is much higher, and you were just talking about performance, not efficiency.... so...

They are an engine with almost the same TWR as LV-909s and poodles, and the same vacuum ISP as them.... yawn.

[EdFred]

I don't find Eve to be any more lumpy than Duna or the Mun, and I land on Duna and the Mun with LV-Ns... so...

Let me know how those LV-Ns work on Eve's surface. Also a 4x cluster of 48-7S engines do not provide the TWR a Aerospike does.

Single lander can +X200-8 + aerospike = Eve TWR of 1.58 and dV of 3493 atmo

Single lander can +X200-8 + TVR-400L + 4x48-7S = Eve TWR of 1.25 and dV of 3489 (I don't 'cheat' and use COS)

Single lander can +X200-8 + LV-T30 = Eve TWR of 2.01 and a dV of 3071

Single lander can +X200-8 + LV-T45 = Eve TWR of 1.80 and a dV of 2881

Single lander can +X200-8 + LV-N = Eve TWR of 0.49 and a dV of 1675

So far the aerospike wins.

Edited October 23, 2014 by EdFred

[Payload]

I use them as lower atmo lift helpers where I may not have enough thrust form the main engine because it is purposefully undersized. I jettison them when my TMR reaches an excessive amount. Engines are incredibly heavy in this game. Dropping them off asparagus style when you don't need them anymore is useful. Doesn't hurt that they have great efficiency in dense atmosphere. They are the most efficient way to do asparagus according to my testing. May not be the cheapest though. Solid boosters are probably the most cost effective.

[Alshain]

It's my "go to" engine for my mid to large plane designs, that or the RAPIER but the RAPIER is not as good as the turbo jets so I prefer separate rockets.

Edited October 23, 2014 by Alshain

[cantab]

I must admit I've never used it much but it does have advantages. It offers the most TWR of the efficient chemical rockets aisde from the much bigger KR-2L, and if we didn't have the LV-N I reckon the aerospike would get used a lot more. It's got the most thrust of any "low profile" engine, though the usefulness of that is partly undone by the inability to put a decoupler under it. Still, you could put an aerospike lander upside-down right on top of the launcher.

[KerikBalm]

Let me know how those LV-Ns work on Eve's surface.

They don't obviously.

I was simply using them as an example of not-low profile engines being usable on a lander. Of course, the engine has to make sense. The LV-N at1atm or greater is a 220 ISP engine with a 2.7 TWR - Terribad.

Also a 4x cluster of 48-7S engines do not provide the TWR a Aerospike does.

That depends entirely on the rest of the craft. A single 48-7s has a TWR of 30, while a single aerospike has a TWR of 14.9.

(I don't 'cheat' and use COS)

Well there are other options, taking advantage of Eve stock aero... just use a cluster of smaller tanks, more attachment nodes, same weight.

It also allows for more asparagus staging.

Its simply a gameplay limitation that we can't attach more than one thing to the bottom, and there is no part adaptor for tiny engines, so you're forced to use small ones.

Therefore I don't consider cubic struts to be cheating (only when used to build massless structural elements and such, not when used to simply add an attachment node. FWIW, I also modded my game so that many adaptors function as 9:1 mass ratio fuel tanks. IRL engine clusters are more mass efficient than single engines)

[The_Rocketeer]

I've used it once or twice but it's only useful for really specific tasks. I far prefer the RAPIER for spaceplanes, and I'm always frustrated by how thirsty the aerospike is.

[Alshain]

I've used it once or twice but it's only useful for really specific tasks. I far prefer the RAPIER for spaceplanes, and I'm always frustrated by how thirsty the aerospike is.

You obviously haven't used it enough, the aerospike is not as thirsty as the rapier. It has a 390 Isp, vs the RAPIER's 360. When paired with the Turbojet it has 2500 Isp vs 800 in the atmosphere.

Edited October 23, 2014 by Alshain

[Wavechaser]

I have used it for just aesthetic purposes before it was the last stage so the transit vehicle had to be upside down but still.

It's indeed useful for Eve ascend purposes (get out from the bottom of Jool's atmosphere isn't really useful and is extremely hard), and small SSTOs. But after the introduce of the RAPIER, many people gave up on it. Still, I personally like to use it as the rocket on SSTOs instead of RAPIER or sometimes even SABRE (B9).

And also useful for vertical cargo SSTOs.

One thing that's bad about it is it doesn't have gimbal device.

[LethalDose]

In addition to Eve vessels, the aerospike is also useful for "Jool diving": Getting low into Jool's atmosphere then coming back up.

