Engines on Eve

[fourfa]

I've been doing some research to make smaller ascent vehicles for Eve. The 3X Mammoth monster I built in 1.0.4 is no longer necessary, a 7X Vector ship can lift a pretty substantial payload. In the course of that, looking at the two engines most suited to Eve blast-off, I found some interesting facts:

[img]http://i.imgur.com/SPbgLvV.jpg[/img]
[URL]http://imgur.com/SPbgLvV[/URL]

In the meat of the ground elevation range (1.5-6km covers most of Eve's surface) the Aerospike and Vector/Mammoth have almost the same specific impulse. In fact the Vector/Mammoth is BETTER in the 6~12km range, which is a pretty critical time! That's very different than the impression you'd have just glancing at the right-click info in the VAB.

(All data just gathered from KER in the VAB, not experimentally, I assume that's still accurate...?)

Just thought some people might be curious. Next I'd like to add the Mainsail and Terrier to the graph, as the Mainsail isn't completely awful at low Eve levels, and the Terrier gets used in some final stages in the outer atmosphere, might be interesting to know how bad it is in the middle atmo.

[A_name]

Sadly I can't see your pic due to being at work, but I wanted to say that the Vector is definitely a game changer in terms of Eve ascent.

[RocketPropelledGiraffe]

Don't forget the Vector weighs 4 tons compared to the Aerospike's 1 t.
It has 6 times the thrust at 1 bar pressure, which is roughly at 10 km altitude. Do we have thrust data for the higher pressure regions in the lower atmosphere?

It would seem the Aerospike wins for low altitudes, higher up it is a matter of how much thrust you actually need. If you have dropped most of your stages and don't need all the thrust of the Vector the Aerospike might still come out on top?

[Fearless Son]

If RPG is correct, then it sounds like it would be a good idea to have Aerospike LFBs in the lower atmosphere that you can detach after getting some kilometers up, and use a Vector as your main thrust engine once that gives out.

[RocketPropelledGiraffe]

I don't have any data, at the moment I am just extrapolating and assuming - since the Aerospike's performance is very constant with pressure changes compared to the other engines it sounds reasonable.
We would need more data to do more than just educated guessing - fourfa's ISP chart is one thing, overall thrust and TWR comparisons would also be needed.

I would guess that at low altitudes the Aerospike is superiour, in the middle altitudes the Vector wins, and in the upper atmosphere maybe the Aerospike's high ISP could be better again, because you no longer need all the Vector's thrust.
In a realistic test scenario, you will have to find a middle ground because you obviously have to carry all the engines with you all of the time - Do you really want to stage and drop your Aerospikes in the low atmosphere if you could benefit from their high ISP later?
Like I said, it's all just guesswork at the moment...

[fourfa]

When I have a bit of time I'll update this with more engines, thrust, and TWR comparisons.

[quote name='RocketPropelledGiraffe']since the Aerospike's performance is very constant with pressure changes compared to the other engines[/QUOTE]

I get that this is the objective of aerospike engines in real life, but by this chart it looks to me to have a pretty similar curve to the Vector. Will be curious to compare to the rest of the field.

[Teilnehmer]

An Aerospike rocket should have several times grater cross section area than a Vector one with the same thrust.
Wouldn’t the drag nullify all the Aerospikes’ advantage at low altitudes?

[KerikBalm]

I think it probably does... I really think the aerospike needs a thrust buff (and a corresponding mass buff)... an aerospike engine is best used on a stage that will be at low altitude, and continue to burn to high altitudes... or when going to really high pressures where you have no de laval nozzle optimized for that atmospheric pressure (like below 2km on Eve)... but the point is... it burns at low altitudes... its a first stage engine... ie it is burning as a/with the boosters... its thrust is rather uninspiring for a first stage engine.
Its TWR seems fine, but is "thrust to node" ratio is not good and doesnt belongs with first stage engines

[RocketPropelledGiraffe]

Well, the Aerospike is more a single stage engine or spaceplane engine, not really for first stage ascending. The only reason we talk about it for staged rockets is because of Eve's thick atmosphere....

I usually use it for spaceplanes and single stage Laythe landers, it is ideal for those purposes. Eve ascend is more like a niche where it can possibly be useful, but certainly not the main use...

[KerikBalm]

Well, I think the aerospike is also ideal for core stage(s) of an asparagus design.. but that is like a single stage design, in that the aerospike burns all the way to orbit or at least a (near) vacuum.
But the point is that it burns with the first stage.
In a single stage design, its not so great because of poor thrust... so it can't lift much fuel. Its not the favored engined for SSTO rockets.

Jool's atmosphere would be even more of a niche... if someone wanted to "Jool limbo", and try and drop deep into Jool's atmosphere before going back to orbit, the aerospike would get more and more attactive relative to the mammoth as the ship descends deeper.
The vector is basically optimized for 1 atmosphere (i'm not 100% sure on this... maybe its 0.8atms or 0.7... I don't know exactly). As pressure gets lower or higher, the aerospike starts to have better Isp.

