Theoretical maximum deltaV and how to get beyond it.

[willow]

So I was thinking, if you bring more fuel on a rocket you get diminishing returns. Doubling your fuel storage doesn't double your deltaV. There must be a limit someplace. What would be the maximum amount of deltaV one could hope to attain in the game? I'm guessing it to be somewhere between 15k and 20k.

And the obvious next question would be: How do you then increase that limit?

Sending out fuel depots to rendezvous with would obviously help. But these are hard to maintain and suffer the same diminishing return problems. Are there other options?

[Spartwo]

If the physics are true than there is no limit the burn times would be far longer.delta v up TWR down.

[UmbralRaptor]

If we're talking about a single stage, then the maximum ÃŽâ€V is based on the mass ratio of the tankage, and the Isp of the engine. For the LV-N on stock tankage, that's 17,261 m/s. For the PB-ION, 36,108 m/s. The most straightforward way to get more ÃŽâ€V is to make a craft multi-stage, giving you effective mass ratios far beyond the limit of the stock fuel tanks. Though like you said, fuel stations get around it.

If the physics are true than there is no limit the burn times would be far longer.delta v up TWR down.

Only if you drop spent fuel tanks. If you don't, there is an upper limit, no matter how much fuel you bring(!)

*digs up a TWR vs ÃŽâ€V chart*

Aerospike

LV-T30

LV-T45

Mainsail

Also, the Tyranny of the Rocket Equation is relevant.

Edited June 5, 2013 by UmbralRaptor

[jebbe]

Actually, there is no limit: To increase your delta-v linearly you need exponentially more fuel. Or, in other words: Increasing your mass linearly only increases your delta-v logarithmically, but you will always get some additional delta-v, without any upper bound. You just need more and more fuel for less and less delta-v, which is probably what you have noticed. So there's only the practical limit of how large a rocket you can design without it getting unstable, or your computer lagging too much.

However, I have built rockets with more than 30km/s delta-v, well capable of doing a grand tour, and there's a neat trick to it: Make your spacecraft as lightweight as possible, ideally it's just a probe core with a minimalistic propulsion system (tiny fuel tank and ant engine / ion propulsion system). The gain from small decreases in the spaceships mass is enormous, try to get rid of everything you don't absolutely need; for example put ASAS and RCS to lower stages and dump them before activating your leightweight spaceship, or even better, don't take them along at all. Actually, if you could make your ship infinitely light, then you would get infinite delta-v.

All this is just inferred from the rocket equation; try to understand it, it will help a lot with building rockets.

[jebbe]

Only if you drop spent fuel tanks. If you don't, there is an upper limit, no matter how much fuel you bring(!)

Sorry, but I have to disagree: delta-v is proportional to the logarithm of your initial fuel, and the logarithm doesn't converge - it grows very slowly, but it never reaches a limit.

Edited June 5, 2013 by jebbe

[UmbralRaptor]

Sorrry, but I have to disagree: delta-v is proportional to the logarithm of your initial fuel, and the logarithm doesn't converge - it grows very slowly, but it never reaches a limit.

Adding more of the same kind of tank does make the mass ratio converge, though.

[jebbe]

Adding more of the same kind of tank does make the mass ratio converge, though.

Hm ok, true, if you consider the dry mass of the fuel tanks. Yeah, I guess that's how it should be done.

[architeuthis]

Adding more of the same kind of tank does make the mass ratio converge, though.

I agree. The maximum delta-v for a single stage is achieved by having an infinite number of the highest mass ratio fuel tanks. The delta-v of a non-infinitely sized rocket logarithmic approaches that value. For stock parts this is done with a PB-Ion to get roughly a KSP single stage maximum of 36,100 m/s delta-v. With staging infinite delta-v is possible in principle, but of course not in reality:)

[Specialist290]

TLDR: The best rocket in terms of delta-v will always be better than the one you actually have. Always.

EDIT: Actually, I've just realized that an infinitely massive rocket would probably be completely useless, as it would have a proportionately infinitely small acceleration when paired with any finite value of thrust. And contemplating an infinitely massive rocket with infinite amount of thrust makes me go cross-eyed.

Edited June 5, 2013 by Specialist290

[Geckgo]

isp * 9.8 * Log(Wet/Dry)

Assuming the same isp, the theoretical maximum has to do with the ratio of wet/dry or full weight/empty weight. That ratio for a fuel tank is 9/1, so, no matter how many fuel tanks you add you can never get a ratio higher than that. The most efficient engine is probably the LV-N, which has an isp of 900 in space. This renders a max of 8416 for a single stage (assuming an infinite amount of fuel to cancel the weight of the rocket).

EDIT:

Sorry about that, should be a natural log, the new ln gives 2.19 instead of .95, so the resulting max Delta-V would be 19380.

Edited August 1, 2013 by Geckgo
Mistake

[Epthelyn]

And contemplating an infinitely massive rocket with infinite amount of thrust makes me go cross-eyed.

Contemplating any rocket with infinite thrust is probably enough. You'd need a rocket capable of burning any amount of fuel instantly.

On Topic: You cannot get beyond the theoretical maximum deltaV of something. Theoretical maximums are (almost) always higher than reality. The only way to get a higher dV is to make something new (e.g. an engine with an isp of a million would work find).

