Delta-V and more rockets?

[Nepos]

I noticed with engineering redux that adding another rocket, be it solid or not, can lower my total Delta-V. forexample adding 2 SRB gave me a higher Thrust to Weight Ratio but lowered my Delta V by about 300. So maybe I am not fully understanding Delta-V.I when to the wiki page on advance rocket designed and was totally confused

So If I have a rocket that has 4000 delta-V and I want another 1000 delta-V.

is it better to add another stage to get more delta-V?

is it better to add more fuel to a current stage, if the rocket can handle it?

How does Thrust to Weight Ratio (TWR) come into play with Delta-V

is 4000 delta-v at .75 TWR the same as 4000 delta-V at 2.5 TWR?

Thanks in advance.

Edited March 18, 2014 by Nepos

[Red Iron Crown]

TWR is not directly related to delta-V, though they are both very important considerations.

Delta-V only takes into account the Isp of the engine(s), the dry mass of the craft, and the fully fuelled mass. It is calculated as follows:

dV= Isp * g * ln(m0/m1)

Where:

dV is the amount of delta-V in meters per second

Isp is the Isp of the engine(s), a measure of its efficiency in seconds

g is the gravity constant, 9.82, in meters per second squared

ln is the natural logarithm function (available on any scientific calculator)

m0 is the mass of the ship fully fueled ("wet weight")

m1 is the mass of the ship empty ("dry weight")

To increase delta-V, use higher Isp engines, increase the amount of fuel on board, or reduce dry mass by eliminating unnecessary non-fuel parts.

Adding more engines generally reduces dV, as it is more dry mass.

Staging affects dV by reducing dry mass by shedding empty tanks and no longer needed engines. Multistage rockets calculate dV for each stage independently, with later stages forming part of the dry mass of earlier ones.

Delta-V is directly comparable between rockets of different TWRs, or masses for that matter.

Whether it is better to increase dV by adding another stage or adding fuel to the current one really depends on the rocket, would you care to share a screenshot of it so we can give you some feedback?

Edited March 17, 2014 by Red Iron Crown
Typos

[cantab]

You'll want to double-check, and maybe rethink, your staging. I'm not sure on the detailed maths, but you want your boosters helping to lift your rocket and not your rocket having to lift your boosters.

[Jimbimbibble]

Research asperagus staging. It can significantly increase your dV. Also, remember that if you TWR is not high enough the rocket won't be able to take off. On Kerbin, you don't want to go much below 2 on takeoff, but your upper stages can be a bit lower. Also, when you're already in orbit, TWR is less important. It just takes a really long time to do stuff with very low TWR.

[capi3101]

Red Iron Crown's got it down - to improve delta-V, you want to either add more fuel, reduce the dead mass (by eliminating unnecessary parts and/or staging), or use more efficient engines. Which method you use is your choice, really.

For asparagus: http://forum.kerbalspaceprogram.com/showthread.php/28248-Is-asparagus-the-best-staging-system-%28might-contain-science%29?p=346702&viewfull=1#post346702

That post is old, but asparagus remains the most efficient staging method in KSP and Temstar's principles still apply (mind you that some find his engine solutions overly complex; I am not one of those individuals).

Delta-V is delta-V; TWR does not factor into it. Now, TWR does measure how well your rocket can lift itself, and you can lose delta-V to either gravity (if the TWR is too low) or drag if it's (too high), so TWR is of some importance when you're launching. Ideally you want your TWR in flight around 2.2 and in practice that means the stage's initial TWR should be somewhere around 1.6 or 1.7 (it'll get to that "sweet spot" as it flies). In general, bottom minimum TWR you want is 1.2; below that the gravity losses become substantial.

[Starstrider42]

I noticed with engineering redux that adding another rocket, be it solid or not, can lower my total Delta-V. forexample adding 2 SRB gave me a higher Thrust to Weight Ratio but lowered my Delta V by about 300. So maybe I am not fully understanding Delta-V.I when to the wiki page on advance rocket designed and was totally confused

This sounds like your boosters are running out of fuel at about the same time as one of your "main" stages (you can check this in Kerbal Engineer; it's the last column on the full display). That means that the boosters+stage act basically as one slightly heavier and less efficient stage (solid rockets have lower I_sp than liquid ones) rather than as two separate stages.

Try rearranging the staging so that the main stage only turns on after the boosters have been dropped away, and see if that still gives you good TWR. In my experience, solid boosters are often overpowered, so they may be able to support your rocket by themselves.

[Poynting]

Just to add:

Thrust to Weight Ratio (TWR) is a much simpler concept than Delta V. It is purely the ratio between the upward thrust force of your engines and the weight (downward force) of the ship. If it is not greater than one your ship cannot lift its own weight.

