Why Are KSP engines and tanks so heavy?

[Halsfury]

Hi I was researching into spaceflight some more and I found two interesting things, firstly a Fuel tank on a modern rocket is about 4% structure and 96% fuel, in KSP an orange tank is 4 tons structure and 32 tons fuel which is a ratio of about 11% structure and 89% fuel. So tanks are heavier in KSP

Then there are the engines themselves

The SpaceX merlin engine has a thrust equal to the thrust of KSP's "skipper" engine at 650KN (At Sea Level)

The Total Weight Ratio for the Skipper is roughly 18g

The SpaceX Merlin engine has a weight ratio of 150g!

To sum up then, the tanks in KSP are 7% more useless (quite literally since I can't burn the structure of them, least not in a "nominal" flight) and the Skipper is 132 times worse than it's real life counterpart.

My question is why did the devs decide on the current arrangement? was it so that our rockets wouldn't rip themselves apart? or just balance since our orbital velocity is only mach 7 ish in KSP

[leax256]

Hi I was researching into spaceflight some more and I found two interesting things, firstly a Fuel tank on a modern rocket is about 4% structure and 96% fuel, in KSP an orange tank is 4 tons structure and 32 tons fuel which is a ratio of about 11% structure and 89% fuel. So tanks are heavier in KSP

Then there are the engines themselves

The SpaceX merlin engine has a thrust equal to the thrust of KSP's "skipper" engine at 650KN (At Sea Level)

The Total Weight Ratio for the Skipper is roughly 18g

The SpaceX Merlin engine has a weight ratio of 150g!

To sum up then, the tanks in KSP are 7% more useless (quite literally since I can't burn the structure of them, least not in a "nominal" flight) and the Skipper is 132 times worse than it's real life counterpart.

My question is why did the devs decide on the current arrangement? was it so that our rockets wouldn't rip themselves apart? or just balance since our orbital velocity is only mach 7 ish in KSP

For balanceing reasons...everything in ksp is 1/10 size.

[UmbralRaptor]

Originally, the parts were even sillier (The old FL-T250 tanks were 12% structure, and the initial LV-T30 had a TWR of ~10.2. Both were made up for with a Ve of 5682 m/s.). Starting with 0.16, there was an attempt to get somewhat realistic tankage and engines. More realistic ones would make getting large payloads into LKO trivial, but more distant locations might be increasingly annoying, depending on if we have access to hydrogen burning rockets.

[Banbite]

Balancing, in the real world it takes almost 10km/s delta-v to get to orbit, in KSP that number is 4.5 km/s due to the scaling. All other maneuvers like TLI and interplanetary burns also takes a lot more IRL. So not to make the game to easy so that the players actually have to design rockets which more closely reassembles the real world counterpart the easiest thing to do is upping the structure/fuel weight ratio as well as a bit heavier engines.

[AmpsterMan]

I've often wondered how much lowering engine mass and increasing tankage would actually change the balance though. would it be a huge difference?

[Kerbol Prime]

Because its kerbal engineering, and we all know how kerbals design their parts...

[Horn Brain]

Consider: If you have a rocket visibly wobbling in KSP, maybe no big deal. If a real life rocket is visibly wobbling, and sometimes even if it's wobbling imperceptibly, you're going to kill everyone.

Kerbal ships are designed to withstand the stress of relatively high speed, poorly-aligned docking. The Space Shuttle weighed over 100 tons fully loaded in space and docked at about 0.03m/s, perfectly aligned. If you come screaming in (relatively speaking) to dock at 0.5 m/s in real life and you aren't perfectly aligned, you're going to kill everyone.

Kerbal engines are designed to support the weight of medium-to-large-sized rockets on their bells while awaiting launch in case the engineers forget to include docking clamps in the design. If you drop a real life rocket on its engine on the pad, you're going to kill everyone.

The "reason" could be that the kerbals design their structures to be much sturdier because they can't trust their pilots to fly the things with the right precision and they can't trust their engineers not to design wobbly, floppy rockets.

Obviously the real reason is that if you had parts with real life mass-ratios you could get to space trivially easy, even with Kerbin's "pease porridge" atmospheric drag physics, but this should make you feel a little less swindled in the technological capabilities department.

[nettcod]

Because its kerbal engineering, and we all know how kerbals design their parts...

The kerbals made those parts? I thought they found them lying on the side of the road...

About the weight of the parts, think it's just to make it balanced, as leax said. Everything is 10x smaller, so it takes a lot less delta-v to get anywhere, and if they didn't make the engines and tanks heavier, it would be way too easy to get to LKO, the mun, Duna, etc.

Edited June 2, 2013 by nettcod

[sjwt]

My question is why did the devs decide on the current arrangement? was it so that our rockets wouldn't rip themselves apart? or just balance since our orbital velocity is only mach 7 ish in KSP

With the fact that the size of the planets/system is scaled down 10 fold, with a 1:1 ratio of the engine mass and weights etc the game would be too easy. RCS to solar escape kind of easy!

[UmbralRaptor]

I've often wondered how much lowering engine mass and increasing tankage would actually change the balance though. would it be a huge difference?

If we go with mid-50s Soviet tech, not really -- the low Isp offsets the other gains. Anything modern (Which for the purposes of this means anything kerolox after 1970, higher performance kerolox engines made after 1960, and all hydrolox engines) would greatly increase performance. As would simply improving tankage mass ratios and decreasing mass/increasing thrust of the current engines.

