Why is my rocket so cost-inefficient?

[deepspacecreeper]

Hello everyone,

This is my 150t (max.170t, i will explain why) rocket launcher.

But I think this thing is so cost-inefficient compared to Temstar's Zenith Supernova. My rocket costs 500,000 vF while his costs 250,000 vF (0.24 or later). The only advantage my rocket has over his, is the part count reduced by half. I know that my rocket designs will never compare up to his, but I just want to learn why is my rocket so expensive.

My rocket uses 6 asparagus boosters and the core stage is a 2-stage design. The entire rocket uses 0.23.5 NASA parts. When it brings 150t to LKO, it hits the TWR bottleneck at 1.1 right after all asparagus boosters are jettisoned. When I add another 20t (total 170t), the TWR goes down to 1.03 and hits the delta-v bottleneck (just enough fuel for deorbiting the 2nd stage).

The download link brings you to a Dropbox folder which includes

My 150t.craft launcher file (made as a perfect subassembly. Thanks to Temstar again.)

The picture below

Temstar's Zenith Supernova subassembly (for conveniency, remember this is an unofficial download link for that craft. i own no rights to it.)

Could anyone tell me why my design is so expensive?

https://www.dropbox.com/sh/50e9xria8jo8s5j/AABfvADOV0v9h6AnJxBHG7Hia?dl=0

Edited November 4, 2014 by deepspacecreeper
[Answered!]

[GoSlash27]

DSC,

Not trying to come off as disinterested, but I'd rather not download both files.

Can you give an overview of both craft? Overview of engines and stage fuel volumes, etc?

-Slashy

[GoSlash27]

Nevermind.

I googled his craft and overview.

The reason his craft is cheaper is because he's using better engines.

Each stage has been optimized to employ the best engines in the game in their most efficient regimes, while your design uses sub-optimal engines and more fuel to get a cleaner overall vehicle and part count.

For all the liquid rocket engine choices available in this game, there are only a handful of them that are ideal. They are:

O-10 monoprop

Rockomax 48-7S

Toroidal Aerospike

LV-N Atomic

Rockomax Mainsail

Each of these either delivers superior ISP for the same t/w or superior t/w for the same Isp as compared to every other offering.

Anything else can be out- performed by these at the expense of part count, which results in a lighter, more efficient overall vehicle. This usually ends up cheaper on the launchpad.

Comparing either of these to my vehicles... it's a whole other level in terms of efficiency. I don't ever design boosters to lift more than 15 tons because there's nothing in this game that can't be accomplished with that payload. I rely on rendezvous schemes to make up the difference, so my launches are dirt- cheap with full recovery and I just make more of 'em.

I put payload in orbit for $73 a ton and just assemble it up there using clamp-o-trons.

Best,

-Slashy

Edited November 1, 2014 by GoSlash27

[UmbralRaptor]

I'm er, not seeing the download link.

[capi3101]

'>Link to the Zenith family. The Supernova is the big one on the right, obviously.

At a glance, I'd say a big difference between y'all's designs is in the cost of the individual parts, in particular the engines. You've got seven KS-25x4 engines there, right? Those things are √32,400 a pop, for a total cost of √226,800 just in engines. In contrast, a Supernova has nine Mainsail engines at √5650 each, for √50,850 total engine cost...22.4% the cost. I might also mention that '>Temstar's design philosophy comes from the days prior to thrust limiters, so it is possible nowadays to build a simpler design that's less reliant on engine clusters. Engine clusters remain more efficient in terms of cost of the design, though. I designed a booster for a 165 tonne payload just this evening, and I used Temstar's philosophies to do it, so they're still very much valid. My booster came in at just over √450,000 if anybody's interested.

Using the extra large fuel tanks is doing you no favors...their full mass-to-dry mass ratio is 8.2:1, as opposed to 9:1 for the FL-T and X200 series tanks (including the Jumbo64, the orange one).

Edited November 1, 2014 by capi3101

[deepspacecreeper]

I'm er, not seeing the download link.

Link added. you have to copy and paste the link cause i am doing this edit on my smartphone.

