So one Atomic Rocket Motor is better than four?

[antbin]

Granted that's an incorrect assumption and may have an effect on what we're talking about.

Yup, that's the meat of what we're talking about, that low TWR means more of the burn occurs off-periapsis, at a less-than-optimal speed. But you have set an upper maximum bound, and that's something!

Edited November 20, 2013 by antbin

[capi3101]

I would integrate the equation, but Calc IV was fifteen years ago and I haven't had much use for it sense (lousy job market). I may look into the derivation of the rocket equation itself and see if there's anything usable. And then I'll have to look into how to do integration again.

Meantime, I've worked up a chart to solve the earlier equation I posted for delta-V bonus from the Oberth effect for a set of different altitudes and differing transfer delta-Vs:

Hopefully that's of some use too.

EDIT: Terms on the chart - that might be needed. GM is Kerbin's Gravitational Parameter, R-SFC is the surface radius of Kerbin, V-ESC is escape velocity at the listed set of altitudes, V-ORBIT is the orbital velocity at the given set of altitudes (which I use for the initial velocity in the equation). V-FINAL is just V-ORBIT plus the transfer delta-V requirement for the listed destinations, and there's an array of those values (MATRIX). DELTA-V REQ., the delta-V required, is the solution to the equation; this is the amount of delta-V that must be applied to acheive the actual delta-V necessary for the transfter, and OBERTH BONUS is just the difference between the two values.

Edited November 20, 2013 by capi3101

[allmhuran]

This is ~450 tons heading out for Tylo at 0.1g. Is there any reason to be in a bigger hurry than that?

Let me quickly destroy this line of comments by asking you, simply: Is there any reason to be in that much of a hurry? Why did you add more than one LVN?

[fatfluffycat]

I understood 0.00%.......Too much math........can someone get me a ambulance?

[capi3101]

I understood 0.00%.......Too much math........can someone get me a ambulance?

Sure thing, but Jeb's driving, so I'd expect it to explode once it arrives......or before it arrives.......you know what? Just expect it to explode......

[Vanamonde]

Let me quickly destroy this line of comments by asking you, simply: Is there any reason to be in that much of a hurry? Why did you add more than one LVN?

This is the problem with discussions on the internet; half of the people in them forget what they're about. To clarify for the third time, OP asked whether having more than one engine reduces delta-V, and some of us agreed/explained that it does reduce delta-V. That is one topic of discussion in this thread. Then some other people started arguing that ships should have high T/W ratios because that reduces the game-time the player spends on burns. That is the second, completely unrelated topic of discussion in this thread. In reply to this second topic, I posted an example of a ship that can go anywhere in the game despite having a low T/W, thus demonstrating that burn times have little to do with efficiency and are mostly a matter of player convenience. Yep, that ship I posted does have multiple engines to reduce the time spent burning (thread topic 2). This does not change the fact that fewer engines are more efficient in terms of delta-V (thread topic 1), because those are two separate topics. Are we all on the same page now? More engines = the convenience of shorter burns. But also, more engines = less delta-V, and delta-V is what the OP was asking about.

[wilt57]

Assuming your ship is large enough and you can time a spiral transfer to intercept your target, yes. Assuming you are using a Hohmann transfer with a ship with a really low TWR, you may actually have more "usable" delta-v by adding an engine. There is a point where long burns lose more delta-v than the additional mass takes away.

[allmhuran]

This is the problem with discussions on the internet; half of the people in them forget what they're about.

I am not forgetting what the OP's question is about. I am not disagreeing with the obvious truth that adding more dead mass reduces delta V.

When you wrote this:

Is there any reason to be in a bigger hurry than that?

It was in response to this:

The idea that TWR isn't important in space is an opinion only batted around by those who let MechJeb play the game for them.

And I am suggesting that the answer to your question is provided by asking yourself "is there any reason to be in that big of a hurry?"

[Vanamonde]

I am suggesting that the answer to your question is provided by asking yourself "is there any reason to be in that big of a hurry?"

I have no idea what you are arguing for or against, but good luck with it.

[gm537]

Delta V is proportional to logarithm(M_start/M_end). Adding more engines you increase M_end and decrease Delta V.

You may drop 2 engines (if space convention allows you dropping nuclear stuff ) when needed and continue with the rest of them.

This is what I did once. Had 3 LN-Vs, dropped a tank and two of them. I couldn't just drop them, b/c thrust would be asymmetrical, so I had to use docking. (Very carefully, to not touch the engine, otherwise docking with a spinning object is very hard.)

Interesting design... As a point of reference the 4-way structural piece is also kinda heavy so that adds even more unnecessary mass which reduces dV. Which I suspect the docking ports do as well. A better design would be to do this:

As opposed to:

I know it only says you gain 29 m/s but the first design allows easy drop of two of the engines while maintaining symmetry. You could even adopt a 5 engine design which would allow 5,3 or 1 engine operation.

Basically to address the original question the less non-fuel mass you have the better your dV; and engines are lots of non-fuel mass.

Here is that design with just one engine:

As a note if I simply add the 4way structural piece and keep one engine dV goes down by 75m/s.

[Mmmmyum]

Just my two cents but long burn times can mess up your ejection angles, as you're orbiting around the planet as you burn

[Iansoreta]

Just my two cents but long burn times can mess up your ejection angles, as you're orbiting around the planet as you burn

It doesn't matter, you just have to adjust ejection time, even modify launch windows to make up for low TWR. Is there a reason to have additional TWR by sacrificing efficiency just to get it done? Wait a second, THIS THREAD is old!!!!! Oh, mother of Christ, why do I end up in grandpa thread?

Edited November 11, 2014 by Iansoreta
noticed that this thread is old

[technicalfool]

It doesn't matter, you just have to adjust ejection time, even modify launch windows to make up for low TWR. Is there a reason to have additional TWR by sacrificing efficiency just to get it done? Wait a second, THIS THREAD is old!!!!! Oh, mother of Christ, why do I end up in grandpa thread?

An easy enough mistake to make.

For this undead thread though, its time is up!