My dumb a** deltaV question

[maceemiller]

Hi all....OK, so im gunna look like a fool here but I dont understand something.

The scenario is this, Im trying to get to Moho. I have built "straight" rockets and have seen the deltaV go down in stages as more parts were added, that I understand, so I thought I'd try something different, ie. assemble a rocket in LKO but attatch the 2 parts side-by-side (the pictures I have provided should give you a better idea of what I mean).

What I did was make "the left, or top side of the craft" which gave me a dV total of 5025 when in orbit. I basically sent up a duplicate of this (with a few more parts added but no biggy) expecting that when I docked with the first craft I would have another 5000 or so of dV, except I've ended up with under 5000 dV when docked together?

Im sure there is a clear and obvious answer to this but atm I'm just not seeing it

Edited August 15, 2015 by maceemiller

[SAI Peregrinus]

Try working out the math:

delta-V = v_e * ln(m0/m1)

If you keep the mass ratio (mass with full tanks / mass with empty tanks) the same, and the exhaust velocity the same (same engine type), you get the same delta-V. If you change the mass ratio (say, by having an extra engine and extra parts) then your delta-V changes, in this case for the worse.

[maceemiller]

Try working out the math:

delta-V = v_e * ln(m0/m1)

If you keep the mass ratio (mass with full tanks / mass with empty tanks) the same, and the exhaust velocity the same (same engine type), you get the same delta-V. If you change the mass ratio (say, by having an extra engine and extra parts) then your delta-V changes, in this case for the worse.

I understand that however what im saying is from having 1 side in orbit with 5000 dv then sending up a duplicate (even with the extra small parts I put on it I could live with losing say 1000dv) I would expect that my dv would go up (5000+say 4000) would equal 9000dv, not drop to under just having 1 side? Where has all that dv gone? Its like I may as well never bothered sending more up to add to it?

Edited August 15, 2015 by maceemiller

[Mastikator]

I understand that however what im saying is from having 1 side in orbit with 5000 dv then sending up a duplicate (even with the extra small parts I put on it I could live with losing say 1000dv) I would expect that my dv would go up (5000+say 4000) would equal 9000dv, not drop to under just having 1 side? Where has all that dv gone? Its like I may as well never bothered sending more up to add to it?

That's the tyranny of the rocket equation, the more fuel you add the less each kg of fuel is worth in terms of dV. What you want then is the highest possible Isp, so if you want some 15km/s+ dv you'll want the ion propulsions.

[Red Iron Crown]

If you are meaning to stage away the smaller module when it is empty: It's the docking ports, dV calculators like MechJeb and KER do not treat them as decouplers so they think it is a single stage design. You can get them to calculate it for you in the VAB by loading your main module, then mounting the other module to it with a decoupler and connecting with a fuel line. In flight you have to do it by hand or exit out to the VAB and check it your prototype with your current fuel state.

If you are meaning to keep it all as a single stage: You are running into the tyranny of the rocket equation. If there was no payload and the modules were identical, one module would have the same dV as two, or ten, or a thousand modules put together (you can check this quickly in the VAB by building clusters of SRBs). Because there is a payload, there is a small gain to be had from adding additional modules as that payload mass is effectively split among them, but those gains are small and get smaller with more and more modules.

Edit: Ninja'd about the tyranny.

[maceemiller]

If you are meaning to stage away the smaller module when it is empty: It's the docking ports, dV calculators like MechJeb and KER do not treat them as decouplers so they think it is a single stage design. You can get them to calculate it for you in the VAB by loading your main module, then mounting the other module to it with a decoupler and connecting with a fuel line. In flight you have to do it by hand or exit out to the VAB and check it your prototype with your current fuel state.

If you are meaning to keep it all as a single stage: You are running into the tyranny of the rocket equation. If there was no payload and the modules were identical, one module would have the same dV as two, or ten, or a thousand modules put together (you can check this quickly in the VAB by building clusters of SRBs). Because there is a payload, there is a small gain to be had from adding additional modules as that payload mass is effectively split among them, but those gains are small and get smaller with more and more modules.

Edit: Ninja'd about the tyranny.

Told you it was a dumb question to ask. If im honest it still makes no sense to me......all that dv I sent up just doesn't exist? All that fuel is effectively unusable? Anyway, thank you for your reply, I do appreciate your time, even if I dont get it.

BTW, I was intending to keep them as a single stage....a waste of time that was

[GoSlash27]

Told you it was a dumb question to ask. If im honest it still makes no sense to me......all that dv I sent up just doesn't exist? All that fuel is effectively unusable? Anyway, thank you for your reply, I do appreciate your time, even if I dont get it.

BTW, I was intending to keep them as a single stage....a waste of time that was

The short answer is DV is not additive like you were expecting. It's proportional to the natural log of the wet mass over dry mass.

You effectively doubled the mass of the fuel, which is good. But then in addition you also doubled the mass of the tanks (which you have to do to hold the fuel) and engines (which you didn't have to do). Plus throw in the random equipment to connect them together/ etc.

You haven't improved your mass ratio at all. In fact, you made it worse, so DV goes down. Your acceleration did improve, but that's not what you were after.