[deepspacecreeper]

Comparing either of these to my vehicles... it's a whole other level in terms of efficiency. I don't ever design boosters to lift more than 15 tons because there's nothing in this game that can't be accomplished with that payload. I rely on rendezvous schemes to make up the difference, so my launches are dirt- cheap with full recovery and I just make more of 'em.

I put payload in orbit for $73 a ton and just assemble it up there using clamp-o-trons.

Best,

-Slashy

Well my most common payload is around 25t. For Mun missions the rockets payload for LKO is 60 to 75.

ARMs which involve moving around e type asteroids need the largest rockets ever created, to haul that asteroid grabber into orbit. This is why i designed this rocket.

[deepspacecreeper]

At a glance, I'd say a big difference between y'all's designs is in the cost of the individual parts, in particular the engines. You've got seven KS-25x4 engines there, right? Those things are √32,400 a pop, for a total cost of √226,800 just in engines. In contrast, a Supernova has nine Mainsail engines at √5650 each, for √50,850 total engine cost...22.4% the cost. I might also mention that '>Temstar's design philosophy comes from the days prior to thrust limiters, so it is possible nowadays to build a simpler design that's less reliant on engine clusters. Engine clusters remain more efficient in terms of cost of the design, though.

Thanks for the advice. i am thinking of switching the KS25x4s in the boosters to dual clustered mainsails (with cubic octagonal struts). Yeah, they cost 10000 for 2 and have an equally good Isp, but slightly lower TWR.

Do you think this is possible? cause without part clipping you can't fit two 2.5 engines below a 3.75m tank.

[Wanderfound]

Well my most common payload is around 25t. For Mun missions the rockets payload for LKO is 60 to 75.

ARMs which involve moving around e type asteroids need the largest rockets ever created, to haul that asteroid grabber into orbit. This is why i designed this rocket.

Whatever you're having fun with is cool, but:

1) LV-N plus docking port.

2) Fuel tank plus several docking ports.

3) Klaw plus docking port.

Voila: arbitrarily large asteroid mover. You can get the whole thing into orbit using nothing more than a single Mk2 cargo bay.

[UmbralRaptor]

Thanks for the link. After messing around with things a bit:

1) Temster's Supernova isn't *that* cheap. 393769 is better than your 547468, but the difference is rather less dramatic.

2) Deleting nosecones and adapters make for an easy way to save funds. 20900 in your case.

3) As others have mentioned, rockomax tanks have better mass ratios than kerbodyne tanks. The S3-14400 is the only one worth using, and that is because it is almost 21% cheaper than it "should" be.

4) Also as others have brought up, Mainsails are quite cheap for the thrust that they provide, while 4x KS-25s are rather expensive. There is a TWR difference, but I think it ends up not increasing the payload fraction enough.

4) You have too many engines for this. In particular, the vertical staging (keeping the KR-2L as dead weight for most of the ascent). The best thing would probably to use a KR-2L as a core stage with a "normal" amount of tankage (say, 1 S3-14400), and other engines around it. Above 3500 m, the KR-2L is likely to have better Isp than anything else you were considering. Also, getting rid of an entire engine plus switching to cheaper tankage should save you ~38400 funds.

I think I found something that can get 135 t into 70-80 km orbits with a bit of fuel leftover for deorbiting. Amusingly, it runs <225k funds.

Clicky download!

[Jouni]

For all the liquid rocket engine choices available in this game, there are only a handful of them that are ideal. They are:

O-10 monoprop

Rockomax 48-7S

Toroidal Aerospike

LV-N Atomic

Rockomax Mainsail

Each of these either delivers superior ISP for the same t/w or superior t/w for the same Isp as compared to every other offering.

The 48-7S and the Mainsail don't belong in there. If you consider vacuum Isp, the Pareto frontier is formed by the LV-N, the Aerospike, the KR-2L, and the O-10. By atmospheric Isp, the optimal engines are the Aerospike, the KS-25x4, the LFB, the KR-2L, and the O-10.

Edited November 1, 2014 by Jouni

[Ruedii]

Seriously is this a joke?

