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practical dV ceiling for rockets?


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I've been tinkering with a rocket for the past couple days, revising and redoing it over and over, and I'm always ending up in the same dV range. It seems like, no matter what I do, I can't seem to get past a certain amount.

Now mind you, this is a very heavily modded scenario (using RSS), so I won't go into details since this is not what this forum is for. No, I'd like to ask in general: is there such a thing as a dV ceiling? A limit that, unless you gain access to better engines and fuels, you cannot exceed no matter how large a rocket you build? I've never before pushed the envelope like this (because it was never necessary before), so this comes pretty much as a surprise to me. If there is no such thing and I just do something wrong on a very basic level, then I need to figure out where my mistake lies.

Edited by Streetwind
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Yes. You get diminishing returns from adding more fuel after a while, as per the Tsiolkovsky's Rocket Equation. So in a way, there's a ceiling. You probably reached the point where you have to add a lot more fuel to get a meaningful increase in dV. Either use different fuels (hydrolox), more efficient engines or remove some payload mass.

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According to Tsiolkovsky equation there is no limit theoretically. Practically you can increase your launch mass only as long as your engines will have enough thrust to get you TWR of at least 1.2. And the equation is logarthimc so with rapidly increasing launch mass dV will increase quite slowly.

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Indeed there is a practical ceiling if you want to get the rocket into space.

There are some theoretical bounds that depend on the engines used.

Recently i held a Delta-V-Maximization challenge.

You can find some ships there that try to maximize the dV.

Tavert and Seanner used a trick to get an increased dV value for a single stage.

Edited by mhoram
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Indeed there is a practical ceiling if you want to get the rocket into space.

There are some theoretical bounds that depend on the engines used.

Recently i held a Delta-V-Maximization challenge.

You can find some ships there that try to maximize the dV.

Tavert and Seanner used a trick to get an increased dV value for a single stage.

Oh, that will be an interesting read, thanks! (For tomorrow, now it is time to sleep.)

Actually there isn't any ceiling if you use staging. I would recommend you to make your upper stages bigger to be able to contain more fuel and your lower stages wider to be able to contain more engines to keep the TWR up.

Yeah... except I am at the point where making my upper stages bigger will decrease the dV of all lower stages so much that the total dV goes down.

Also very interesting! Definitely makes me wish I had access to LV-N's :P But alas, hydrolox is the best I can do for now.

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For each stage the absolute limit is about 22*Isp where the mass of the rocket is comprised almost exclusively of fuel tanks.

A rocket can have unlimited stages with this quantity of delta-V, but each stage needs to have a much larger mass than the next. The high part count, and therefore lag, limits this realistically being achieved. I find that generally there is always going to be a compromise between delta-v, TWR, mass of science equipment/utilities and part count. If you attempt to design a craft specifically for delta-V, then you will manage to go a long way, but that's about it.

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Definitely makes me wish I had access to LV-N's :P But alas, hydrolox is the best I can do for now.

I was really really wanting the LV-N (for transfer) and fuel lines (for launcher) when I made this monster (and playing stock):

11973065625_5e21836539_b.jpg

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Hah, okay... my rocket isn't quite as crazy as that :D Since I have FAR installed, I unfortunately can't resort to ultrawide pancake thrust plates to launch stuff. As it is, I'm already suspecting I'm near the sanity border for atmospheric drag with this design: https://dl.dropboxusercontent.com/u/44754370/screenshot18.png I'm getting mach effects while in the solid booster stage even at a relatively low TWR of 1.4 on the pad (in RSS, thrust scales with atmospheric pressure, not fuel consumption).

I find it interesting that the limit seems to be per stage, though (unless I completely misunderstood that part). My stages are all over the place in terms of how much dV they offer, maybe I could tweak those which have a lot less than the rest. I'm fairly hard limited by TWR though as you can see.

Maybe I should try a three-column design in the vein of the Delta IV Heavy / Falcon Heavy...