TWR = Thrust/Weight

(Where weight is in newtons and is Weight=mass*g)

So to illustrate the the difference between TWR and Delta V imagine two ships in space (Max thrust 100 and mass 10 [we'll also round g to 10m/s/s]). The first ship goes to 20% throttle (or 1/5th thrust) and burns all of its fuel. It is now travelling at 4000m/s relative to the second ship. In other words it had 4000m/s Delta V. At 20% throttle its TWR was 20/(10*10) = 0.2

The second ship then burns all of its fuel but at 100% throttle. As it had the wet weight, dry weight and ISP as the first ship it too is now travelling at 4000m/s. However we can already see, TWR = 100/(10*10) = 1, that its TWR is greater. So a change in TWR need not affect the Delta V of the ship. However weren't we running the first engine at 20%? Coundln't we have just used a smaller engine? Yes and a smaller engine weighs less, therefore our wet and dry mass both reduce, which in turn give us more delta v!

(The increase in Delta v arises from the fact that we now have a greater percentage of our ship being fuel. Therefore the mass ratio in the Dv equation gets larger)

Edited March 17, 2014 by Poynting

[Nepos]

Thanks for the answers. I will post a pic of my rocket that I noticed it on (wont happen till after work in about 6 hours). Better than starting Manned mod limites you early on as to what you can and cannot do so I do not have any side decouplers so asparagus staging is out. one the very basic stage decoupler. It has been an interesting play as I am learning more and more how rockets design works in this game. So smart rocket design has become very important to me.

When I post my images I will include the data for the engines as well.

Thanks again

[Red Iron Crown]

Since you're using Engineer Redux anyway just have the window open when you take the screenshot, it will give us hard data on your design.

[Starstrider42]

I do not have any side decouplers

Ok, then forget my previous advice. New advice: don't use boosters unless you really need the TWR, they'll just increase your dry mass. Use vertical staging instead.

[wilt57]

is 4000 delta-v at .75 TWR the same as 4000 delta-V at 2.5 TWR?

Thanks in advance.

This matters where you're at. If you have already achieved orbit, 4000 delta-v is 4000 delta-v. Both rockets will be able to get to the same places, assuming you don't have too long of a burn with the .75 TWR rocket.

It's different when considering landing or launching from a gravitational body. If your TWR local to the body you are trying to launch from is lower than 1, you will have to burn off reaction mass to lighten your ship before it lifts off. You have lost that available delta-v because you were not able to use that reaction mass to either increase or decrease your rocket's velocity. If your TWR is above 1 but still very low, you will incur excess gravity losses during your ascent. If your TWR is too high and you are launching from a body with an atmosphere, you will incur excess drag losses. In either case, you will incur both gravity and drag losses, the key is to minimize their sum. That occurs at terminal velocity. So to reduce the amount of delta-v lost during ascent, you need to match your TWR to a value that allows you to fly as close to terminal velocity as possible. This is at least important early in the launch, it is generally too cost prohibitive to maintain terminal velocity in the later stages of your launch when you are traveling through the thin upper atmosphere.

[Guest]

Rule of thumb: boosters for insufficient TWR, upper stages for insufficient dV. You may start with adding upper stages until you run out of TWR, then add boosters. Since you've got KER, you can do this in VAB, and not test every configuration in-flight.

[EtherDragon]

As was hinted at in this thread by others:

A craft's Total Delta-V has everything to do with it's full to empty mass ratio and the efficiency of the rocket motor. It has nothing (that is zero, that is mathematically verifiably nada) gained from TWR. Increasing your TWR lowers your Delta-V because you have to add rocket motors (which contributes to dry mass).

Simply put, the less dry mass you have, the more Delta-V you get.

However, consider where Delta-V is really useful - the main purpose of having lots of Delta-V is to go really far, in a vacuum. The more Delta-V you have, the larger your mission can be, from orbit. But first you have to get your ship into orbit, that's where TWR comes in.

During ascent, from the ground, some portion of your thrust is spent overcoming gravity. The longer you spend reaching a stable orbit, the more time you spend wasting fuel against gravity - which is lost Delta-V. You must have a TWR > 1 in order to get off the ground. The more TWR you have during ascent the less energy is "wasted" getting into orbit (to a point).

From the ground, it actually takes more Delta-V to reach orbit the closer to 1 your TWR is. The higher your TWR, during ascent, the less Delta-V required to reach orbit. So, ascent stages are complex things where you balance the Total Delta-V against the TWR to achieve a design that has the most Effective Delta-V. You may find a design that has less "Total" Delta-V, but is able to achieve more because of a better TWR.

