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There's some "fun" math there for you if you want to figure out mass flow rate of fuel in your designs and spreadsheet your way into orbit. Generally, an engine with a higher specific impulse (Isp) will burn its fuel more efficiently.
The specific impulse of an engine depends on a number of factors, but in KSP the main thing to care about is atmospheric pressure. This is why there's two Isp statistics for each engine: At sea level, and in vacuum. Some engines are better in atmosphere, some are really vacuum engines and give you no useful thrust at sea level.
Higher ISP means the engine is more efficient.
ISP figures into the Trivlosky rocket equation...
dV=ISP*G*Ln(Mw/Md)
dV is your total delta velocity
ISP is the average specific impulse of all engines you're using on your stage or rocket
G is the acceleration due to Earth's gravity at surface level which is a constan 9.8m/s^2
Mw is the mass of your rocket (or stage) when it's full of fuel
Md is the mass of your rocket (or stage) when it has no fuel
Ln is just the natural log function and any scientific calculator can perform it
Your parts list in the VAB and SPH and the tech tree will tell you how much force your rocket thrust is giving and exactly how much fuel that engine is using per second.
I like to use a different equation for my dV calculations...
dV=((total engine force)*(total fuel))/(((Mw+Md)/2)*(fuel consumption))
Both equations spit out the same dV values but they just ask for different inputs.
There are mods out there like Kerbal Engineer and mechjeb(maybe?) that do all this math for you.
Just remember you can go pretty much anywhere and land on everything except the sun and Jool without any math at all.
Math is optional but I recommend it.
Not that I can't see the use of that spreadsheet, but I was more out to find the general efficiency.
In the describtion of the engines I see two numbers one is named (vac.) and I don't recall the other one, but I asume that those values are what you refer to as "specifik impulse" and (vac) is the vacum value, right?
So can I expect thos values to reflect the engine efficiency?
Say engine 1 has a specifik impulse of 100 in vacum and engine 2 has a specifik impulse of 200 in vacum. So engine 2 is burning fuel twice as efficiently as engine 1.
Am I understanding this correctly?
Thanks
All propellants used total, thus fuel plus oxidizer mass for respective engines. It may be based on N/g per sec as SI units though (not the kN or t) and you can easily get those exact numbers dividing them yourself. kN=t*gravity. That's why the result will be sec for Isp, since gravity has sec² in formula.
Since everything in space is about mass anyway, that's the perfect efficiency measurment. You use mass to move mass.
VAC=vacuum
ASL=at sea level (on bodies without sea it is ground zero)
Going deeper on efficiency sometimes it is not all about Isp. Because there is such thing as theoretical delta v in your tanks and the real one you will use efficiently. Because all rocket launches and landings and even orbital burns include losses due to gravity hovering or burning at imperfect spot or angle.
Vacuum engines also have much lower power (except Rhino), thus you would waste a lot more engine mass trying to launch with vacuum engines than with "bad Isp" solid rocket boosters (SRB), but having huge TWR or simply explosive power which helps you escape gravity.
"Vacuum" in rocket engine terms starts at roughly 15km altitude on Kerbin! Some VAC engines beat their "rather ASL" counterparts at 6..10km already in terms of Isp. Though not even close in thrust (power) values!
While perfect physical vacuum is above 70km, the air is thin enough much sooner and engines perform 99% of their vacuum at those 15km already. That's why some launch engines with higher vacuum values save more fuel (they have higher Isp growth potential) and quicker launch does so too - all engines gain more Isp (=efficiency) with higher altitude.
Isp VAC>ASL for all of them.
Good drag optimization and well chosen flight curve is basically as important for efficiency.
Also, delta v cares about dry mass a lot. That's why very heavy engines (per thrust) like Nukes can't beat much Isp-worse engines for close distance targets. They are meant exclusively for slow, long haul transfers with huge fuel amounts. They waste more mass on engines than save on fuel for closer distances. As an example you need at least 4.5t fuel tank for each Nuke engine to beat Terrier at total efficiency even though Terrier has Isp of just 345 and Nuke more than twice that - 800.
So, efficiency:
1. Isp
2. flight (Thrust to Weight Ratio, trajectory curve, drag optimization in atmosphere)
3. dry mass
However, with all that new info, new questions arise :)
About the trajectory curve (that is the curve the ship moves from launch till orbit or reentry, right?) does it differ, depending on the shape and mass of the ship?
As for shape.. the game engine takes some shortcuts, or at least it has in previous versions.
More drag will slow you down quicker.
More mass will require more force (drag) to slow down by the same amount.
An example might be a capsule with a fuel tank attached.
If the tank is full on re-entry the higher mass will mean it takes longer to decelerate than if the capsule was empty, you will have a faster, straighter & hotter entry.
If the tank has some fins on it these will create drag causing the capsule to slow down faster giving a slower more curved re-entry.
Obviously if you don’t slow down fast enough you will either get too hot and explode or be going too fast to deploy chutes in time.