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If you're starting to get painfully diminishing returns from tweaking your rocket the next place to look is your payload. Most vehicles designed for operations in space can be broken down into sections, split over several launches and reassembled in orbit if need be.

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My payload is a 2.66t unmanned science lander. That's what you see as "S1" in the Engineer readout, with some 1650 m/s out of the total dV attributed to it. There is a "S0" on top of it, which is a small 250kg SCANsat with another ~600 m/s dV by itself (for some odd reason that doesn't show up in KER). But it's not really relevant, as it'll be relying on the rest of the rocket to brake it into Moho orbit anyway. It will only be decoupled once all the fuel but that for the landing itself has already been spent. Screenshot: https://dl.dropboxusercontent.com/u/44754370/screenshot19.png

So yeah, technically I could send the lander and the satellite in two separate rockets. I'm fairly sure that if all this rocket had to carry was a 250 kg payload, it's dV would be crazy. But the real trick is getting the science lander there with enough fuel left to actually land, and well, I'm not sure how much dV I gain by dropping the lightweight satellite. As it stands, my tests reveal that starting with 22,500 m/s dV, I have about 13,600 left after reaching orbit, the ejection burn will cost around 5,100, then there'll be a plane change of unknown magnitude, then the orbit injection, and finally a powered descent costing maybe as much as 3k if I compare Moho to the Mun... I'm honestly doubtful. Which is why I keep hemming and hawing and trying to redesign the rocket, which ultimately led me to make this post.

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If you're starting to get painfully diminishing returns from tweaking your rocket the next place to look is your payload. Most vehicles designed for operations in space can be broken down into sections, split over several launches and reassembled in orbit if need be.

In addition to payload, check the efficiency of your lifter. I put together this comparison a while back when I was struggling with some interplanetary launches (namely Moho):

uF7gNg0.jpg

Regardless of design, the rolloff happens around the same payload, it's the efficiency of the lifter that determines the radius of curvature at the inflection points. Changes in ÃŽâ€V naturally result in changes in efficiency, as the capacity to change ÃŽâ€V necessarily impacts your fuel fraction no matter how it is incorporated. (Plain English: lower ÃŽâ€V on a modular lifter gives you a wider payload range within a given ÃŽâ€V range)

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Nice looking design, Streetwind. My guess is that you don't want to cut your payload down any further. Of course, the solution is to add more dV by adding the appropriate fuel and possibly engines (where TWR is a concern). Good luck with it!

Edit: I usually don't try to rival Whackjob. :) I can do minimalist craft:

13259375964_d0d23fc9e6_b.jpg

Interesting graph, PUNiSH3R, thanks for sharing it.

Edited by Dispatcher
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My payload is a 2.66t unmanned science lander. That's what you see as "S1" in the Engineer readout, with some 1650 m/s out of the total dV attributed to it. There is a "S0" on top of it, which is a small 250kg SCANsat with another ~600 m/s dV by itself (for some odd reason that doesn't show up in KER). But it's not really relevant, as it'll be relying on the rest of the rocket to brake it into Moho orbit anyway. It will only be decoupled once all the fuel but that for the landing itself has already been spent. Screenshot: https://dl.dropboxusercontent.com/u/44754370/screenshot19.png

KER is known to have various bugs/shortcomings in the deltaV calculation code. What other mods are you running? I see some dual nozzle RCS thrusters. Is stage 0 ion-powered (or RCS powered)? Does it display correctly (or at all) if you remove the rest of the rocket? Can you post a shot with the "Show all stages" button selected? Have you tried the test version of KER in this thread?

