A bunch of LV-N motors in one stage does not equal same amount of LV-N motors staged seperately?WAT!

[Veraster]

So I have a ship with 25 of those LV-N atomic rocket engines. Here is a picture of it.

It's got a total delta v of like 16000 m/s. That big array of engines is separated into 3 stages.

However, I was trying to find a way to get that 16 km/s of delta v into one stage.

When I just merge all the engines into ONE STAGE, it says I only have 7km/s of delta v.

WHY?!? They all have the same isp! Each engine has it's own fuel tank. All the fuel tanks are exactly the same size. How can combining a bunch of these together be less efficient than only using some of them at the same time?

[Red Iron Crown]

It is because when you stage the empty tanks and their engines away the next stage doesn't need to accelerate all that mass. As the craft gets lighter from staging things away, it becomes easier to accelerate so the same fuel delivers more delta-V.

There are four basic ways to increase delta-V:

1. Decrease any mass that is not fuel.

2. Increase fuel mass.

3. Use engines with higher Isp.

4. Split the craft into stages.

It looks like you have 1 through 3 taken care of (it's nearly all fuel tank, very little payload, and using high efficiency nukes). Number 4 is where you are seeing gains in your two configurations.

[Taki117]

The most efficient way to get the most dV is to run fuel lines from your outer tanks to your inner tanks, this way your inner tanks contribute to your initial TWR, but remains fuled for when you stage (If you're not doing that already)

[Wanderfound]

And the other point: that many LV-R's in one stage are counterproductive. Less engines, more ÃŽâ€V.

[maltesh]

Aye, to get 16 km/s of delta-v in a single stage, using the LV-N,your spacecraft will need to be roughly 87% fuel by mass. The limit of what you can get from standard 1.25 and 2.5 meter fuel tanks would be just shy of 89% fuel by mass. Chasing that level of diminishing returns is probably not worth the effort.

[Taki117]

Alright, time to go in a different direction. Why do you need 16km/s vacuum dV? Instead of brute forcing whatever you're doing (Like a Moho Transfer) try optimizing your flight plan.

[Deutherius]

What the guys above me said. If you're interested in the math behind it, it's pretty simple. The deltaV you get from one stage is calculated as:

dV = I_sp*g₀*ln(Mass_full/Mass_empty)

When all the stages are consolidated (all engines fire at once - the entire craft is one stage):

dV = 800*9.82*ln(194310/74311) = 7856*ln(2.6148) = 7856*0.9612 = 7551 m/s

When you have different stages, each having their own nukes active, you just calculate the dV of the separate stages and sum the results:

dV = 800*9.82*ln(194310/107904) + 800*9.82*ln(54630/25828) + 800*9.82*ln(8820/4020) = 4621 + 5885 + 6173 = 16679 m/s

[Skorpychan]

That's because you're hauling around all that dry mass of heavy engines. Either make a longer burn, or use a higher-thrust engine.

A big orange tank with one or two LV-Ns would work fine for 16K dV.

[maltesh]

That's because you're hauling around all that dry mass of heavy engines. Either make a longer burn, or use a higher-thrust engine.

A big orange tank with one or two LV-Ns would work fine for 16K dV.

That's not a high enough mass ratio, actually. A spacecraft composed of a single LV-N, three full Rockomax Jumbo-64s and a Probodobodyne OKTO-2 just barely scrapes past 16 km/s of vacuum delta-V in a single-stage.

[maccollo]

The highest amount of delta V you can actually cram out of any stage on the ISP of your engine(s) and the fuel fraction of the tanks.

In KSP most tanks have a fuel fraction of about 0.889, so with the LVN the absolute most your can get is about 17 km/s.

To get this amount of deltaV out of the stage you need to have a whole bunch of fuel tanks, almost no payload, and a single engine.

This can never actually be an efficient setup, because in the fuel tanks will actually absorb the vast majority of impulse that the engines generate. In your it's just as bad, because you are pushing almost 43 tonnes worth of engines in addition to the 12 tonnes of fuel tank, whole your payload (the capsule) represents less than 2% of the stage mass. Terrible!

When trying to make an efficient stage you should avoid this. The majority of the mass that you are pushing should be fuel and payload. When the dry mass of the stage begins to approach 30% of the total mass of your space stage you should probably think about staging.

Edited March 29, 2015 by maccollo

[OhioBob]

When the dry mass of the stage begins to approach 30% of the total mass of your space stage you should probably think about staging.