High drag ships probably won't fly very far but can be designed to fly sort of straight.
High mass ships need lots of engines to get off the ground.
During reentry, high mass ships take longer to slow down because of the high mass.
Newton's second law, F=MA plays a huge role in KSP.
F is force and your engines provide force in kilonewtons
Drag is also a force
M is mass
KSP measures mass in metric tons which is 1000 kilograms
A is acceleration in meters per second^2
It all works out so you can ignore the units and get your acceleration in m/s^2.
Drag will always act in the retrograde direction in KSP relative to the surface.
Wind doesn't really exist so the air is stationary.
About the take-off trajectory, sometimes it's best to get out of the atmosphere as fast as possible to avoid wasting fuel.
All it means is go up until your engines have a more reasonable ISP before starting your gravity turn.
You might have to wait to start your turn for high drag ships too.
Some ships like spaceplanes need to stay in the atmosphere until they're going fast enough to launch themselves up to fire up the rocket engines for the final kick into orbit.
Jet engines are the most efficient engines the game has but they only work on Kerbin and Laythe.
I was thinking about the general procedure to perform just after launch. So I will try to refrase my question. :)
Say the most optimal course would be to go straight east and then alter the pitch by 10% for every 100m/s that the ship increases the velocity (this is most likely not the most optimal course, I'm just trying to make an example).
So if above course would be the most optimal course, would it change if the mass of the ship changes?
Yeah that was what I was thinking too, it seems to me like the heavier ships needs to go up prety high before starting the turn. Thanks for confirming that. :)
The best launch trajectory is going horizontal with very hight TWR and extremely quick propulsion right from the launch. That way you have least impact from gravity and waste less on hovering (which is -9.8m/s²) or vertical climb. Planetary curvature will take you to space anyway.
The problem is, you can't!
1. Kerbin atmosphere is too thick to pull that through without exploding or losing too much on drag when it becomes extreme and the proportion of drag growing as velocity² starts to matter.
Kerbin also has too big radius for escape to be quick enough. So it is about something in between, a best balance for particular ship.
2. There are no engines capable of impulse as short. You can get close, but then you encounter another issue - max TWR=>min delta v. So actually you look for a balance. Quick launch will save delta v, but it also requires much less onboard. One curve grows quicker than another.
So you still have to add some vertical component. Like on Mun the recommended pitch is ~6° (tutorial). You may do less with most powerful engines at that point, but you would hit next mountain eventually.
3. Even while looking straight, your launch still does a curve due to gravity. Depending on the TWR, you can be capable to sustain a shallower one or not. With some less TWR ships you would dive back before reaching any reasonable velocity except you use a lot of steering surfaces... which would add drag and waste too.
Short - you do as shallow trajectory as possible with current ship staying stable (just using thrust without any steering input after initial one) while still being able to avoid dive (building up final Ap at least above 48km) and also avoiding explosion from the heat before reaching that.
And you should gain velocity as fast as possible with your current mass and total delta v requirements.
SRB. Decouplers will waste a lot of money though. So if you want to build also cheap, there are even more variables. Also going for higher TWR usually means overpowered (more expensive than necessary) engines. Which is a waste too.
I also presumed your ship has proper rocket design and sharp nose cones everywhere. Unlike a flying brick. Because bricks have to be launched differently due to that drag (amount and instability) while wasting a lot of delta v. Just to be able to reach orbit at all.
Yeah I was asking very generaly and now I understand that there so many variables that I did not take into account.
From what I understand, there is no single best trajectory and to find the best one for a certain ship, trial and error is most likely the best option.
That is good enough for me. :)
I know this is just a simulator, but imagine what real rocket-guys have to do to prepare them selves for a launch. I mean we can just burn up a few kerbals or reload the game. That is just amazing!
Actually slightly less since gravity declines with altitude, but not too quickly. And then drag would increase it back a bit. So - close to that number.
Unfortunately vertical speed has no impact on orbit*. You need orbital (horizontal) speed to stay on orbit. Roughly 2290m/s on 72-72km orbit.
You can't launch rocket into orbit with just 2300m/s on Kerbin. Not even close to that because of the reasons mentioned above. 3400m/s used for LKO is a great rocket. 3600m/s is fine for common ones.
You can reach delta v values as low as 3200m/s or slightly less with extreme launch tests. But that's rather close to limits of what you can reach due to heat limiting minimal angles and robbing you of some delta v too.
While on Mun iirc you can launch into 550m/s orbit using just 570..580m/s. If Mun were as limiting as Kerbin, that minimal value would be closer to 800m/s. Which it is not. Because there is no atmosphere.
So yeah, test and balance. Can be done with math and quite complicated optimization models. Eventually ;/
----
you can go for straight escape with proper timing. Which isn't much more expensive than regular one in KSP universe. Skip Kerbin orbit completely going straight for Mun landing or solar orbit instead. In this case it is very important to have extreme TWR at launch to reach declining gravity altitudes soon enough. Or that hovering will punish you.