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In addition to payload, check the efficiency of your lifter. I put together this comparison a while back when I was struggling with some interplanetary launches (namely Moho):

*snip interesting chart*

Regardless of design, the rolloff happens around the same payload, it's the efficiency of the lifter that determines the radius of curvature at the inflection points. Changes in ÃŽâ€V naturally result in changes in efficiency, as the capacity to change ÃŽâ€V necessarily impacts your fuel fraction no matter how it is incorporated. (Plain English: lower ÃŽâ€V on a modular lifter gives you a wider payload range within a given ÃŽâ€V range)

How would I "check the efficiency" of my lifter? Which efficiency do you mean? I'm already using the highest Isp and TWR engines I have available, and making sure that I'm not starting to burn precious hydrolox until I'm out of the lower atmosphere. LH2 fuel requires gigantic tanks, but tests showed that I still get slightly more dV than with common kerosene this way despite the extra dry mass I'm lugging around. (Fun fact: this rocket would be a quarter its current size if I was using kerolox, but even heavier.)

KER is known to have various bugs/shortcomings in the deltaV calculation code. What other mods are you running? I see some dual nozzle RCS thrusters. Is stage 0 ion-powered (or RCS powered)? Does it display correctly (or at all) if you remove the rest of the rocket? Can you post a shot with the "Show all stages" button selected? Have you tried the test version of KER in this thread?

It's the whole RSS suite and all its changes, so I wouldn't be surprised if KER tripped over something weird. S0 has a monopropellant tank with one of RLA Stockalike's tiny monopropellant engines; maybe KER just thinks that that's an RCS setup and doesn't calculate it. Come to think of it, I'm not sure if KER ever gave me dV for RCS equipped sections, like command pods and stuff, before.

The four dual-nozzle engines on the lander are not actually RCS, they're small liquid engines meant for orbital maneuvering and lightweight vacuum environment landers. Don't know which mod adds the model, but it's either NovaPunch or AIES, which are the two in the RSS suite that I haven't played with before.

Edited by Streetwind
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I've found that the best way to maximize dV is refueling. You could create an impressive asparagus staged rocket (preferably with only 1.25 m parts) and launch it to orbit without doing any staging (so it's intact) and then run a few trips up to refuel it. You can also ship fuel to your destination to use for the return. I managed to land on Eve and return using 3 launches this way and non-whackjobian proportions.

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I'm currently looking at fixing various RSS issues in KER (mainly engine thrust varying with Isp though there are other issues too). KER doesn't include RCS thrusters in deltaV calculations but an actual engine powered by monopropellant should work in the existing code as long as it only has one ModuleEngine[sFX] module (it should work in the new code even with multiple engine modules). Can you post your craft file (PM me a link if you don't want it public and/or cut it down to the top sections) so I can take a look?

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How would I "check the efficiency" of my lifter? Which efficiency do you mean?

Sorry, I guess efficiency is a bit of a generic term; LV efficiency is measured as the ratio of initial mass (LV + propellant + payload) to final mass (LV + payload). From Tsiolkovsky's, you can define it as eÃŽâ€v/ve where ve=g0Isp. This is why engineers obsess about "Mass Fraction" so much when doing napkin math; Euler's number, g0 and the needed ÃŽâ€v are fixed, so the only thing you get to consider is the ratio of fuel mass to dry vehicle mass and the specific impulse. Isp suffers from TANSTAAFL in most cases, so increases in Isp increase dry mass, which reduces efficiency. This is why the LV-909 is a good choice on some lifters vs. the LV-N on others. So, you optimize your design to the fuel mass fraction and then 'scooch' the efficiency up with your Isp, if there's a little wiggle room.

I'm already using the highest Isp and TWR engines I have available

For the raw ÃŽâ€v efficiency, TWR doesn't matter (see above). For an (efficient) atmospheric launch, you need only enough TWR to get you to the terminal velocity at a given altitude. For Kerbin, I eyeball a linear relationship from rest at sea level to 250 m/s at 10km (despite the fact that I know it's not linear, it makes for a easily remembered target velocity vs. altitude) and throttle accordingly. More than that and you're wasting fuel. After 10km, it's usually enough of a race between altitude and velocity that you can pitchover, find a happy throttle setpoint and let it run until your apogee hits 72km, cut throttle, coast to 65km, maneuver for your insertion burn, throttle up, pop your solar panels, verify transfer orbit, cut throttle, open beer.