[Jouni]

The easiest way of building a sea-level Eve lander is a central stack of 2-3 stages and 9 pairs of asparagus boosters. If the payload is a Mk1 command pod or lander can, the Aerospike just happens to be the perfect engine for the boosters. Of course you could replace each Aerospike with 5-6 48-7S engines, but my computer can't handle a lander with 100 engines and their attachment points.

[KerikBalm]

In addition to Eve vessels, the aerospike is also useful for "Jool diving": Getting low into Jool's atmosphere then coming back up.

Hmm, Jool diving may indeed be a good use, its atmosphere is even thicker, but it has less than half the "surface gravity" of Eve- the TWR issue isn't so bad.

You obviously haven't used it enough, the aerospike is not as thirsty as the rapier. It has a 390 Isp, vs the RAPIER's 360. When paired with the Turbojet it has 2500 Isp vs 800 in the atmosphere.

Huh, rapiers and turbojets have the same airbreathing ISP

This is the curve for both of them:

atmosphereCurve

{

key = 0 1200

key = 0.3 2500

key = 1 800

}

[Rainbowtrout]

I want to know why people use the aro spike, personly I never use it but idont get why others use its just a rocket engine.

So's a mainsail.

Now that pedantic me has been forced back into it's cage, I use it because it looks nice

[Laie]

Of course you could replace each Aerospike with 5-6 48-7S engines, but my computer can't handle a lander with 100 engines and their attachment points.

In that case, you may be interested in that quadruple Kerbodyne thingy. It can serve as a drop-in replacement for 100 48-7S -- actually, it has better stats, so it might still be worthwhile even if you'd need only 70-80 of the smaller engines. You can't stage it in pieces, though.

[LethalDose]

Hmm, Jool diving may indeed be a good use, its atmosphere is even thicker, but it has less than half the "surface gravity" of Eve- the TWR issue isn't so bad.

TWR should also less of an issue because when "Jool diving" you should never be thrusting straight up. You want to maintain horizontal speed as best you can, just like you're flying an SSTO on Kerbin, and then kick up out of orbit. But I have to admit, none of my Jool dive missions have made it beyond the planning stage.

But really, powered flight around Eve and Jool are only uses for which the aerospike is the best engine choice.

I would concede that they're reasonable for use on spaceplanes around Kerbin and Laythe, but there are RAPIERs and other Air breathing/LFO combos that provide better ISP and TWRs. Finally, Duna's atmosphere is so thin that many LFO engines are still very efficient, even at sea level, and have better TWRs.

[Jouni]

In that case, you may be interested in that quadruple Kerbodyne thingy. It can serve as a drop-in replacement for 100 48-7S -- actually, it has better stats, so it might still be worthwhile even if you'd need only 70-80 of the smaller engines. You can't stage it in pieces, though.

I used it as the lower stage engine in my 7-kerbal Eve lander. Even there, it was too big for the boosters.

[Wanderfound]

A LV-T30 can also give you that lower atmosphere afterburner effect... for that matter, so can a 48-7s cluster.

Or a KR-2L:

Sure, they burn a bit more fuel, but you can get a lot more thrust, because their TWR is much higher, and you were just talking about performance, not efficiency.... so...

While efficiency isn't the prime consideration, it isn't totally irrelevant either. With non-aerospike rockets, I have to lug around big tanks if I want to run them more than a few seconds in atmosphere. With the 'spikes, I have the option of sustained atmospheric burns while still flying an ultralight ship:

Each to their own, if you don't like 'em, don't use 'em. But there are valid reasons why other folks might.

[kujuman]

One of the problems with the aerospike is that none of the stock engines' ISP gets worse as pressure goes above 1 ATM (nor does the flow become unstable). So most engines on Eve should have even lower ISPs than they do since their nozzles are not sized correctly; the aerospike would be absolutely unmatched in this regime.

[Dispatcher]

... For those that said spaceplanes:

By the time you light your rockets in Space planes, you are so high up, it might as well be vacuum ISP ...

Notice that the spaceplane I mentioned is "rocket engines only"; it doesn't have any jet or rapier engines. It launches and lands using the runway. I had experimented with a variety of engines (and combinations) before settling on the aerospikes.

[Northstar1989]

You obviously haven't used it enough, the aerospike is not as thirsty as the rapier. It has a 390 Isp, vs the RAPIER's 360. When paired with the Turbojet it has 2500 Isp vs 800 in the atmosphere.