Edited August 1, 2013 by Epthelyn

[UbioZur]

Here are all the links to the Engine data that can randomly found in the forum

http://forum.kerbalspaceprogram.com/showthread.php/30989-Nuclear-engine-interplanetary-transfer-sloooooowwwwwwwww?p=382503&viewfull=1#post382503

http://imgur.com/a/kfnng

http://imgur.com/a/okuO4

Max liquid fuel Dv = a bit over 17k (good luck getting it)

Max Xenon Dv = more than 90K (good luck having the patience for it)

To extend it, the best way is adding stages and so dropping dead weight.

Another way is to use the infinite fuel cheat (ALT+F12 menu)

Note that RCS have a nice Dv for very light craft (check

of course) but I don't think it's possible to calculate it.

[SkyRender]

If you don't mind using mods, you can theoretically have effectively infinite delta-V using Kethane. Just set up mining operations at each stop-over you plan to make, and you basically have no real limits on how far you can go.

Getting Kethane mining operations set up all over the place, however, is a lot more time-consuming than you might be up for...

[Ralathon]

Other posters pretty much summed it up: dV is limited by the mass ratio between wet and dry mass for fuel tanks.

You can get around this by dropping tanks as you empty them, allowing you to get theoretically infinite dV, but your acceleration will be so low in the start you're limited by boredom instead.

[mintme]

I spent the early stages of my kerbal space program experience making my own fuel to combat exactly this. i managed to get to duna in 6 days and had fuel to spare after a some 14km/s retrograde burn. this is great to learn how to get to planets etc. but what i have found after nearly 2 years of playing is that managing fuel cans and a space plane that can get orbit with 90 liquid fuel, thats the most fun. This is the game, these are pretty much the realitys of real space flight. Although the nuclear rocket is probably your best bet.

[Xaiier]

This old thread has a thorough discussion of it. The important thing to remember is that TWR and delta-v are largely inversely related, and thus delta-v is a meaningless statistic without TWR.

Depending on patience, the maximum with what we have lies somewhere around 50k. To reduce burn times to tolerable levels and allow for thrust that is meaningful and useful, the maximum is maybe 30k. Personally, my best is a ship that has 22k at LKO, while maintaining the ability to land on any of the non-special case bodies.

[problemecium]

As long as we're rambling about this, how do gravity assists play into the equations? If I understand correctly, a fortuitous gravity assist equals free delta-V (effective delta-V anyway, I know your ship doesn't literally get more thrust). But how far can this go? Do gravity assists always contribute the same amount no matter how many times you do it, or is there also a falloff there? Can I get unlimited travel range out of a ship by surfing up and down gravity wells a bunch of times?

[Rakaydos]

As long as we're rambling about this, how do gravity assists play into the equations? If I understand correctly, a fortuitous gravity assist equals free delta-V (effective delta-V anyway, I know your ship doesn't literally get more thrust). But how far can this go? Do gravity assists always contribute the same amount no matter how many times you do it, or is there also a falloff there? Can I get unlimited travel range out of a ship by surfing up and down gravity wells a bunch of times?

Voyager was launched from a Titan 3 balistic missile. While I may be a bit loose on the math, I suspect voyager's "grand tour" slingshots did allow quite a bit of delta V, with the perfect arrangeent of planets.

[Jimbimbibble]

From a more practical perspective, using a single LV-N with drop tanks is the best way to maximize dV without painfully slow burns. Set it up kind of like asparagus staging, except the outer tanks have no engine. You can then take this assembly and mount it on a large asparagus staged rocket with enough dV to get the upper assembly to escape. Another cool trick is to do a powered gravity assist around Jool to drop your solar Pe to almost 0 and then burn the rest of your fuel close to the sun. By the time you get back to Jool you can be going upwards of 50km/s.

[Silverchain]

Are the full stats for the new ARM parts available anywhere? Just wondering if the new big tanks keep the 9:1 ratio.

[EtherDragon]

There are two different considerations when talking about maximum Delta-V, or any limit on Delta-V.

First, in the equation, it is true that as long as you increase your fuel you increase Delta-V, but Dv increases logarithmically. So long as you can attach more fuel mass (without attaching any other empty mass) it is theoretically possible to have infinite Dv.

In practicality, this can never be achieved, because fuel must be stored in a container - which has mass. That means, at best, your Delta-V is governed by the constraints of the fuel container (90% fuel 10% container in KSP). This sets a hard limit to the maximum Delta-V of any single stage.

PS, you can again, approach infinite Delta-V if you have an infinite number of stages.

[Rakaydos]

Hmm. It seems like there would be an ideal maximun DeltaV Ion stage, an ideal NERVA stage to get it moving, and an ideal turbojet stage to get it into orbit.

[James_Eh]

In practicality, this can never be achieved, because fuel must be stored in a container - which has mass. That means, at best, your Delta-V is governed by the constraints of the fuel container (90% fuel 10% container in KSP). This sets a hard limit to the maximum Delta-V of any single stage.

Make the container out of frozen fuel that sublimates into the emptying tank as the pressure drops. (And use magic to stop the outside from sublimating to space. Or foil.)

[Rakaydos]

Make the container out of frozen fuel that sublimates into the emptying tank as the pressure drops. (And use magic to stop the outside from sublimating to space. Or foil.)

Does the magic have mass? I know the foil does...

[TheDarkStar]

Make the container out of frozen fuel that sublimates into the emptying tank as the pressure drops. (And use magic to stop the outside from sublimating to space. Or foil.)

Use two chemicals that are stable when separated, have a high melting point, and violently combust when they come into contact with each other. You'll have a payload mass, but adding more fuel shouldn't increase dry mass as long as you have a method for storing, moving, and reacting it that can accept a large volume of fuel.