First thing, by the time you are in orbit you should be down to a small high-efficiency rocket. Remember, you can build stuff in space to make bigger ships. However, sometimes smaller is better. You should bring the minimum you need with you on interplanetary missions. You also should refuel larger ships in orbit, so you can bring up the fuel separately on a specialty ship.

Smaller is better if you want cost-effective rockets. I cannot stress this enough. Send the smallest ship that will complete the mission.

As of how to get more bang for your buck (hopefully in the figurative sense) I have a few tips. I highly recommend use of lots of solid boosters in initial takeoff, as even without recovering them they are far more cost-effective. This is not to mention the punch they pack. The LFBx2 is also a very nice way to asparagus in more takeoff power. Ideally you want to break atmosphere as fast as possible without exceeding the terminal velocity for your craft. This usually means having a thrust to weight ration of 2-3. You may want to configure your ship that you have to initially throttle down a little to maintain this, so you can punch it once in the upper atmosphere.

Remember, you can refuel your ship once it's in orbit if you are launching bigger interplanetary vessels. This is important, because there is a limit to the maximum cost-effective weight, and using your final stage tanks twice can really save on weight. Finally, while making massive launch platforms SSTO is a real pain, it's quite easy to send up fuel in an SSTO or an SSTO with boosters. This can save you LOTS of money.

You already discovered that fins aren't necessary, so that's good. Fins are only necessary if you have no other means of control. Gimbling with works fine so long as you turn on SAS and don't do anything stupid. Fins are really just added drag and weight after that.

[magnemoe]

As i know its two ways to make cost effective rockets, simplest is make an SSTO, take your rocket equal fuel in all stage and no crossfeed.

More effective is to use SRB in the start they are cheaper than liquid fuel and let you lift off even if core has an TWR less than 1.

Has an rocket with a 24x4 and four large SRB it can lift 40 ton to orbit, as an SSTO it can only lift 20.

For higher payloads first increase the number of SRB, they can be put together on one decopler making costruction and seperation easier and you save decoplers.

Then stack the 3.75 meter parts together. 3,4,5 and 7 for increased capasity, an 7 24x4 with SRB help can lift over 200 ton.

All of this can return to KSC so it only cost fuel SRB and seperators.

Last why do you use the huge RCS tank? 2-4x40 liter should be enough.

[Ikaneko]

Because you use 3.75m LFO boosters for getting above the soupy lower atmo (and a load of them at that). It's giving you masses of Rocket equation tyrany.

Pro tip: use the SLS SRBs. Dirt cheap and high TWR!!

There ya have it folks.

[deepspacecreeper]

I quickly slapped together a rocket that uses dual clustered mainsails and no nosecones. Results in a vF 150,000 drop. Well, I'm still experiencing the TWR bottleneck when all boosters dropped. Talking of TWR, there was a slight sacrifice when using mainsails (from 2.0, 1.9, 1.7 to 1.8, 1.7, 1.6).

Why the TWR bottleneck? And the cost is still high, 400,000 vF.

Same dropbox folder, added the modified craft files.

https://www.dropbox.com/sh/50e9xria8jo8s5j/AABfvADOV0v9h6AnJxBHG7Hia?dl=0

Edited November 1, 2014 by deepspacecreeper

[GoSlash27]

The 48-7S and the Mainsail don't belong in there. If you consider vacuum Isp, the Pareto frontier is formed by the LV-N, the Aerospike, the KR-2L, and the O-10. By atmospheric Isp, the optimal engines are the Aerospike, the KS-25x4, the LFB, the KR-2L, and the O-10.

I'm also including t/w ratio. Both must be considered when designing a stage for minimum overall mass and thus operating/ initial cost.

If you plot out t/w vs Isp for all the liquid engines, you can see that the engines I cited form the vanguard with the others falling behind them in terms of Isp, t/w, or both.

As an aside, I just figured out this morning that Isp (atm) vs Isp(vac) is highly misleading in this game. On Kerbin, by the time you hit 5Km altitude you're already closer to the vacuum value than the atmospheric. At 10k, you may as well use the vacuum number in your calculations.

Best,

-Slashy

Edited November 1, 2014 by GoSlash27

[deepspacecreeper]

Last why do you use the huge RCS tank? 2-4x40 liter should be enough.

