Toroidal Aerospike Engine overhaul

[desert]

I was trying to use Dart engine for some time and didn't find any applications for it. Recently I was tinkering with my Eve ascent rocket and finally got an understanding that Dart engine is completely useless in its main application and Vector is almost the only way to go.

This is strange because the idea of aerospike is to work independently from ambient pressure. Turns out, Dart has ISP 290s-340s vs Vector's 295-315s. Its thrust is also more affected by an atmosphere: 85% of vacuum thrust, compared to Vector's 94%. So, the aerospike is affected by atmosphere more than a bell nozzle engine. Maybe I shouldn't compare it with Vector, but still, I've seen and made many Kerbin VTOL SSTOs and Eve rockets on Vectors and these cases should be applications for the aerospike. 340s ISP makes it a good thing for vacuum stage, but there are better variants. It is also unlocked pretty late in the game, so it is too small to be the first stage of late game big rockets.

In real life aerospikes were studied for J-2 engine. And they gave it better sea level performance, as well as increased vacuum ISP and thrust. Maybe in the game Dart should also be considered as a version of another engine, for example, Swivel, as they have the same diameter and close in thrust.  And as it is a late Kerbal technology, it may have better characteristics in general. Swivel has a mass of 1.5t, 168 - 215kN thrust, 250-320 ISP. Maybe Dart should have something like 1.5t, 220 - 230kN thrust, 310 -330 ISP. Of course, the downside of it should be greater cost, and maybe, overheating if a burn is too long.

It would be also cool to remodel it, add some plumbing which would resemble Swivel's and of course add linear variant, which would be cool to recreate X-33 by placing them in a row.

[hemeac]

@desert, the applications for the Dart would be in atmospheres that have higher pressure than Kerbin.  The Dart has an ISP of 230s in 5atm and 170s in 10atm while the SSME would have an ISP of around 26s in 10atm.  I haven't played with stock atmospheres for awhile, so I am not sure if that is relevant for stock Eve's atmosphere.

If you use mods, you should check out:

 

[RyanRising]

It's a bummer, the one place that would best use the Dart's efficiency, Eve, has that advantage mostly neutered by the requirements for TWR Eve demands and which the aerospike can't provide. You just have to use the SSME/Vector if you want to get anything significant off the ground there. I still use it on occasion for when I want a low-profile engine, but I have to admit its best feature is one that's largely irrelevant to the stock bodies. OPM's Tekto would probably love it, though.

[Bej Kerman]

4 hours ago, RyanRising said:

It's a bummer, the one place that would best use the Dart's efficiency, Eve, has that advantage mostly neutered by the requirements for TWR Eve demands and which the aerospike can't provide.

I don't think jet engines work and the Aerospike came way before propellers. If you bring wings, they should work.

[desert]

On 10/28/2020 at 1:35 PM, hemeac said:

the applications for the Dart would be in atmospheres that have higher pressure than Kerbin.

Well, it doesn't work great in this application. Eve has 5atm pressure. I've found good plots with engine comparison in different pressure: https://github.com/mueslo/KerbalPlot

Even at 5atm Mammoth (and hence Vector) has better ISP than Aerospike. Same with TWR. That's not mentioning Vector's insane area density of thrust, which allows to spam the whole bottom of tank with them to get enormous thrust. So either Vector needs to be nerfed (no one will like it) or Dart needs a bit more love and respect.

[jimmymcgoochie]

The Dart needs to have higher thrust to be able to compete as a launch-stage engine, and possibly better ISP in high-pressure atmospheres too as aerospikes are supposed to have. A 2.5m stock aerospike would be an interesting addition to the game, if it had enough thrust and a decent ISP curve to be a viable alternative to the Vector/Mammoth on Eve.

[king of nowhere]

On 10/29/2020 at 7:10 PM, desert said:

Well, it doesn't work great in this application. Eve has 5atm pressure. I've found good plots with engine comparison in different pressure: https://github.com/mueslo/KerbalPlot

Even at 5atm Mammoth (and hence Vector) has better ISP than Aerospike.

no, it has not. aerospike has by far the best Isp at eve sea level.

the problem is the low TWR. you need a crapton of darts to get off of eve, so the weight of the extra engines more than compensates for the fuel saving. not to mention the aerodinamic problems caused by so many engines.

personally, i like it for ssto planes, they can get by with a low twr and the dart works better in vacuum than other ssto engines.

[Pds314]

Dart is intended as Jack of all trades. Not a specialized sea level engine like Vector is. That being said, you are right that in a lot of situations the vector is the clear winner.

From a raw stat for stat comparison standpoint, first in a vacuum:

Dart:

Mass: 1000 kg

Vac Thrust: 180 kN

SL Thrust: 153.53 kN

Vac ISP: 340 s

SL ISP: 290 s

Cost: 3850 funds

Node: 1.25 m

Gimbal: 0°

Vac TWR: 18.35

SL TWR: 15.66

Vac Cost/Thrust: 21.39 funds/kN

SL Cost/Thrust: 25.08 funds/kN

 

Vector:

Mass: 4000 kg

Vac Thrust: 1000 kN

SL Thrust: 936.51 kN

Vac ISP: 315 s

1 atm ISP: 295 s

Cost: 18000 funds

Node: 1.25 m

Gimbal: 10.5°

Vac TWR: 25.49

SL TWR: 23.87

Vac Cost/Thrust: 18.00 funds/kN

SL Cost/Thrust: 19.22 funds/kN

 

Scenario stress tests:

Liftoff stage scenario: 800 m/s SL, 800 m/s Vac, 2.0 SL TWR:

 

Dart:

Payload fraction: 41.53%

Fuel fraction: 40.62%

Tankage fraction: 5.08%

Engine fraction: 12.77%

Cost/payload: 1.38 funds/kg

Burn time: 58.89 s

Burnout TWR: 3.95

 

Vector:

Payload fraction: 44.98%

Fuel fraction: 41.46%

Tankage fraction: 5.18%

Engine fraction: 8.38%

Cost/payload: 1.02 funds/kg

Burn time: 61.16 s

Burnout TWR: 3.65

 

As we expect, the Vector claps the dart at sea level.