The Turbojet does NOT perform at 2500 ISP in the upper atmosphere (it's ISP approaches 1200s in the upper atmosphere)

The ISP value you're listing is at optimal atmospheric pressure (30% pressure, just under 1 scale-height, or around 4-5 km above sea-level in the stock Kerbin system) and speed (1000 m/s), at HIGHER altitudes and speeds its ISP *plummets*.

If you actually look at the velocity-curve of the RAPIER in atmospheric mode, it is a LOT better than the Turbojet[s/] (this has changes since 0.23.5, when I last checked...) At anything above or below the "optimal" velocity of 1000 m/s, you suffer from a turbojet's velocity-curve. Velocity-curve is a "stealth" destroyer of ISP, since it causes the engine to produce less thrust for the same fuel flow, but doesn't actually show up as reduced ISP when right-clicking on the engine.

The RAPIER also has a better ISP in the upper atmosphere (anywhere above about 24 or 25 km, if my memory serves me correctly).

The RAPIER almost always beats having separate jets and rockets, because it can act as BOTH. That is, if you think of it as getting 85% of a turbojet and 50% of a LV-T30 for the mass (the percentages due to the slightly lower performance for both- though the RAPIER in atmo-mode clearly beats the turbojet above 25 km and 1200 m/s), then it always makes sense to rely on RAPIER engines for the best mass-efficiency.

The most mass-efficient small to mid-sized spaceplanes basically only have 1 jet engines anyways, and then rely on rockets for the rest of their thrust to make it to orbit- that's because near altitude-ceiling (you want to build up most of your velocity a bit below it for the lower AoA, but not too much lower), you will basically only have enough IntakeAtm to run one engine at full thrust (more intakes raises this altitude ceiling, but does not eliminate it unless you MASSIVELY abuse intake-spam...) And if you only have 1 jet, you want it to be the type that performs best at really high altitudes, which is the RAPIER (just dial down the thrust when ascending through the lower atmosphere or you'll waste a lot of fuel!)

But if you want to have multiple jets, for better TWR in the lower atmosphere, then having multiple RAPIER engines in a *HUGE* advantage over multiple turbojets, because they will also be useful as rockets when you switch over to rocket-propulsion... (it may also be necessary to have multiple jets to climb high enough to where you can only run one jet engine near full thrust anyways...)

Regards,

Northstar

Edited October 24, 2014 by Northstar1989

[Northstar1989]

OK, so did a little checking of the current (0.25) engine configs. Here are the atmosphere and velocity-curves for RAPIER. I've added in notes on what each term means...

ATMO:

key = 0 1200 (ISP at vacuum-pressure)

key = 0.3 2500 (ISP at 30% sea-level pressure)

key = 1 800 (ISP at sea-level pressure)

VELOCITY:

key = 0 0.5 0 0 (% maximum thrust at standstill- 50% here)

key = 1000 1 0 0 (this engine reaches maximum thrust at 1000 m/s?)

key = 2000 0.5 0 0 (and 50% thrust at 2000 m/s)

key = 2200 0 0 0 (and stops producing ANY thrust at 2200 m/s)

And for the Turbojet...

ATMO: (the atmosphere-curves for the RAPIER and Turbojet are now IDENTICAL in 0.25- so they have *EXACTLY* the same ISP based solely on altitude...)

key = 0 1200

key = 0.3 2500

key = 1 800

VELOCITY: (however the velocity curve is now *slightly* more generous than the RAPIER above 2000 m/s- so I guess it got changed since 0.23.5...)

key = 0 0.5 0 0

key = 1000 1 0 0

key = 2000 0.5 0 0

key = 2400 0 0 0

Basically, you can ignore what I said about the RAPIER performing better than the Turbojet at high-altitude and speed, because that's no longer the case (EDIT: I went and struck out those phrases, just to make it clear). However, the differences in performance are *minor*, and below 2000 m/s (who would ever fly at 2000 m/s in the stock Kerbin system? I can only foresee see this adversely affecting extreme air-hogging or spaceplanes with both jets and rockets firing simultaneously...), the performances are in every way identical. Which means, performance-wise, the RAPIER is basically a turbojet below 2000 m/s, by which point you probably should have switched to rocket-mode anyways (because at that point in a spaceplane ascent you probably don't have the IntakeAir to run more than a single jet engine at more than 20% thrust- and you're losing over half of that thrust to the velocity-curve with either type of jet engine...)

And obviously, once you're out of the atmosphere (which is your goal with a spaceplane- right?), a turbojet engine is completely worthless, whereas a RAPIER is not.