[Streetwind]

You'll want to double-check, and maybe rethink, your staging. I'm not sure on the detailed maths, but you want your boosters helping to lift your rocket and not your rocket having to lift your boosters.

That's probably the post which most directly addresses the problem reported.

A solid booster is a bunch of fuel with an engine that produces more thrust than it needs for itself, and thus it can help move the rocket. The worst case scenario is that the booser is not strong enough to to overcome gravity, in which case your rocket sits still on the launchpad. Your net gain is 0 dV and your net loss is also 0 dV. In all other cases you should gain at least some dV.

There are only two ways for solid boosters to reduce total delta-V, and both have to do with improper staging:

- You continue carrying the spent booster housings after they burned out for so long that the total effort required to carry them is greater than the benefit they brought

- You are using solid boosters on an upper stage, meaning you are required to lift the fully fueled booster up out of the gravity well, which reduces the dV of your lower stages by more than the boosters add to the upper stage

Generally, the rule of the thumb for maximizing dV goes "burn your least efficient fuel first". For example, if you have the choice between using your main liquid fuel engine and the RCS thrusters for an orbital maneuver, using the RCS thrusters first even if you have lots of liquid fuel left will maximize your craft's dV. The rocket will be at its lightest by the time it starts burning its most efficient fuel, which will give it the biggest possible benefit. For the same reason, it's usually a bad idea to have a liquid launch stage and then a solid booster assisted upper stage, because solid boosters are a lot worse in terms of Isp than liquid engines. You're effectively spending your best fuel simply to push a load of crap fuel through atmospheric resistance while fighting gravity. Better to do it the other way round, no?

And of course, drop the spent boosters as soon as they burn out. If you have only vertical staging available, but still need to build outward with boosters, then design the stage so that either the center stack is also an identical booster or so that all active components in it burn out at the same time. That way you can decouple the entire lower part of the rocket at once and don't need radial decouplers.

[KerikBalm]

Generally, the rule of the thumb for maximizing dV goes "burn your least efficient fuel first". For example, if you have the choice between using your main liquid fuel engine and the RCS thrusters for an orbital maneuver, using the RCS thrusters first even if you have lots of liquid fuel left will maximize your craft's dV. The rocket will be at its lightest by the time it starts burning its most efficient fuel, which will give it the biggest possible benefit. For the same reason, it's usually a bad idea to have a liquid launch stage and then a solid booster assisted upper stage, because solid boosters are a lot worse in terms of Isp than liquid engines. You're effectively spending your best fuel simply to push a load of crap fuel through atmospheric resistance while fighting gravity. Better to do it the other way round, no?

Except... it can get even more complicated. For example, consider the case of a final stage consisting of a command pod with 2x sepratrons, and a liquid fuel engine underneath on a decoupler. If you burned the sepratrons first, then fired the liquid engine, you'd not get much benefit from the sepratrons. But if you burn the liquid engine, jettison it, and then fire the sepratrons, you'll get more dV. In this case, it goes back to mass ratio.

Also I'd say we should talk about engine efficiencies, not fuels. We could do another case with 2 stages of FL-T200's and an engine - assume a lander can or even a command seat. One is a nuke, the other is a 48-7s.

Burning the least efficient engine first is only good if the mass ratios are equal. Due to the mass of a nuclear engine, its more efficient to burn the nuke, then jettison it, and then burn the 48-7s.

Or we could do a more reasonable case of a 48-7s and 2x oscarB/round-8 toroidal tanks, and a FL-T400 tank + nuke.

For such a system, you'd want to use the 48-7s last.

Also, with RCS, you could put (a) RCS port(s) on a command pod, in which case you'd want to not use the rcs it carries for maximum dV (assuming its just hte command pod, and nothing else that you are trying to push)

Its ok to have a lower ISP upper stage, if its mass ratio makes up for it, and you can't get a similar mass ratio with a higher ISP engine.

However, given the mass of all solid boosters except sepratrons, there are better options. SRBs should only be in the bottom stages... pretty much just like mainsails

[Red Iron Crown]

Increasing your TWR lowers your Delta-V because you have to add rocket motors (which contributes to dry mass).

This is not universally true, though it is commonly the case. Example: Probe core, orange tank, one 48-7S vs Probe core, orange tank, one LV-N; the LV-N powered ship has higher TWR and delta-V.

[Streetwind]

Except... it can get even more complicated.

Yeah, that's why it's just a rule of the thumb

If the entire upper stage weighs less than the engine in the one below it, then you'll of course want that engine to work itself dry first so you can jettison it.