If you were to descend down to 20km above the "surface" of jool, where its nearly 10 atmospheres.... the aerospike would be way better than the vector....
Although I have yet to see anyone return from Jool after going that far down... you only need to go down to 120km to get the "flying low" science.

[fourfa]

On 11/26/2015, 9:02:31, RocketPropelledGiraffe said:

Eve ascend is more like a niche where it can possibly be useful, but certainly not the main use...

Well, that is the purpose of this thread...

Charts updated for all the relevant liquid-fuel engines, and some irrelevant ones for comparison.

I think it's the TWR chart that's the most telling  The Aerospike does indeed suffer less change from vacuum to low elevation and has perhaps the best overall Isp, but its TWR is poor compared to the Vector, Mammoth, and Mainsail.  I wish there was a single metric that encompassed both Isp and TWR, that would represent how much fuel each engine would need to (for instance) reach orbit.  I suppose it's just the rocket delta-V equation?

Edited November 30, 2015 by fourfa

[OhioBob]

19 minutes ago, fourfa said:

I wish there was a single metric that encompassed both Isp and TWR, that would represent how much fuel each engine would need to (for instance) reach orbit.

I tried to do something like that a few months ago.  What I did was to take each engine and add to it a nose cone and fuel tanks until it's Eve sea level TWR was 1.4.  I then computed how much dV each configuration could produce.  It's just a hypothetical experiment, but I figured it gave me some idea of the combined effect of ISP and TWR.  Adding the Vector to my old results, I find that the best engines in order of performance are: (1) Mammoth, (2) Vector, (3) Aerospike, (4) Twin-Boar, and (5) Mainsail.  The drop off in performance after these top 5 was so large that I wouldn't even consider any other engines.

[fourfa]

Which reminds me, I missed the Twin Boar in my charts.  Tried to update the above post with the new chart, didn't work - let's see if it works here.  

Interesting.  The Twin-Boar, for the specific case in which you require exactly its one orange tank full of fuel, has terrific TWR compared to everything else, Isp is not far off, and total thrust is quite high.  (For consistent comparison, I calculated it at 6.5t - its 42.5t minus the 36t of an orange tank). It could be an excellent core stage, though it's a lot of dead tank weight to take halfway to orbit, and I've found very tall rockets to be hard to land on Eve's sloped surface without damage.

One last note - I've seen a lot of designs use a Terrier as the final sustainer/circularization stage; be sure it doesn't have to kick in below 30km where it's still very weak.

Edited November 30, 2015 by fourfa

[FlyingPete]

The comparison between the aerospike and vector is interesting. There's a balance somewhere between higher ISp which reduces your fuel for a given delta-V, and a lower TWR which means more delta-V wasted fighting gravity.

[RocketPropelledGiraffe]

The sea level ISP of the vector is acually way better than most other engines, and it has good thrust. I guess you want the Vector's thrust taking you into the upper atmosphere and then have the superiour ISP of the Aerospike get you into orbit. The 10% better ISP high up is actually worth more than an ISP difference at sea level, because you don't need to take as much fuel to the high altitudes.

It seems the efficient way of action would be to get a 1 t centered Aerospike that burns all the way to orbit and combine it with Vector driven liquid fuel boosters? No need to carry all those heavy Vectors all the way up, and once you dropped all your boosters the craft will be light enough to manage with the Aerospike thrust.

Thanks a lot for the effort. I have not done any Eve missions in 1.0.5, but this really sparked some ideas for ascend vehicles.

 

Edit: Now how do I give you rep in the new forums? Argh, so hard getting used to this...

Edited November 30, 2015 by RocketPropelledGiraffe

[Starwaster]

On 11/30/2015 at 6:09 PM, RocketPropelledGiraffe said:

The sea level ISP of the vector is acually way better than most other engines, and it has good thrust. I guess you want the Vector's thrust taking you into the upper atmosphere and then have the superiour ISP of the Aerospike get you into orbit. The 10% better ISP high up is actually worth more than an ISP difference at sea level, because you don't need to take as much fuel to the high altitudes.

It seems the efficient way of action would be to get a 1 t centered Aerospike that burns all the way to orbit and combine it with Vector driven liquid fuel boosters? No need to carry all those heavy Vectors all the way up, and once you dropped all your boosters the craft will be light enough to manage with the Aerospike thrust.

Thanks a lot for the effort. I have not done any Eve missions in 1.0.5, but this really sparked some ideas for ascend vehicles.

 

Edit: Now how do I give you rep in the new forums? Argh, so hard getting used to this...

Like the post. It's not that much different from the old forum except that you only have to click once.

[Vegetal]

I just recently made some experiments with a vector core surrounded by 6 asparagus aerospikes, and a terrier final stage, and the lifter achieved orbit with some fuel to spare. It didn't have any payload, it was meant to be just a test anyway.

The objective was to see the plausibility of a small, relatively cheap Eve lander\ascender to use in career, and I managed 60 tons. With one kerbal it will be bigger, but not by much I guess.

I don't know if it would be more efficient the other way around, aerospike core with vector boosters, but that would be a lot more expensive.