Regards,

Northstar

P.S. If you refuel a RAPIER-equipped spaceplane (say with multiple spaceplane flights, or ISRU), it makes a decent plane for other planets as well, where a standard turbojet would just be deadweight- places like Eve and Duna (yes, it is possible to fly on Duna- go look up the Flying Duna challenge, and submit an entry if you're daring enough...

P.P.S. Sorry for the tangent of RAPIER engines. It is one of the main competitors to the aerospike though. Aerospikes also make for decent Eve exploration planes- although on Duna you're probably better off with one of the tiny Rockomax engines, as you don't need much thrust to stay airborne in such thin atmosphere and low gravity...

Edited October 24, 2014 by Northstar1989

[Red Iron Crown]

I've found that the extra 200m/s of usable speed with the turbojet to be critical for the ascents my spaceplanes fly as it greatly reduces the magnitude of the circularization burn. The rocket engine in the Rapier is almost useless to me: For surface-to-LKO shuttles a pair of tiny rocket engines are enough for circularization and orbital maneuvers; for spaceplanes going further I want something more efficient than the Rapier like an LV-909, aerospike, or LV-N.

As for other uses of the aerospike, I agree with the others above. Good for Eve ascent, or situations where you need a short engine in its thrust class. It's also a good orbital engine if you don't use nukes (whether for tech tree or roleplay reasons).

[KerikBalm]

I too use it on my spaceplanes, even without my personal buff of the thrust to equal the LV-T45 thrust... but I don't use it for its "aerospike" properties.

I use it simply because I want 390 vacuum ISP, and a LV-909 is too weak, and a poodle's dimensions are impractical.

(also, RP reasons, or I'd design the craft to use a nuke engine, but I don't want my orbital shuttles that routinely go to and from destinations in kerbin's SOI and back to the surface of kerbin to be running engines that would release a lot of radioactive material if there is an accident)

I wouldn't call it 200 m/s of usable speed.

A turbojet may produce thrust up to 2,400 m/s, but you won't reach 2,400 surface velocity.

That is over 2,500 orbital velocity... you won't stay in the atmosphere long enough to use it up.

A rapier's 2,200 max (though you won't reach the max due to drag and very weak thrust) won't get you out of the atmosphere much

I think it actually comes out to 100 m/s or less of "useful" velocity.

At any rate, I use NEAR, and they cap out at 1,700 m/s (and most of my designs reach only 1,600m/s or less, due to drag and 0.01 kN of force at 1,699 m/s not being sufficient to maintain velocity), so my designs need more oomph from the rockets.

I basically use it as if it were a 1.25m poodle.

For that matter, the constant atmospheric ISP isn't very realistic.

You can't make atmospheric ISP equal vacuum ISP for a pure rocket.

Suppose you have something like the poodle optimized for vacuum:

270 to 390 ISP

(or even more so, the LV-N, 220 to 800)

You can redesign the nozzle to give you a better atmospheric ISP at the cost of a lower vacuum ISP, like the ks 25x4/mainsail/Rapier

320 to 360 ISP

Then you could do an aerospike, and get both the low end ISP, and the high end ISP

320-390

But it won't get you a better ISP at 1 atm than a rocket nozzle designed for use at 1 atm, which won't get as good of an ISP as a rocket nozzle designed for use at 0 atm.

IRL, we'd have isp ranges something more like

1) 330-350

2) 300-370

3) 220-420

Where in a staged rocket, you'd use nozzle design 1 on the lower stage, 2) on the middle, and 3 on the upper

But with an aerospike, you'd get something like

330-420

Edited October 24, 2014 by KerikBalm

[UmbralRaptor]

The Turbojet does NOT perform at 2500 ISP in the upper atmosphere (it's ISP approaches 1200s in the upper atmosphere)

Not exactly. Air being "free" multiplies their effective Isp by 16x. Sure, the thrust loss at the high end of the velocity curve would suggest a 600 s Isp at 2000 m/s, but it's more like 9600 s. I think this also means that as long as you're getting more than ~2% thrust out of the velocity curve, you should be running jets. (Maybe. At point point do drag losses matter?)

I haven't done enough with RAPIERs to fully judge their performance, but can you get into a stable orbit on ~30 m/s in rocket mode? (turbojets can get you into that situation, and even bring usefully large payloads)

[KerikBalm]

I don't think the thrust loss results in a lower ISP. I thought it was just a thrust scaler, and then ISP considers how much fuel is needed to produce that thrust.

Can anyone confirm?

And yes, the extreme OP'dness of the jets in game is well known. They should either be given an ISP of ~150, and use air:fuel in a 16:1 ratio, or have intake air be massless (like electric charge), and keep the rest as is.