[KerikBalm]

Yes, he should have stupilated: *for a given engine type* or *engines of a given ISP* "Increasing your TWR lowers your Delta-V because you have to add rocket motors (which contributes to dry mass)."

ie, increasing TWR by adding more engines of the same ISP lowers your delta-V

[Nepos]

never been one of the great rocket designer. Most of mine are monstrosities. However I am getting better. Here is a set of screen shot of the 2 versions of the lander I was trying to to MUN.

I can get the lander there if I had solar power however all I have at this moment is battery and I only have 1 engine that produces electricity. Those dang things take up most my delta-V.

In the images you see the final design I tried but alas it is top heavy. back to the drawing board. The info you guy have provided me explains alot of why my previous designed failed. I added more rockets thinking that will get me more Delta-V and it did not because of the dry mass. Maybe I will just settle for a sat in polar orbit around the Mun to finishes off that science. I will figure it out and land something on the Mun soon.

Thanks again for all the useful info on this it has been very helpful

[capi3101]

One for the road - Geschosskopf's got a good tutorial on planning for science, link's here.

What techs exactly have you unlocked? We can work with that and help you build a better lander system; your original lander design would've been light on what it needed for a successful landing and return as it was, looking at your screenies. You can also build radially without adding decouplers - I should mention that.

One of my early designs using demo-level tech. You can build a booster like this without necessarily having fuel lines (if you don't have those yet), though whether it'll get you into orbit is another matter.

Concerns me a bit that you have orange tanks already but no solar panels...unless you were building that design in the sandbox; that would make sense (its not that you don't have panels, you just aren't using them).

Edited March 18, 2014 by capi3101

[Dispatcher]

Another thing which I usually overlook due to lack of need is that you can stage not only engined tanks, but you can stage just fuel tanks, without engines being jettisoned with them. In fact, for really tough loads, I asparagus stage fuel tanks. This works as long as you have sufficient TWR for the specific application, such as launching.

Edit: this assumes that you have radially connecting parts which allow fuel cross feed, such as struts (not strut connectors) or fuel lines.

[cantab]

Worth bearing in mind that even though adding the boosters lowers the nominal delta-v, by raising TWR they will probably raise the effective dV.

A change to the design you might try, for your first stage instead of using 2.5m tanks with 4 engines, use seven stacks of 1.25m tanks (one central and 6-fold radial) with 7 engines. Then a decoupler under each engine, and a booster below that. That should give you better TWR and better dV, though it means a long rocket.

Another thing you might try is mounting an appropriate structural part radially, then a stack decoupler below that, then your booster, thus giving you a pseudo-radial decoupler.

Oh, and move the fins to the bottom, they work better that way.

[Nepos]

Oh in the mod Better than staring manned you are limited in the beginning and some things do not work the same way. Right now I have only one de-coupler with no side de-couplers. Also I finally got the strut.

So I have to go vertical. I also tend to think BIGGER IS BETTER and that is not always the case. As for what I have with no solar panels is that the tech tree is different in BTSM. here is an older image of my tech tree. I currently have all but two of those. None of those have solar panels but do have the orange tank. In vanilla KSP I have no problem getting most places and never concerned myself with Delta-V. In BTSM it is different story since it is harder but for me more fun.

In the end it is all about fun.

Thanks again for all the info it was very helpful

[capi3101]

Ah, that makes sense then. Okay. I remember that the parts get unlocked in different order with that mod, so I'll have to review it and get back to you, see what you can do with the tech you've got.

[capi3101]

Okay......so I've looked a little bit at BTSM, and so far as best I can tell you've got these parts available -

Stayputnik

RT-10 SRB

Thermometer

Communotron 16

AV-T1

RT-20 BACC SRB

POT-1 BATT (50e, MASS 0.0375)

BAROMETER

KKII Unpressurized Cockpit (2t)

QBE

AV-R8

NOSE CONE

GRAVIOLI DETECTOR

LV-T15 (1.5, 215 Kn, 310/350 iSP)

FL-T800

Mk-16 CHUTE

RCS BLOCKS

SMALL RCS TANK

ASAS

FL-T200

FL-T400

LV-909

OCTAGONAL STRUT

LAUNCH CLAMP

BI-ADAPTER

DECOUPLER

Mk0 cockpit (1.5 t)

cylinder rcs

1.25 heatshield (didn't catch the stats on this one)

POT XL-2 BATT (didn't catch the stats on this one)

Where does the orange tank come into play with the BTSM mod? I didn't get that far.

How similar is the LV-T15 to the LV-T30? I know the -T30 will generate electricity; is the same thing true of the -T15? If so, you might be able to work with that a bit.

You might be able to pull some of the magic I'd ordinarily do with Modular Girder Segments with the Modular Girder Adapter instead; I'll definitely need to do some research.

From your rocket screenies, you're definitely further along the tech tree than just the first ten techs...do you have a more up to date tech tree screenshot?