-Thrust -Delta-V?

[O Nerd]

Some time ago, i've made a thread to discuss the utility of TWR in space. But i've noticed a thing: I made a test of a rocket with a payload of 4 tons and one with a payload of 10. Both had the exact same amount of fuel and the same engine, BUT... One had less Delta-v than the other, acoording to Flight Engineer. Wut?

Edited December 19, 2013 by O Nerd

[kurja]

Some time ago, i've made a thread to discuss the utility of TWR in space. But i've noticed a thing: I made a test of a rocket with a payload of 4 tons and one with a payload of 10. Both had the exact same amount of fuel and the same engine, BUT... One had less Delta-v than the other, acoording to Flight Engineer. Wut?

weight matters, the heavier rocket doesn't get 'as far' with the same amount of fuel as the smaller rocket.

[cryptk]

This makes complete sense. I am guessing that the one with a payload of 10 tons had less delta-v. The more mass there is to move, the more force it requires to get the same change in velocity. This means that you will have to burn more fuel in the heavier one in order to get the same change in velocity. Since you have the same amount of fuel in both, the one with more mass can do less of a change in velocity. Delta-V means Delta-Velocity, or "amount of change in velocity that you can accomplish". Mass go's into that equation. http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

[Monkeh]

Imagine a Mini car trying to push an iceberg with a full tank of fuel. It's not going to go very far, this configuration will have very low delta v.

Now imagine a Mini car trying to push an elephant. It will get further than when it tried to push the iceberg, but still not very far.

Now then, take the engine and fuel tank out of a Mini, strap them to a couple of light weight planks, add a wheel or two and then see how far you get. MUCH FURTHER!

I think the concept under discussion here is INERTIA. It takes a lot of energy to move something massive and less energy to move something less massive.

Edited December 19, 2013 by Monkeh
Added link.

[O Nerd]

This makes complete sense, but, considering that the craft is in SPAAACE, there should be no reason for it to lose energy to anything... So, sometimes it's better to have a stronger engine than more fuel?

[Sirrobert]

Inertia. It takes energy to get mass to change speed. More mass means you need more energy for the same change in speed

Things in space may be weightless, but they are not MASS less. Mass is the relavent factor in inertia, weight is just a combination of mass and gravity

[capi3101]

Alrighty - math time!! We've got your two payloads, 4 tons and 10 tons. Now, let's stick an orange tank and a single nuke underneath each one. Nukes come in at 2.25 tonnes, and orange tank is 36 tonnes full and 4 tonnes empty, and we're in space so the nuke's Isp is 800s.

Plug in to Tsiolkovsky -

4 tonnes payload + 36 tonnes full + 2.25 tonnes engine = 42.25 tonnes total.

4 tonnes payload + 4 tonnes empty + 2.25 tonnes engine = 10.25 tonnes dry.

dV = ln (M/Mo) * Isp * Go = ln(42.25/10.25) * 800 * 9.81 = ln(4.12195) * 7848 = 11,115.33 m/s

10 tonnes payload + 36 tonnes full + 2.25 tonnes engine = 48.25 tonnes total.

10 tonnes payload + 4 tonnes empty + 2.25 tonnes engine = 16.25 tonnes dry.

dV = ln (M/Mo) * Isp * Go = ln(48.25/16.25) * 800 * 9.81 = ln(2.96923) * 7848 = 8,541.00 m/s

Note that we've changed nothing else - just the payload mass. The net result: an extra six tonnes of payload cost you 2500 m/s of delta-V.

As far as the notion of "sometimes it's better to have a stronger engine than more fuel", I say no, it's better to have the most fuel efficient engine you can muster that'll do the job you want while adding the least amount of mass (translation: a real NERVA - one that has better thrust and is more efficient than any chemical rocket - instead of the one that the developers added to KSP for purposes of game balance).

Okay - case in point. Let's trade out that NERVA for a Poodle. Comparable weight (only a quarter tonne more) and way moar thrust (almost four times as much). It's Isp in space is only 390 as opposed to the LV-N's 800. So for the ten tonne payload you get:

dV = ln (M/Mo) * Isp * Go = ln(48.5/16.5) * 390 * 9.81 = ln(2.93939) * 3825.9 = 4,125.098 m/s

You haven't jacked with the mass all that terribly much and you've nearly quadrupled your thrust. But because the engine is significantly less efficient, you've halved your delta-V. Thrust-vs-delta-V is always a tradeoff in space, and in most cases the more important factor by far is delta-V. Adding a second, identical engine? That decreases your delta-V - you've reduced the mass ratio while not making the rocket any more efficient (same thing as you would've done by upping the payload - the second engine adds "dead mass"; the only class of part that doesn't add dead mass is fuel tanks).

A lot of people add more engines anyway - my own Ranger-series rockets utilize four NERVAs - because they don't like to have to thrust for an hour to get to where they're going, and that's fair. All you really can do though is find a balance that works for you and then go with it.

Edited December 19, 2013 by capi3101

[Monkeh]

This makes complete sense, but, considering that the craft is in SPAAACE, there should be no reason for it to lose energy to anything... So, sometimes it's better to have a stronger engine than more fuel?

If you have achieved orbit and gravity is no longer pulling you down then the power of the rocket just means how fast you will accelerate and eat up the fuel. Nothing more. If you have 2 rockets that have the exactly the same mass and exactly the same amount of fuel and two engines on those rockets with the same ISP but one is twice as powerful as the other, the only difference you will notice is the rocket with the stronger engine will accelerate twice as fast and burn fuel twice as fast, resulting in EXACTLY the same amount of delta v.