I think that's the point when you should start to think about staging, though I've found it practical to run that percentage to about 20% before staging really becomes necessary. Most of my small launchers are in the 25-30% range, but the larger ones (say >100 t total launch mass) are about 20%.

Another reason for staging is to limit the acceleration loads, though this is more of a real-life issue than it is a KSP problem. Let's say you have a single stage launcher that is 90% propellant and 10% dry mass. If your initial thrust-to-weight ratio is 1.2, you're TWR will max out at 12 by the time the engine burns off the last of the propellant (assuming fixed thrust). That high a g-load will likely cause your crew to blackout. Staging allows you to not only drop unneeded dry mass, but to also switch to lower thrust engines to manage the acceleration loads. Since all engines in KSP are throttleable, you can avoid this problem by simply dialing back the thrust. Most real-world engines don't have a throttle.

[Technical Ben]

What is the theoretical max? I think it's around the 16k mps... as that's what most my interstellar stock designs top out at before I add payloads.

[OhioBob]

What is the theoretical max?

For a single stage?

The highest possible mass ratio for just a propellant tank, without engine or payload, is 9. This gives a theoretical ÃŽâ€v of a little more than 17 km/s. Of course this is impossible to achieve because we must add the mass of the engine and at least a probe core. From my experience, I believe a mass ratio of 5 is about the highest that can be practically achieved. With an LV-N engine, a mass ratio of 5 yields a ÃŽâ€v of 12,630 m/s; however, this also gives a paltry TWR of only 0.27.

If we consider just the weight of the LV-N engine and fuel tank, and assume a fuel tank propellant fraction of 8/9, we can compute the ÃŽâ€v and TWR ratio for any given mass ratio.

[TABLE=width: 500]

[TR]

[TD=align: center]Mass

Ratio[/TD]

[TD=align: center]ÃŽâ€v (m/s)[/TD]

[TD=align: center]TWR[/TD]

[/TR]

[TR]

[TD=align: center]2[/TD]

[TD=align: center]5,440[/TD]

[TD=align: center]1.19[/TD]

[/TR]

[TR]

[TD=align: center]3[/TD]

[TD=align: center]8,622[/TD]

[TD=align: center]0.68[/TD]

[/TR]

[TR]

[TD=align: center]4[/TD]

[TD=align: center]10,880[/TD]

[TD=align: center]0.42[/TD]

[/TR]

[TR]

[TD=align: center]5[/TD]

[TD=align: center]12,631[/TD]

[TD=align: center]0.27[/TD]

[/TR]

[TR]

[TD=align: center]6[/TD]

[TD=align: center]14,062[/TD]

[TD=align: center]0.17[/TD]

[/TR]

[TR]

[TD=align: center]7[/TD]

[TD=align: center]15,272[/TD]

[TD=align: center]0.10[/TD]

[/TR]

[TR]

[TD=align: center]8[/TD]

[TD=align: center]16,319[/TD]

[TD=align: center]0.04[/TD]

[/TR]

[/TABLE]

[thadisjones]

They really need to implement some harsh disincentives for people to stage FISSION REACTORS and then drop FISSION REACTORS with no recovery parachutes or guidance on ballistic trajectories onto an inhabited planet. You'd think the high cost, bad atmospheric performance, and abysmal TWR would make people stop this insanity by default, but just from looking at this abomination, this guy is obviously dropping unguided FISSION REACTORS onto the KSC area from a few kilometers up with no intent of landing them intact or recovering them.

[How2FoldSoup]

They really need to implement some harsh disincentives for people to stage FISSION REACTORS and then drop FISSION REACTORS with no recovery parachutes or guidance on ballistic trajectories onto an inhabited planet. You'd think the high cost, bad atmospheric performance, and abysmal TWR would make people stop this insanity by default, but just from looking at this abomination, this guy is obviously dropping unguided FISSION REACTORS onto the KSC area from a few kilometers up with no intent of landing them intact or recovering them.

The nuke engines have a TWR <1. It's impossible to launch from kerbin using purely nukes..If you looked at the KER readout his TWR is only .69. I don't see how he's launching anywhere from Kerbin. It would make sense if this rocket was his payload.