If you were to try it again with just an additional fuel tank, you would see an improvement in DV. Not a doubling, but an improvement. If you were to fire the attached rocket by itself and then discard it when empty, you would also see an improvement. Again, not a doubling.

And no, not a dumb question. You have to ask questions like this to get a handle on how the rocket equation works. It would only be dumb if you didn't ask.

HtHs,

-Slashy

[KrazyKrl]

Told you it was a dumb question to ask. If im honest it still makes no sense to me......all that dv I sent up just doesn't exist? All that fuel is effectively unusable? Anyway, thank you for your reply, I do appreciate your time, even if I dont get it.

BTW, I was intending to keep them as a single stage....a waste of time that was

Think of it like this:

Each module has a "no payload" maximum dV it can provide. As you add more modules, the efficiency of each one drops significantly. You're never really adding dV, you're making the fuel fraction so large; that your payload becomes a small part of the fuel you are using.

It's the same reason any engine can never reach the engine's max TWR, because that would require a 0kg mass to be on the engine.

[LordCorwin]

Imagine you have a car that weights 1000Kg and a engine of 100HorsePower. You put 50l of gasoline and you can travel 500 kilometers, ok?

Now you change this car for a different one that weights 2000Kg has an engine of 200HP and brings 100 litres. In this conditions, you also will travel 500 kilometers.

You are maintaining the same ratio of mass / engine power / engine efficiency / fuel. The situation won't change.

That's because you're merging the 2 ships in paralel. Try to put 1 after the other, as a wagon. The idea is to ignite first one, then the other. You won't travel 500 + 500 km, but may be 200 km + 500 km.

I know that the simile is crude, but may be it will help you to understand the situation.

Edited August 15, 2015 by LordCorwin

[Sharpy]

If you have a payload of 10 tons on a rocket with 5000m/s dV, you can double the payload by doubling the rockets, accelerate 20 tons to 5000m/s with twice as many boosters.

But think about this: you have a payload of 10 tons on top of a booster which weighs 10 tons with its fuel and all. If you send another similar booster, it will have to push both the payload and the first booster. It's calculated to produce 5000m/s with a payload of 10 tons, and now its payload is 20 tons. For the former, the *average* weight pushed was 15 tons, the middle between full and empty booster plus the constant payload. For this one, the average is 25 tons! It will get maybe 3500m/s. Then the other booster will engage producing its full 5000, and so you'll end with 8500 instead of 10.

And it doesn't matter if the boosters fire together instead of in order. That will change the time they take to reach the final speed, but won't change the resulting speed.

To overcome this, you need your stages increasingly more powerful. As your final stage produces 5000m/s for 10 tons, your earlier stage must handle 5000m/s for a payload of 20 tons, and be correspondingly larger, maybe 15 tons itself. And if you want to propel it by a third stage, it will need to handle a payload of 10+10+15 tons, not just the same initial 10. Or just accept that the earlier stages will provide much less delta-V than the later ones if you want to reuse the same design.

Let me show you what I mean. This thing is meant to deliver some 25,000m/s to bring a kerbal from a distant retrograde sun orbit:

The first stage is meant to deliver it into orbit. It has only some 4000m/s in it for a payload of 150 tons. That's about the weight of the second stage, the orange tank and the two MK3 tanks.

The second stage can accelerate the payload in the fairing by some 8000m. Payload of 17 tons.

Now what's under the fairing? This:

A single nuke with three MK1 LF fuselages. It can produce about 3000m/s, which should allow me to bring the final part to encounter with the stranded kerbal.

Now what's left is the way back.

KER estimates the delta-V at 10,000m/s but it doesn't understand the kind of staging I'm going to use by manually transferring fuel. It's going to be at least 15,000 m/s across four stages, ion engines and fuel cells, extremely efficient. The last stage is a sliver above 1 ton and holds 3300m/s in it.

Compare to the first stage, with some 4000m/s across three mammoths and huge Kerbodyne tanks.

Edited August 15, 2015 by Sharpy

[Jason Patterson]

Told you it was a dumb question to ask. If im honest it still makes no sense to me......all that dv I sent up just doesn't exist? All that fuel is effectively unusable? Anyway, thank you for your reply, I do appreciate your time, even if I dont get it.

BTW, I was intending to keep them as a single stage....a waste of time that was

It's not a dumb question at all.

Delta-v is a measure of the change in velocity that a rocket can cause if it were in a totally gravity-less environment. Its value is based on how much fuel you bring, the mass of the rocket, and the mass of any additional payload you might have.

For an analogy that might help this make more sense:

Suppose I have a loaded semi truck that can travel 200 miles on the fuel that it has. If I weld a second truck with an identical load and fuel to its side, it's not going to be able to go 400 miles. Instead it will move 200 miles with twice as much load as the original. That's what you've done here, given yourself the ability to move twice the payload through the same velocity change as the original ship.

If you want to add delta-v to a vessel, the easiest way is to add booster rockets that are as minimal as possible: fuel tanks, engines, and whatever aerodynamic or control devices are absolutely necessary to get it where it needs to be.