In KSP the stronger the engine the more it weighs. As weight goes up then delta v goes down due to inertia. This is why you often see interplanetary stages consisting of a HUGE orange tank and only a single LV-N engine to push it all. The thrust to weight is tiny, resulting in very long burn times, but it's so efficient, in a vacuum, that as long as you can sit around waiting for a half hour burn it's the best way to do things.

[Guest]

Exactly. Note, though, that there's another factor in play: transfer efficiency and precision.

An usual dV estimate for a Hohmann transfer assumes burn time to be zero. KSP Patched Conics system usually does, too. However, this is never the case. It takes time to achieve the required velocity due to limited acceleration. Now, this effect is negligible in most "normal" cases, but if your TWR is too low, you'd need to use a Low Thrust Transfer instead of Hohmann, which is noticeably less efficient. According to Wikipedia, it requires up to 141% of Hohmann dV! You need to find a point at which you get the best efficiency of the entire transfer, which is a function of maneuver efficiency and engine efficiency.

Also, KSP can only calculate Hohmanns, not LTTs (nor brahistochrones, which an LTT can become in an extreme case). So, if you want to use the Maneuver Planner for a precise insertion, you'd be better off with a non-abysmal TWR, because otherwise, you'd end up off mark.

[purpletarget]

This makes complete sense, but, considering that the craft is in SPAAACE, there should be no reason for it to lose energy to anything... So, sometimes it's better to have a stronger engine than more fuel?

Remember in SPAAACE, in KSP you don't generally lose energy to much, and but you also can't gain any. So all the rocket motors are Reaction Engines. That means they use the Newtonian principle of throw stuff out one direction, you'll get the same reaction in the opposite direction. All the fuel you have on board, isn't like a car where it turns gears and wheels to get that force on the ground. The rocket basically is throwing stuff out the nozzle behind your ship with a certain force, and that imparts the same force on the ship to go the opposite direction(forwards). The more mass it throws (Fuel Flow), and/or the faster it accelerates the mass it's throwing (ISP), the more force you get for your ship (Thrust).

So, if you have 1/2 your ship mass is fuel, and you throw it all out the back at once you get force X on the fuel going backwards, and force X on the ship forwards as well, and that will change your velocity (delta-V) by Y.

If however, you use the same mass of fuel, but double the mass of your ship, the fuel only accounts for 1/3 of the ship's overall mass. (Similar to your case above, increasing payload from 4 - 10) If you throw the fuel out as before, you still get force x, and it still imparts force x on the ship. But because the ship has twice the mass, it will only accelerate half as much, so the delta-V of the ship will be 1/2Y.

That's why you're dV went down...if you increase the payload, and do not increase the reaction mass to move said payload, you aren't going to be able to push that heavier mass to as high a velocity as the lighter payload.

[O Nerd]

Now there are some complete answers! So, i lost my Delta-v because of the low thrust generating less energy, and the way to solve it is to find a balance! Thank you guys, you're

[arq]

Exactly. Note, though, that there's another factor in play: transfer efficiency and precision.

People always seem to neglect transfer efficiency. There is a point where TWR is so low that adding another engine will give greater 'range', even though there is less 'delta-V'. If your payload is 50t and you have 450t of fuel tanks (400t of fuel), using two LV-Ns instead of one will drop your delta-V from 12491m/s to 12297m/s, while essentially doubling your TWR. Those lost 194m/s will be made up for in improved maneuver efficiency (and even with those two engines those burns will be agonizingly long).

[phunk]

Now there are some complete answers! So, i lost my Delta-v because of the low thrust generating less energy, and the way to solve it is to find a balance! Thank you guys, you're

No, you lost delta-v because you added payload. The thrust and energy did not change, you just need more energy to get the same d-v out of a heavier payload.

[Sirrobert]

Now there are some complete answers! So, i lost my Delta-v because of the low thrust generating less energy, and the way to solve it is to find a balance! Thank you guys, you're

No, thrust has nothing to do with it.

Thrust only effects how fast you accelerate.

You lost deltaV because you added more mass to your ship. That's how inertia works.

Your engine generates the same amount of energy on both ships.

However, it costs more energy to move a ship that weighs 10 tons, than a ship that weighs 4 tons

[Soda Popinski]

Delta v is a measure of acceleration x time (aka energy). This may make more sense.

F=ma

Force = mass * acceleration

Same Force from the same engine, say Mainsail with 1500 kN. On the other side of the equation, you have mass of the ship and acceleration. If you increase the mass (more payload), to get the equation to balance out (same Force), acceleration is reduced.

[capi3101]

^^^

Incidentally, we should mention that Tsiolkovsky is Newton's Second Law. It's generally unrecognizable as such because it has been modified to account for the variable mass of the system (i.e. the fact you have to burn fuel to accelerate).

[Ravenchant]

Easy way to visualize it: you have two identical hikers, both are equally fit, rested, hydrated... and weighing the same. Now you give one of them a backpack with a couple of bricks inside to carry, and give both of them an energy bar.

The one without the backpack will get much further before he collapses of exhaustion, with the same amount of initial energy, though they have equally strong legs. How much further depends on the amount of bricks you stuff inside.

Okay, so SPAAAACE! doesn't exactly work like that, but in this particular case it's a useful analogy