Sluggish steering.

[GoSlash27]

I quickly slapped together a rocket that uses dual clustered mainsails and no nosecones. Results in a vF 150,000 drop. Well, I'm still experiencing the TWR bottleneck when all boosters dropped. Talking of TWR, there was a slight sacrifice when using mainsails (from 2.0, 1.9, 1.7 to 1.8, 1.7, 1.6).

Why the TWR bottleneck? And the cost is still high, 400,000 vF.

Same dropbox folder, added the modified craft files.

https://www.dropbox.com/sh/50e9xria8jo8s5j/AABfvADOV0v9h6AnJxBHG7Hia?dl=0

http://i.imgur.com/S2xIhjyl.png

DSC,

The entire launch assembly needs to be redesigned to take advantage of the new engines. You don't need all the tankage in the upper stages you did before, which lowers payload demand on the lower stages, thus cascading the weight (and thus monetary) savings back down the launch vehicle.

As for why you're running into the t/w bottleneck, it's partially what I mentioned above and partially a by-product of asparagus staging. Thrust is reduced at each staging event more than payload is reduced.

The trick is to balance the fuel load distribution among the stages such that the t/w after staging is where you need it to be at that portion of the launch profile. 1.2-1.5 at initial boost, approximately 1 at the gravity turn, and .75 or even less at insertion is adequate to get the job done.

There's no reason why all your asparagus boosters need to be clones of each other (or even use the same engines, necessarily).

Best,

-Slashy

Edited November 1, 2014 by GoSlash27

[GoSlash27]

These are some examples from the way-back machine of how small and light you can make an asparagus booster and still have it work:

In this case, the mass- mover has just enough fuel to complete it's own insertion burn.

Here, the stack of rovers is empty and the booster does everything including rendezvous and docking.

Here, the payload is all dead weight and fully loaded. A supply pod destined for transfer to a space station.

I don't use asparagus staging much these days, but this shows what even simple arrangements are capable of.

Best,

-Slashy

[Red Iron Crown]

As an aside, I just figured out this morning that Isp (atm) vs Isp(vac) is highly misleading in this game. On Kerbin, by the time you hit 5Km altitude you're already closer to the vacuum value than the atmospheric. At 10k, you may as well use the vacuum number in your calculations.

That's been my observation, too. Atmospheric Isp is almost irrelevant anywhere but Eve. Even the woeful atmo Isp of the LV-N isn't a big deal, by 1700m or so on Kerbin its Isp has reached 390 and keeps getting better. The TWR though...

[Wanderfound]

That's been my observation, too. Atmospheric Isp is almost irrelevant anywhere but Eve. Even the woeful atmo Isp of the LV-N isn't a big deal, by 1700m or so on Kerbin its Isp has reached 390 and keeps getting better. The TWR though...

I've been having a lot of luck with LV-N powered spaceplanes lately. Up to Mach 5/30,000m on jets, then spend ten minutes cruising around the world on the nuke at 45,000m. Works well.

[Jouni]

I'm also including t/w ratio. Both must be considered when designing a stage for minimum overall mass and thus operating/ initial cost.

If you plot out t/w vs Isp for all the liquid engines, you can see that the engines I cited form the vanguard with the others falling behind them in terms of Isp, t/w, or both.

I already included TWR in the considerations. It's been 7 months since the 48-7S and the Mainsail were the liquid fuel engines with the highest TWR. The KR-2L and the O-10 beat them by a wide margin, and even the KS-25x4 and the LFB have a higher TWR.

[Tortoise]

Because you use extremely expensive fuselages and engines w/ liquid fuel & oxidizer.

I might make a 150t launcher, just do show that I can do it cheaper.

May I ask, do you use FAR?

[Norcalplanner]

I'll second those who suggest using the big SLS SRBs for cost-effective launches. Each puts out the same thrust as a Skipper for only a few thousand funds. My medium lifter is now an 82 ton ARM tank, a mainsail, and between 6 and 14 of the big S1 SRBs. For a really heavy lifter I'll have 4 of these (with slightly fewer SRBs) asparagused around a core with a KR2 to keep the TWR up. Keep in mind that Mainsails were buffed recently, and now have an Isp of 320 / 370.

