Another Rocket Ascent Question

[Spaced Out]

When a rocket is ascending, do you want to clear the lower atmosphere before gaining any signifigant horizontal velocity to avoid heat?

[p1t1o]

A gravity turn is an efficient ascent - a gentle curve all the way up with almost constant acceleration, not a vertical section followed by a turn and a more horizontal section. A gravity turn begins almost as soon as you clear the tower, but you will have cleared the "lower atmosphere" before gaining too much of an angle.

Its drag though, that you are avoiding, heat should not be a concern since the lowest part of your trajectory is also the slowest. In KSP the heat/flame visuals display at quite low velocities though, but IIRC do not necessarily represent a damaging heat flux. 

Having said that, with a proper gravity-turn ascent and acceleration profile, you shouldnt even get the heat/flame visuals as the acceleration should be fairly gentle (~2-3-4 Gees).

There is no point in blasting away at many Gees, then you WILL lose a lot of dV to drag.

[Spaced Out]

@p1t1o  Thanks, and by the way is there a website where I can learn more about rocket ascents instead of just filling up the forum with questions?

[Green Baron]

Actually, quite a lot, when i search "rocket ascent profiles" i get a lot of links and documents.

And i once found this program but i have never tried it myself.

[p1t1o]

1 hour ago, Spaced Out said:

@p1t1o  Thanks, and by the way is there a website where I can learn more about rocket ascents instead of just filling up the forum with questions?

People here generally happy to answer questions, or they dont bother commenting, you won't hear "RTFM nuube!!" very much here

Pretty much all the questions have been asked at some point, by someone, so dont feel bad.

If you ask me, wikipedia is a good start for most things, and most pages will have plenty of links to further info.

https://en.wikipedia.org/wiki/Gravity_turn

 

There are two links you need though, if you are playing KSP. They are a bit more advanced, but if you get into KSP for more than a couple of weeks you will want the info contained within. (and they are by far THE most referenced links on this entire forum - ok I cant prove that, but you get it...)

http://www.projectrho.com/public_html/rocket/ - This website, intended for people writing science fiction stories, contains a huge amount of material describing real-life and near-future technology in terms of space travel. Also has extensive sections on hypothetical combat in space. The propulsion tech section is also highly relevant.

https://library.sciencemadness.org/library/books/ignition.pdf - This is a book. Its a legit link, dont worry about downloading it. It is a history of research into rocket fuels, written by a scientist who worked on the forefront of it. It is actually quite a fun read and is written in a very easy to digest manner (its not full of maths or theory you need a degree to understand). It brings a lot of rocketry into real perspective.

 

Welcome to KSP!

 

Edited August 18, 2017 by p1t1o

[Bill Phil]

Most of the atmosphere is just a few kilometers above your head. It's not hard to get above it. The gravity turn optimizes the ascent, going up initially for a small amount of time and then turning over, gaining both horizontal and vertical speed, and eventually almost all of the gains are horizontal. Having to add that vertical portion increases Delta-v by large amounts, as well as having to burn engines for long periods.

[sevenperforce]

I've seen it phrased this way:

If you could drive the average sports car straight up through the air, without a roadway, then it would take just a little over two hours of driving to reach the altitude of the International Space Station. But you'd still only be moving at 0.7% the speed of the International Space Station.

[Lukaszenko]

My understanding is that losses due to gravity are much greater than aerodynamic losses, so it generally does make sense to punch it as hard as you can as low as you can.

[p1t1o]

36 minutes ago, Lukaszenko said:

My understanding is that losses due to gravity are much greater than aerodynamic losses, so it generally does make sense to punch it as hard as you can as low as you can.

In a perfect world, a well-executed gravity turn perfectly balances gravity losses against aerodynamic losses. You are flying at the point where any faster and you lose more to drag than you gain from spending less time in the gravity well.

In KSP, you very quickly pass through the layers of atmosphere that are thick enough to illustrate this balance (as pressure drops away, so does drag, so the gravity term takes sole control) so it is more possible to do 15G launches from ground level without too much loss in terms of dV.

 

[sevenperforce]

1 hour ago, Lukaszenko said:

My understanding is that losses due to gravity are much greater than aerodynamic losses, so it generally does make sense to punch it as hard as you can as low as you can.

Yeah. Losses due to gravity can be calculated very simply: t x g, where t is flight time and g is the acceleration due to gravity. For TSTOs this typically holds true up through staging. So if you take 100 seconds to get to staging, your gravity losses are 981 m/s. If you take 50 seconds to get to staging, your gravity losses are 491 m/s. Thus, gravity losses are (effectively) inversely proportional to TWR. It's really quite dramatic.

Instantaneous drag is proportional to the density of the air times the square of the velocity. But air density decreases exponentially with altitude. At first, the square of velocity ramps up faster than decreasing air density, but most rockets hit MaxQ fairly early. As long as your ascent isn't almost completely flat, air density will be dropping faster than the square of velocity after MaxQ, and so you have only gravity losses to worry about.

[Deddly]

4 minutes ago, sevenperforce said:

Yeah. Losses due to gravity can be calculated very simply: t x g, where t is flight time and g is the acceleration due to gravity. For TSTOs this typically holds true up through staging. So if you take 100 seconds to get to staging, your gravity losses are 981 m/s. If you take 50 seconds to get to staging, your gravity losses are 491 m/s. Thus, gravity losses are (effectively) inversely proportional to TWR. It's really quite dramatic.

Just to clarify this point - @sevenperforce is talking about what happens if you launch straight up, fighting gravity all the way. This shows very clearly that begining your gravity turn early is important for fuel efficiency.

[sevenperforce]

44 minutes ago, Deddly said:

Just to clarify this point - @sevenperforce is talking about what happens if you launch straight up, fighting gravity all the way. This shows very clearly that begining your gravity turn early is important for fuel efficiency.

Err...unless I'm wildly off somewhere, this is still true whether you do a gravity turn or not.

Of course, by the time you get close to staging, you've started to get a centrifugal (not centripetal) assist, but that really starts showing up after staging in most TSTOs.

[wumpus]

23 hours ago, p1t1o said:

In a perfect world, a well-executed gravity turn perfectly balances gravity losses against aerodynamic losses. You are flying at the point where any faster and you lose more to drag than you gain from spending less time in the gravity well.

In KSP, you very quickly pass through the layers of atmosphere that are thick enough to illustrate this balance (as pressure drops away, so does drag, so the gravity term takes sole control) so it is more possible to do 15G launches from ground level without too much loss in terms of dV.

If you want to experiment with TWR balances, expect to spend a lot of time adjusting mechjeb to create repeatable (or ideally optimal for that TWR) flight paths.  You can get wildly wrong data by simply firing sounding rockets vertically into the sky (I think those work best ~2.0 TWR for a single stage.  This didn't help me for getting to orbit).

[YNM]

IRL atmosphere is very thin compared to the planet's curvature and most parking orbit height. Clearing it isn't as much as a problem like in KSP. The biggest hurdle is going fast enough to miss it entirely.

[magnemoe]

On 19.8.2017 at 1:30 AM, Bill Phil said:

Most of the atmosphere is just a few kilometers above your head. It's not hard to get above it. The gravity turn optimizes the ascent, going up initially for a small amount of time and then turning over, gaining both horizontal and vertical speed, and eventually almost all of the gains are horizontal. Having to add that vertical portion increases Delta-v by large amounts, as well as having to burn engines for long periods.

You might also start turning a bit early for safety reasons so an fail will not hit pad or complex. This is also important in KSP hard mode as our rockets are less reliable, however they tend to fail in other ways, mostly loosing boosters than engine fails.