[Red Iron Crown]

They really need to implement some harsh disincentives for people to stage FISSION REACTORS and then drop FISSION REACTORS with no recovery parachutes or guidance on ballistic trajectories onto an inhabited planet. You'd think the high cost, bad atmospheric performance, and abysmal TWR would make people stop this insanity by default, but just from looking at this abomination, this guy is obviously dropping unguided FISSION REACTORS onto the KSC area from a few kilometers up with no intent of landing them intact or recovering them.

FISSION REACTORS are not as dangerous as you are implying, and those same FISSION REACTORS are in heavy casings and a non-critical state. The cleanup of a splashed or dropped FISSION REACTOR is not nearly as difficult as the cleanup after a FISSION REACTOR has gone supercritical or melted down. Frankly, I think you are buying into the real-world hysteria about FISSION REACTORS and nuclear things in general, when the simple fact is FISSION REACTORS are clean and safe in comparison to almost every other power source. (Fun fact: Wind generators are extremely dangerous and have killed far more people than FISSION REACTORS despite being in general service for far less time).

Also, why do you care what other people do in their game? Handle your own FISSION REACTORS however you'd like and stop trying to enforce your rules on others.

The nuke engines have a TWR <1. It's impossible to launch from kerbin using purely nukes..If you looked at the KER readout his TWR is only .69. I don't see how he's launching anywhere from Kerbin. It would make sense if this rocket was his payload.

Check your math, an LV-N has a Kerbin-relative TWR of 2.7. While the OP's rocket is TWR<1, it is possible to get to space on LV-Ns alone.

Edited March 29, 2015 by Red Iron Crown

[Streetwind]

What is the theoretical max? I think it's around the 16k mps... as that's what most my interstellar stock designs top out at before I add payloads.

The theoretical maximum - in KSP, with liquid fuel - is 21.5 times your Isp. This directly derives from the ratio of fuel mass to dry mass in LF/O tanks. Other values do not influence it.

This assumes that all the weight of the vehicle is in fuel tanks. In other words - either your engine and other non-tank parts are massless, or you are carrying the entire observable universe's mass in fuel.

In practical application, since there is no massless engine or control equipment, what you can actually achieve will be lower than 20 times your Isp.

[Mr. Scruffy]

[...] (Fun fact: Wind generators are extremely dangerous and have killed far more people than FISSION REACTORS despite being in general service for far less time) [...]

[...]

The statistics this ´fun fact´ is based on, must be so heavily skewed, it´s not funny anymore. It must count all death in which wind generators are somehow involved (like crazy people trying to climb them or whatnot), while skipping all cases of cancer that can not be attributed to nuclear power with 100% certainty (which is quite impossible).

If done the other way around, wind generator would come out in absolute numbers close to zero (despite having been used for centuries - windmills are not exactly new), while nuclear energy would rake up hundreds of thousands, the first from bombs alone. Skewed statistics. They dont prove anything and 83% of people know that.

To add something on-topic: Maybe there could be a ´strategy´ (´clean atmo´ or something) which would penalize you for using and dropping LVNs in kerbin´s atmosphere, but rewards you in some other way, when you manage to avoid it. Compromise between both sides.

[Red Iron Crown]

I don't want to drag the thread any further off topic, Mr Scruffy, PM sent.

[John FX]

Also the NERVA is only the best engine for a TWR between 0.4 and 0.8. Below 0.4 and you are better off with the ION engine and above that the surprise contender is the KR-2L for a lot of cases when you are seeking the maximum Dv for a particular payload.

[Deutherius]

In practical application, since there is no massless engine or control equipment, what you can actually achieve will be lower than 20 times your Isp.

The O-10 monoprop engines are massless, as are RCS ports and Vernor engines. No massless control parts that I know of, though, so your point still stands.

[UmbralRaptor]

...bad atmospheric performance...

Isp parity with the Aerospike is achieved at 1717 m on Kerbin. (Though TWR is still a bit of an issue at that altitude.)

[rkman]

Another approach: the entire point of staging is to get more dV for the same amount of fuel. Without that, staging would not be a thing.

[Streetwind]

The O-10 monoprop engines are massless, as are RCS ports and Vernor engines. No massless control parts that I know of, though, so your point still stands.

That's also why I qualified my statement as "in KSP, with liquid fuel"

Monopropellant tanks have different (worse) fuel mass fractions than LFO tanks, and therefore have a different (lower) maximum performance number, and a different number in the best possible realworld case as well. However I have not done the math on it, so I couldn't tell you which number it is right now.