Edited by PUNiSH3R
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I've found that the best way to maximize dV is refueling. You could create an impressive asparagus staged rocket (preferably with only 1.25 m parts) and launch it to orbit without doing any staging (so it's intact) and then run a few trips up to refuel it. You can also ship fuel to your destination to use for the return. I managed to land on Eve and return using 3 launches this way and non-whackjobian proportions.

It'll be a one-way trip only, so no need for return fuel. But you're right, I hadn't considered orbital refueling. I would still not get the dV much higher, I wager, but I could at least replace the over 9k that I'm using to reach orbit. The only problem is the giant size of those LH2 tanks, it'll be a pain in the rear to refuel that much volume. Still, I'll think about it...

I'm currently looking at fixing various RSS issues in KER (mainly engine thrust varying with Isp though there are other issues too). KER doesn't include RCS thrusters in deltaV calculations but an actual engine powered by monopropellant should work in the existing code as long as it only has one ModuleEngine[sFX] module (it should work in the new code even with multiple engine modules). Can you post your craft file (PM me a link if you don't want it public and/or cut it down to the top sections) so I can take a look?

Sure, here's the craft file for the lander. I stripped the lifter portion out because it is so large that it makes scrolling around the VAB a real chore :P I did a quick check on the parts used, and you'll definitely need AIES, NovaPunch, KW Rocketry, RLA Stockalike and SCANsat to load this craft. And KER of course. The data circuit is taped to the narrow decoupler between the lander and the satellite, which by the way is also the core part of the whole assembly.

Edited by Streetwind
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Sorry, I guess efficiency is a bit of a generic term; LV efficiency is measured as the ratio of initial mass (LV + propellant + payload) to final mass (LV + payload). From Tsiolkovsky's, you can define it as eÃŽâ€v/ve where ve=g0Isp. This is why engineers obsess about "Mass Fraction" so much when doing napkin math; Euler's number, g0 and the needed ÃŽâ€v are fixed, so the only thing you get to consider is the ratio of fuel mass to dry vehicle mass and the specific impulse. Isp suffers from TANSTAAFL in most cases, so increases in Isp increase dry mass, which reduces efficiency. This is why the LV-909 is a good choice on some lifters vs. the LV-N on others. So, you optimize your design to the fuel mass fraction and then 'scooch' the efficiency up with your Isp, if there's a little wiggle room.

I'm inferring from this description that LV here stands for dry mass... Right, I need to go plug some numbers into this, else I won't understand what this does at all :P Do I have to do this for each stage individually? And that will show me which stage is underperforming, basically?

For the raw ÃŽâ€v efficiency, TWR doesn't matter (see above). (...)

What I meant with that is that I aim to use engines which weigh very little for the thrust and Isp they offer. Think about it like the Poodle vs. LV-909 discussion; four LV-909's have the same thrust but less weight than one poodle, so their TWR is higher. I meant to say I am paying attention to that kind of stuff. (Besides, many engines in Realism Overhaul cannot be restartet and cannot be throttled, and the atmosphere goes to 105km, so my launch strategies are completely different anyway. :P)

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I'm inferring from this description that LV here stands for dry mass...

Sorry, more acronyms (at least it's not a TLA, right? ;) ), LV = Launch Vehicle, indicating the lifting system as a whole (fuel, engines, structure) so LV would be total mass minus payload. Initial mass (LV + Payload) is AKA wet mass, fueled mass, start mass, m0, ms, mstart. Final mass ((LV - Fuel Mass) + Payload) is AKA dry mass, m1, mf, mend, mfinal.

What I meant with that is that I aim to use engines which weigh very little for the thrust and Isp they offer.

Exactly, but if you can maintain a TWR>1 for any given launch phase and remove engines, you're reducing dry mass without sacrificing specific impulse, thus you're getting extra ÃŽâ€v. However I'm not familiar with your mod's thrust sources so that may not be an option for you.

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