[maceemiller]

The short answer is DV is not additive like you were expecting. It's proportional to the natural log of the wet mass over dry mass.

You effectively doubled the mass of the fuel, which is good. But then in addition you also doubled the mass of the tanks (which you have to do to hold the fuel) and engines (which you didn't have to do). Plus throw in the random equipment to connect them together/ etc.

You haven't improved your mass ratio at all. In fact, you made it worse, so DV goes down. Your acceleration did improve, but that's not what you were after.

If you were to try it again with just an additional fuel tank, you would see an improvement in DV. Not a doubling, but an improvement. If you were to fire the attached rocket by itself and then discard it when empty, you would also see an improvement. Again, not a doubling.

And no, not a dumb question. You have to ask questions like this to get a handle on how the rocket equation works. It would only be dumb if you didn't ask.

HtHs,

-Slashy

Ok, id just like to say straight off the bat to all members here that have replied to my question a massive thankyou from a not-so-confused guy from England. By reading your replies a few times over I now can see the error of my "logic". When I first posted my question I felt frustrated and disheartened but now I'm looking forward to rethinking and redesigning my rockets.

This is the beauty of everyone on this forum.....which is without doubt the best forum this side of LKO

Thanks once again.

- - - Updated - - -

It's not a dumb question at all.

Delta-v is a measure of the change in velocity that a rocket can cause if it were in a totally gravity-less environment. Its value is based on how much fuel you bring, the mass of the rocket, and the mass of any additional payload you might have.

For an analogy that might help this make more sense:

Suppose I have a loaded semi truck that can travel 200 miles on the fuel that it has. If I weld a second truck with an identical load and fuel to its side, it's not going to be able to go 400 miles. Instead it will move 200 miles with twice as much load as the original. That's what you've done here, given yourself the ability to move twice the payload through the same velocity change as the original ship.

If you want to add delta-v to a vessel, the easiest way is to add booster rockets that are as minimal as possible: fuel tanks, engines, and whatever aerodynamic or control devices are absolutely necessary to get it where it needs to be.

Your analogy has just made it click in my head

[kiwi1960]

If you push a car, then you will go so fast.... if two people push, the speed of the pushing isn't doubled....

put it this way..... your rocket doubled its mass.... but the thrust remained the same... even with both engines... because there is only so much an engine can give....

if not for this, then man would be able to assemble (in real life) a massive rocket in orbit and go to Mars in a month..... sadly, it doesn't work that way.

[Red Iron Crown]

It really is nothing to be embarrassed about. The tyranny of the rocket equation, like so many things in spaceflight, is really counterintuitive if you try to apply the rules we are all have learned here on the ground. I remember trying to wrap my head around the Oberth Effect and gravity slingshots and how they don't violate conservation of energy, for example. Most of us around here are not real rocket scientists or physics majors (though there are some of those here, too), a learning curve is to be expected and most of us are happy to share our hard-earned knowledge.

The only stupid question is one that isn't asked. Better to feel humble for a little while than be ignorant forever.

[GoSlash27]

maceemiller,

No worries, and you're very welcome. Answering questions like this one makes us cantankerous old farts feel useful

You now have a better understanding of how the rocket equation works than probably most people who play this game, and first-hand experience of why "moar boosters" doesn't work.

You'll run into more confusing and counterintuitive stuff in the future (I sure as heck did). When you do, give us a holler. We don't mind.

Best,

-Slashy

[Jason Patterson]

Your analogy has just made it click in my head

I'm glad it helped. There's a lot of this that is in no way obvious; don't feel bad if you get confused. The Q&A forum is usually very patient with these sorts of things.

[MarvinKitFox]

Hi all....OK, so im gunna look like a fool here but I dont understand something.

..... I basically sent up a duplicate of this .....

Im sure there is a clear and obvious answer to this but atm I'm just not seeing it

Run this though experiment:

Your car, on a full tank of gas, can drive 300 miles.

How much further will you be able to go, if your brother drives his identical car alongside yours on the same route, and you get into his car when yours runs out of gas?

(remember there are no gas refills along this route!..)

[_stilgar_]

If you had one rocket, it would have some dV. If you had two identical rockets flying side by side, they would be able to cover exactly the same distance as the first one. It doesn't change anything if your rocket has a company or not.

If you then connect those two rockets flying side by side... again, nothing would change

That's exactly the same reason why heavier objects fall at the same speed as light ones. One rock falls the same as another one. If you connect them by a string, the new object would have the mass of two rocks, but the string doesn't change anything

[maceemiller]

If you had one rocket, it would have some dV. If you had two identical rockets flying side by side, they would be able to cover exactly the same distance as the first one. It doesn't change anything if your rocket has a company or not.

If you then connect those two rockets flying side by side... again, nothing would change

That's exactly the same reason why heavier objects fall at the same speed as light ones. One rock falls the same as another one. If you connect them by a string, the new object would have the mass of two rocks, but the string doesn't change anything

Yet again this is one of them answers that really makes sense to me. In fact my question now seems obvious lol! Cant wait to start construction on some new craft with my new found knowledge thank you all once again