[capi3101]

I quickly slapped together a rocket that uses dual clustered mainsails and no nosecones. Results in a vF 150,000 drop. Well, I'm still experiencing the TWR bottleneck when all boosters dropped. Talking of TWR, there was a slight sacrifice when using mainsails (from 2.0, 1.9, 1.7 to 1.8, 1.7, 1.6).

Why the TWR bottleneck? And the cost is still high, 400,000 vF.

Same dropbox folder, added the modified craft files.

https://www.dropbox.com/sh/50e9xria8jo8s5j/AABfvADOV0v9h6AnJxBHG7Hia?dl=0

http://i.imgur.com/S2xIhjyl.png

Okay...so we're trying to isolate the cause of the TWR bottleneck for 150 tonnes, right? That's easy - your core is too weak for the amount of fuel in it.

Temstar's Supernova - in fact his entire Zenith series - was based on a set of design formulas and philosophies which his booster set follows. Based on that philosophy, we'd design an asparagus staged rocket as follows:

1)150 tonnes is the desired payload. Assume 15% payload fraction - that means our final vehicle will be around 1000 tonnes (150/0.15 = 1000)

2)Shoot for a TWR between 1.6 and 1.7 at liftoff. So we'll need between 15,680 - 16,660 kN of thrust (1000*9.8*1.6=15680, 1000*9.8*1.7=16660).

3)Shoot for 22% of your launch thrust in your core and divvy the rest among the boosters. That works out to 3449.6-3665.2 kN in the core (just multiply the values time .22) and for three booster pairs, each booster should have between 2038.4-2165.8 kN ((15680*.78)/6 = 2038.4, ((16680*.78)/6=2165.8).

4)Subtract the mass of the engines you need and the mass of the payload from the estimated weight. The remainder is your estimated fuel mass.

So let's ignore the cost factor for a moment. Thrust wise, a KS-25x4 is what you should have in the core for the payload you want. For boosters, I'd recommend a single KR-2L set to about 87%. There's your thrust requirement. Now, the total mass of your engines with that configuration is 9.75+(6*6.5) = 48.75 tonnes. So our rough fuel mass should be 1000-150-48.75 = 801.25 tonnes. Divide that evenly among seven stacks; you get 114.46 tonnes each.

Looking at your design - you have an S3-14400 and an S3-7200 in each booster stage. I assume you've also got that in the core, plus another S3-7200. If they're full tanks, that's 123 tonnes of fuel in the boosters and 164 tonnes in the core. And if they're not full, the assumptions of the design might not hold because the mass ratio of the tanks you're using isn't 9:1 - it's 8.2:1. Your options are to change your tank setup - three orange tanks and an X200-16 will give you what you need - or to reduce the amount of fuel in the S3-7200s to about 90% (that should be, if I'm doing the math right, 2916 units of liquid fuel and 3564 units of oxidizer each) and take out that topmost S3-7200 in the core. This is assuming you swap out the dual Mainsails with a single KR-2L in any case (two mainsails are 12 tonnes, a single KR-2L is 6.5 tonnes, so the KR-2Ls fall in the "more efficient" category in this case).

I'll tell you what - I'll go ahead and replicate your booster with the tweaks I've proposed, to see if just adjusting the fuel will do the trick or not (I'm concerned about hitting an early delta-v bottleneck).

Bear in mind that Temstar's designs generally wind up with a 1.3 TWR in the core once all the boosters are separated. If you've got that, you're golden.

Now, SSTO transporter spaceplane might be an option as well. DocMoriarty has put out what I consider to be the definitive guide to transporter spaceplanes and his guidelines have proven themselves most useful in my own fights with flight. A good rule of thumb for a transporter spaceplane is a 25% payload fraction - so for a 150 tonne payload you'd want to shoot for a 600 tonne spaceplane. His guide says that the max take-off weight per RAPIER engine is 13 tonnes...so you'd need 47 engines. All I can say there is "good luck" if you wanted to try that route...

Edited November 2, 2014 by capi3101