To expand on Horent_Brain's comments, consider that KSP engines can reach 1% throttle without much effort, and are fully efficient over the *entire* throttle range. Real rocket engineers would be rather impressed by the Kerbals' pumps.

Edited June 2, 2013 by UmbralRaptor

[AmpsterMan]

I guess I wish mass of engines was decreased to encourage serial staging a bit more.

[DisarmingBaton5]

I guess I wish mass of engines was decreased to encourage serial staging a bit more.

That, and stronger RCS Tanks and Engine shrouds. I am currently trying to launch a station core, and those two parts are clipping through everything!

[N_Molson]

Hi I was researching into spaceflight some more and I found two interesting things, firstly a Fuel tank on a modern rocket is about 4% structure and 96% fuel, in KSP an orange tank is 4 tons structure and 32 tons fuel which is a ratio of about 11% structure and 89% fuel. So tanks are heavier in KSP

If you look at things more globally, the 11% of structure mass is quite realistic. "Tanks" IRL are only the metallic sheet that holds the propellant, you have to take into account all the structure around (the "skin" of the rocket and the structural stringers). In KSP, all is included in the part.

In the Delta IV Heavy case (figures from Encyclopedia Astronautica)

Boosters : Gross Mass: 226,400 kg. Empty Mass: 26,760 kg (x2 each).

Stage 1 : Gross Mass: 226,400 kg. Empty Mass: 26,760 kg (boosters & first stage are identical, hence the "Common Booster Core" concept).

Stage 2 : Gross Mass: 30,710 kg. Empty Mass: 3,490 kg.

Payload : up to 25,800 kg.

If we add everything up :

Gross Mass = 226,400 kg * 3 + 30,710 kg + 25,800 kg

Gross Mass = 735,710 kg

Empty Mass (with payload) = 26,760 kg * 3 + 3,490 kg + 25,800 kg

Empty Mass = 109,570 kg

Empty Mass (without payload) = 26,760 kg * 3 + 3,490 kg

Empty Mass (without payload) = 83,770 kg

So, for the real-life Delta IV Heavy :

Empty Mass / Gross Mass ratio (%)

With payload : 14.89 %

Without payload : 11.39 %

Edited June 2, 2013 by N_Molson

[tstehler1]

For balanceing reasons...everything in ksp is 1/10 size.

Actually, I believe it's 1/11. That's the size ratio of Earth to Kerbin

[AceGamer]

Its making it fair every thing is very scaled down.

[AlternNocturn]

Because then the game wouldn't be fun.

[Fel]

I've often wondered how much lowering engine mass and increasing tankage would actually change the balance though. would it be a huge difference?

I did a large number of tests with this, and settled to recreate Saturn V tech via part.cfg but adjust the isps to provide a proportional delta-v. (Same fuel mass / Engine Stats... just different isps [i tried to explain in a thread, a while back that it SEEMED like "true balancing" would be isp adjustment while keeping thrust the same... and no one understood what I was saying -__-])

First problem was that the attachment nodes were significantly weaker than I needed them to be (the rocket also didn't like to "recover" from bad maneuvers). (It also exploded a few times until I practically disabled all "impact collisions")

Second problem was that flights were actually significantly longer. I may have some 50kN? (I forgot) but I had only 1.1 TWR at launch and 0.8 TWR after second stage (and it weighed ~3,000Mg [payload attached]). I tend to do this now with regular KSP, but I've heard so many people give advice to use wicked large second stage TWRs so they could burst through the (thin) atmosphere.

But interesting to note that I can follow the Saturn V formula with regular KSP parts and get a BETTER "Mass to Orbit Ratio" than the Earth Equivalent.

*** Effectively ***

KSP is not "100% Balanced" but you aren't really hindered by the extra mass; so lowering it would just make things even easier ;p

I guess I wish mass of engines was decreased to encourage serial staging a bit more.

Well, yes... they need a new model for their balancing; doing some "kiddy physics" I found ISP to be the most direct approach, but people are right in that Engine TWRs are too low and tanks are too heavy... srbs are too heavy... and well, balance is "completely undecided"

Edited June 2, 2013 by Fel

[AmpsterMan]

Yeah I think lowering ISP and lowering mass on the lifting engines might be a good idea. Purpose isn't to increase delta-v but rather TWR

[Khrissetti]

I quite like the 'sturdy rockets' hypothesis. Considering the punishment we all subject our rockets to, is it any wonder they're built a bit stronger than RL Equivalents?

[sadron]

Actually, I believe it's 1/11. That's the size ratio of Earth to Kerbin

I would go with maybe 1/10.5, the Earth is only around 12.7km in diameter or so, which is slightly above 10 times Kerbin's diameter.

[Xeldrak]

The Total Weight Ratio for the Skipper is roughly 18g

The SpaceX Merlin engine has a weight ratio of 150g!

That's because Kerbal itself is smaler, actually the whole Kerbol-system is much smaller than our starsystem.

PS: It's thrust-to-weight ratio and it has no dimension....so it's just 18 or 150 - no g needed

[Merinsan]

I would go with maybe 1/10.5, the Earth is only around 12.7km in diameter or so, which is slightly above 10 times Kerbin's diameter.

I think you might mean 12700 km (12.7 Mm ?) If it were only 12.7km, the airline industry would be out of business.