Thrust efficiency

[TheBlueGuy]

Hello,

 

   I'm encountering some counter intuitive issue while trying to make a rocket reach the escape velocity.

Indeed, as the ISP is supposed to be lower and the atmospheric drag higher, it sounds logical not to use the rocket maximum thrust at sea level for saving fuel.
But in practice, using the maximum rocket power at sea level allows to reach higher altitude at an higher speed than carefully controlling the thrust power and the rocket's altitude.

Ex :

• When controlling thrust power: trajectory apogee : 116,000 m / speed: 800 m.s-¹ at 50,000m
• Using max thrust for the whole launch sequence : trajectory apogee: 488,000 m / speed: 2000 m.s-¹ at 50,000m

Has someone any idea about this ?

[Snark]

Hello, and welcome to the forums! 

25 minutes ago, TheBlueGuy said:

Has someone any idea about this ?

There are two main potential sources of "loss" when you're climbing to orbit from the surface of Kerbin, i.e. ways that you "waste" dV on your rocket:

1. Gravity losses, i.e. fuel you spend fighting gravity rather than accelerating your rocket.
2. Aerodynamic losses, due to drag.

(There's a third category that can happen, cosine losses, if you spend significant time thrusting any direction other than .  However, assuming you're doing a reasonably well-executed gravity turn, you can keep cosine losses close to zero, so I'll assume those aren't an issue here and will ignore them.)

Ever since the new aerodynamics model arrived in KSP 1.0, it's been the case that for most "normal" rockets, gravity losses are much bigger than aerodynamic losses.  Therefore, your piloting strategy should focus on the gravity losses.

You minimize your gravity losses by being high TWR, at least during the initial part of your ascent when you're going mostly vertical.  (Easy way to see this:  Imagine if you had a TWR of 1.0001:  you'd rise sloooooooowly off the launch pad, basically just hovering.  Burning tons and tons of fuel, but going nowhere and getting no velocity out of it.)

Climbing fast like that will mean you get more aerodynamic drag, sure.  But since gravity losses tend to be so much bigger than aero losses, it ends up being a net win.

The optimum-efficiency case for a vertical ascent is when your rocket is traveling right at terminal velocity all the way up (bearing in mind that terminal velocity rises with altitude, due to the thinning air).  So if you built a rocket with stupid-crazy-huge TWR like 5 or something, then yeah, you'd be wasting way too much dV to aero losses and should slow it down.

But if you did that... you'd be inefficient due to having far more engine than you need, and lugging around all that dead weight.  Most rockets aren't designed like that.

Most rockets have a saner TWR, either less than 2.0, or at least not a lot higher than 2.0.  And for such a rocket:  you should put the hammer down and accelerate at 100% throttle all the way.  Why?  Because you never catch up to your terminal velocity, which means "the faster the better" and higher throttle = higher efficiency.

So that's the theoretical explanation of the results that you have already experimentally observed.  Congratulations on your successful experiment.  You're officially a rocket scientist! 

[TheBlueGuy]

Thank you so much for this very quick and detailed answer ! That makes things a lot clearer !

Edited October 3, 2017 by TheBlueGuy

[Snark]

Incidentally, I don't mean to belittle aerodynamic losses.  Just that the way to minimize them is via ship design (i.e. making it streamlined) rather than by piloting (i.e. trying to rise slowly, which is generally not the answer).

The amount of aero loss depends on your ship design, of course-- the less streamlined you are, the bigger the losses.  However, for most "reasonably" sleek rockets, you don't have to worry about it much.

The exception happens if you've got some funky vehicle that has really horrible aerodynamics and there's no way to fix it (e.g. it's a big ungainly thing that's too big to fit in a fairing).  If that's the case, and if the design of the craft is really draggy, then sometimes it may be that you just have to bite the bullet and rise a bit more slowly and just accept that you're going to waste a lot of fuel.

But for the large majority of rocket designs, that's generally not a concern.

[Aegolius13]

@Snark hit the nail on the head, but one more minor thing to add.  Even putting aside drag issues, you can't avoid the specific impulse penalty at low altitudes by throttling down.  Specific impulse is the ratio of thrust produced to fuel consumed, so if you throttle down you're using less fuel, but you're getting correspondingly less thrust.  Which means you'll be spending longer at low elevations. In fact, this will be counterproductive, because as mentioned above, the higher your TWR, the less total gravity loss you will incur, and the faster you'll climb out of the bad-ISP lower atmosphere.  

Of course, you can use design choices (especially engine choices) to minimize the atmospheric ISP hit.  Depending on what you're trying to optimize, you can either go with a very efficient ASL engine like the Vector (which is great for keeping mass low, but is expensive), or you can use big dumb cheap thrust like SRBs to blast yourself out of low atmosphere (which is generally cost-efficient but not so mass-efficient).