Ascent profile efficiency

[MAFman]

How do you measure the efficiency of an ascent profile in terms of gravity and aerodynamic losses?

[Vanamonde]

This is actually a remarkably complex question involving aerodynamics, physics, and piloting, and much of the math will be the same as for the real world. With that in mind, your question has been moved to the Spaceflight sub, because that's where the kind of people who do this sort of math for fun hang around. 

[cubinator]

I would start with guessing that 

Energy required to be in orbit at the desired altitude = Energy you spent getting there - Energy you lost to drag, gravity, inefficiencies, etc.

The energy required comes naturally from Newton's laws and can be calculated from the mass of Earth, your spacecraft's mass, etc.

Your rocket is generally going to get its energy from chemicals inside propellant tanks. It's pretty easy to calculate the theoretical energy contained in those tanks, and be sure that there's more than enough of it to lift the rocket to the desired orbit. 

When you fly your rocket, you can see how much propellant is left in the tanks and calculate the energy you spent to get into orbit. Typically it's expensive to fly rockets, so while engineers to perform test flights to determine stuff like this a lot of it is worked out by simulation. 

From there you can easily know just how much energy you lost. The harder part is finding out where exactly the losses happened. 

For gravity vs aerodynamic loss, I would say you'd have to run simulations of your ascent that take into account air flow around the vehicle, and from that you might be able to figure out how much the air is slowing you down. It's going to be pretty complicated, though, because it depends on the exact shape of the vehicle and the properties of air change pretty drastically over the wide range of speeds and air temperatures the rocket goes through. 

[kerbiloid]

Start the gravity turn as soon as possible. Real rockets do it almost from launch.

Keep the rocket nose at the current velocity vector. Ideally, the angle of attack should be always zero.
To achieve that, find the optimal initial pitch angle and take it asap, while the velocity and thus the air drag are low. (see p.1.)

(Always irl, but hardly in KSP due to much smaller planets).
Finish acceleration exactly in apoapsis, no pause and apoapsis burn.

Find the optimal initial thrust-to-weight ratio for you rocket.
Irl space rockets have ~1.2, ICBM ~2. In KSP probably 2.
The 2nd stage is 0.8 .. 1.2.

Set the full thrust and don't change it. The underthrusted engine means carrying excessive mass of the engine when you could use a weaker and a lighter one.

Attach a fuel tank to your test rocket as a test payload. 
Looking at the rest of the fuel in it, adjust your stage fuel and payload mass.

Edited January 26, 2022 by kerbiloid

[mikegarrison]

On 1/25/2022 at 11:44 PM, kerbiloid said:

Set the full thrust and don't change it. The underthrusted engine means carrying excessive mass of the engine when you could use a weaker and a lighter one.

Well, no. At least, not always.

Because the weight of the engine isn't the only issue. There is also the weight of the structure. And this has to be set by the peak loads on it, which will come when most of the fuel is burned and the mass is the lowest.

Saturn V could not turn down their thrust, so they did their thrust limiting by simply shutting off the middle engine (20% reduction in thrust). More modern rockets often do throttle down toward the end of their stage's burn, in order to limit the loads on the now-empty structure.

[kerbiloid]

1 hour ago, mikegarrison said:

More modern rockets often do throttle down toward the end of their stage's burn, in order to limit the loads on the now-empty structure.

They do it since 1960s at the very end of the stage burn to make separation softer, easier, and less affecting the trajectory.

The Saturn-like cases exist but not always. Most of the payloads are uncrewed and see no difference between 4 g and 6 g.

[Meecrob]

I think Mike is trying to say that if you push too hard on the equivalent of an empty popcan, it crushes. Or you have to waste payload mass to add structural mass.

 

[kerbiloid]

20 minutes ago, Meecrob said:

I think Mike is trying to say that if you push too hard on the equivalent of an empty popcan, it crushes.

The popcan carries everything on top and the fuel hydrostatic pressure, unless it's early Atlas.

[mikegarrison]

I just want to make it clear that while I am an aerospace engineer, rocket launch profiles are not my particular field of expertise.

But everything in engineering involves tradeoffs. I am pretty sure (but not certain) that many modern rockets are designed to throttle down in order to limit the structural loads they would otherwise face when they are light. This is a trade between structural weight and engine complexity.

---------------------------------

Did you know that airplanes have a minimum weight? If they get too light, they can actually be too responsive to gust loads. Complex flight vehicles are designed to operate within flight envelopes. Making the envelopes bigger usually adds weight, and so sometimes you have to limit what they can do so they stay in the envelope you design for them.

[Starhawk]

Some content has been removed from this discussion.

Please do not allow emotions to lead to behaviour such as sniping at one another.

Also please do not respond to inappropriate posts.  Just report the post and move on.

Thank you for your understanding,
KSP Moderation Team

[magnemoe]

On 1/28/2022 at 5:24 PM, kerbiloid said:

The popcan carries everything on top and the fuel hydrostatic pressure, unless it's early Atlas.

This is true so an major leak will create an structural collapse, you have some abort systems running wires down the body and if multiple wires breaks abort system activates automatically. 
Who is smart as this is very fast, in early KSP telescoping was an know issue, here typically second stage coupling broke so you had the fun situation of first stage ramming 3rd stage and the payload, it was very fast. 
Part of why asparagus was so popular was as you could make an pyramid of an rocket where the boosters supported the upper stage who would be shorter because of all the boosters.