 

Second stage scenario: 1600 m/s Vac, 1.0 Vac TWR.

 

Dart:

Payload fraction: 51.67%

Fuel fraction: 38.11%

Tankage fraction: 4.76%

Engine fraction: 5.45%

Cost/payload: 0.55 funds/kg

Burn time: 129.55 s

Burnout TWR: 1.62

 

Vector:

Payload fraction: 50.60%

Fuel fraction: 40.43%

Tankage fraction: 5.05%

Engine fraction: 3.92%

Cost/payload: 0.51 funds/kg

Burn time: 127.35

Burnout TWR: 1.68

 

Vector surprisingly ok at high altitude thanks mostly to crazy TWR. It even managed to get better cost per payload, albeit at a payload fraction disadvantage. It is worth pointing out, however, that the Dart is actually more cost effective than vector upper stage stop vector lower stage, although the difference is within a rounding error.

 

SSTO scenario: 800 m/s SL, 2400 m/s Vac, 2.0 SL TWR:

 

Dart:

Payload fraction: 16.07%

Fuel fraction: 63.25%

Tankage fraction: 7.91%

Engine fraction: 12.77%

Cost/payload: 3.85 funds/kg

Burn time: 91.69 s

Burnout TWR: 5.44

 

Vector:

Payload fraction: 18.35%

Fuel fraction: 65.13%

Tankage fraction: 8.14%

Engine fraction: 8.38%

Cost/payload: 2.76 funds/kg

Burn time: 96.07 s

Burnout TWR: 5.74.

 

Dart got completely clapped! So much for "SSTO engine." That's  pretty shocking result and it shows that a high TWR with the dart is pretty much inevitably not a good idea.

 

Single Stage Tylo Lander scenario: 2700 m/s any TWR, 2300 m/s Vac, 1.6 Vac TWR:

Dart:

Payload fraction: 8.73%

Fuel fraction: 77.68%

Tankage fraction: 9.71%

Engine fraction: 3.88%

Cost/payload: 3.49 funds/kg

Burn time: 370.87 s

Initial TWR: 0.72

Burnout TWR: 3.19

 

Vector:

Payload fraction: 7.18%

Fuel fraction: 80.18%

Tankage fraction: 10.02%

Engine fraction: 2.62%

Cost/payload: 3.88 funds/kg

Burn time: 378.35 s

Initial TWR: 0.67

Burnout TWR: 3.37

 

Dart easily wins. A low initial TWR 5 km/s burn is a pretty optimal scenario for it, but then again we should be comparing against dedicated vacuum engines. Note the dart finds it cheaper to do a single stage Tylo landing and return than to orbit SSTO with an initial TWR of 2.

 

Pretty much the dart loses cost for cost in every scenario with a high TWR and a low DV.

 

It's probably worth mentioning, however, that in almost any scenario where they start anywhere near the ground these are both very expensive engines.

As a comparison, the Swivel is pretty well known for being a bad engine, but let's compare it as an upper stage? It only has a 48.22% payload fraction which is not great, but it has a cost of 0.28. consequentially it's very slightly better than the dart as an upper stage for the vector, at least from a cost standpoint (though the reliant would still be better).

Edited November 20, 2020 by Pds314

[Pds314]

Interestingly, it looks like the dart actually is ok at competing with dedicated vacuum engines on payload fraction as long as TWR requirements are decently high. So things like Tylo landers and reuseable upper stages should probably prefer darts to poodles for example. The darts cost a lot more though.

[Laie]

On 10/27/2020 at 12:44 AM, desert said:

Maybe I shouldn't compare it with Vector,

Yeah, maybe you shouldn't. The Vector is essentially an ARM engine, those were and are quite OP compared to the other engines in the game.
Then again, if you can't compare two engines in the game to each other...?

As for it's upsides: for one thing, it looks good on planes.

The other: it's compact and has a high TWR for a vacuum engine. Three LV-909s worth of thrust at the mass of two, and the size of one. I find it quite useful on Tylo landers.

Edited November 21, 2020 by Laie

[king of nowhere]

8 hours ago, Laie said:

The other: it's compact and has a high TWR for a vacuum engine. Three LV-909s worth of thrust at the mass of two, and the size of one. I find it quite useful on Tylo landers.

yep. that's one advantage it has. as a vacuum engine, if a terrier doesn't have enough power, the dart is the superior option to two terriers, or one poodle. those few seconds of isp are totally compensated by the lower mass.

i'd say it's the best vacuum engine in the stock game for a mid-sized ship. and it's also effective for a small ssto. you know, when a vector would be way too big. I can totally see a laythe lander in the 5-10 tons range using a dart.

[Pds314]

On 11/21/2020 at 3:14 AM, Laie said:

Yeah, maybe you shouldn't. The Vector is essentially an ARM engine, those were and are quite OP compared to the other engines in the game.
Then again, if you can't compare two engines in the game to each other...?

As for it's upsides: for one thing, it looks good on planes.

The other: it's compact and has a high TWR for a vacuum engine. Three LV-909s worth of thrust at the mass of two, and the size of one. I find it quite useful on Tylo landers.

Yeah long burn Tylo lander was where my scenarios showed the Dart beating the Vector pretty convincingly. Even managing to beat it in cost at Tylo.

[desert]

Well, it seems it has an application, but unfortunately, not in atmosphere, as it supposed to be.