Fuel burn rate question

[Epic DaVinci]

Hi Guys,

Good to see the forums back up, shame im back to 0 posts nevermind.

Anyway, my question.

Say for instance an Engine burns 10l per second at 100% throttle, would it burn 5l per second at 50% throttle (an all the obvious inbetween rates)

If the answer to this is no, and the half throttle burn rate is lower than one half of full. would adding an engine with twice the power and running it at 50% be more efficient?

[sal_vager]

Your first guess is right Epic DaVinci, at 50% throttle you use 50% fuel

[EndlessWaves]

If the answer to this is no, and the half throttle burn rate is lower than one half of full. would adding an engine with twice the power and running it at 50% be more efficient?

The opposite is true.

The engine consumes the same amount of fuel but an engine with twice the thrust will generally be twice the weight so the rocket is heavier overall and consumes more fuel to get the same distance.

[Epic DaVinci]

Thanks guys, im just struggling with fuel efficiancy for some reason, i used to be able to get into orbit without thinking and now i can get my rover up there, even with asparagus staging. I guess my rover may be a tad heavy. Oh well, back to the simple designs i guess.

[Freefly101]

Thanks guys, im just struggling with fuel efficiancy for some reason, i used to be able to get into orbit without thinking and now i can get my rover up there, even with asparagus staging. I guess my rover may be a tad heavy. Oh well, back to the simple designs i guess.

hmmm just add more srb , or small tanks with mainsail . that always helps .

[Ivan Ivanovich]

As a rule of thumb, you're usually better off if using as much thrust as you can during ascent. The only exception to this seems to be the first 10,000 meters when the atmosphere is thick and drag may actually kill more speed than gravity.

[Epic DaVinci]

What would you say is too fast? I usually try not to exceed 300m/s below 10,000m.

[Freefly101]

As a rule of thumb, you're usually better off if using as much thrust as you can during ascent. The only exception to this seems to be the first 10,000 meters when the atmosphere is thick and drag may actually kill more speed than gravity.

hey ivan wanted to ask , do could you maybe explain to me in more detail how the engines work and what all the stats stand for , by that meaning the vacuum and power , so i can then compare and know which engine to use at what point . You could PM as not to spam this post .

THanks

[EndlessWaves]

ISP is fuel efficiency (higher is better), thrust is how much weight they'll lift per engine (to get weight you multiply mass by local gravity - 9.81m/s² at KSC).

[Jason Patterson]

hey ivan wanted to ask , do could you maybe explain to me in more detail how the engines work and what all the stats stand for , by that meaning the vacuum and power , so i can then compare and know which engine to use at what point . You could PM as not to spam this post .

THanks

There are only three stats for an engine that really matter:

Mass - for any engine less mass is better, as you likely have guessed.

Maximum thrust - allows you to fit the engine to the job so that your ship's thrust to weight ratio is about right (2 for an atmospheric ascent, for instance)

Isp - This is basically the fuel efficiency of the engine. Engines have two values for this, one at sea level and one in vacuum. High Isp is good.

Generally speaking, you want to find an engine based on the thrust you need for a job, then find the engine with the best combination of Isp and mass for the role you want. Each of the engines has a role that it fills best.

LV-T30/LV-T45 - Engines for ascent/descent of medium sized rockets. Can be stacked to make a great heavy lifter, the 45 provides thrust vectoring in exchange for decreased thrust and increased mass

Mainsail - Heavy lifter. A better option is the LV-T30/45 combination, stacked, but that can cause part count problems

Poodle - Large lander engine. It's not really very good at that job; this is one that doesn't get used often.

LV-909 - Good small lander engine. Sometimes an LV-N will be better. Also useful for spaceplanes.

LV-N - Excellent for transfer stages between planets or other large delta-v maneuvers. It can be a good lander engine when the lander also has significant maneuvering to do, but its length is problematic.

Mark 55 Radial - Liquid engine for increasing TWR. It's kind of awful compared to the other engines.

24-77 - Can be used to increase TWR for landers or spaceplanes. It's not a great engine by itself, usually, but it's good when you'll need a burst of extra power for a short time.

Aerospike - Good rocket for use near sea level. Practically speaking this could be stacked to make an efficient heavy lifting rocket for a first stage on Kerbin and is pretty standard for Eve ascent vehicles. It's also useful on spaceplanes.

LV-1 - Surprisingly effective engine for probe maneuvering.

What would you say is too fast? I usually try not to exceed 300m/s below 10,000m.

For an ideal launch you want to ascend at the terminal velocity for a given altitude. At sea level that's something like 110 m/s, and at 10km it's about 250 m/s. You can go a bit above or below that throughout without wasting too much fuel.

[Ruedii]

It depends on the size and design of your rocket, but 145-160m/s will see a drastic drag reduction.