Gravity turn and TWR

[villetuomas]

Hello,

Yesterday I was trying to launch with a proper gravity turn. My vehicle was a mk2 shuttle with an external tank and its TWR was a little low after the separation of SRB’s (about 0.8-0.9). However I noticed that the AP continued to rise even with such a low TWR. I then continued with a launch profile where I tried to keep the AP within a minute away. I had the TWR in about 0.9-1.1 during much of the ascent. All the time my AP was rising. I assume this is because as I increase my horizontal speed the ground “falls” away from me even if my TWR is below 1.0. Is this the way it works? 

Anyway, I got a stable orbit around 95km but I was wondering that such a low TWR during a ascent is probably not the most efficient way of launching (previous test run with about 1.5 TWR gave a 250km orbit). What kind of ratio should I aim at after the separation of SRB’s?

Even if it wasn’t the most efficient profile it was a real pleasure getting a nice orbit with such a low TWR! And if nothing else, it had me cracking my brain for the rest of the evening, so all in all a massive success!

[Foxster]

Depends on just where the lower TWR kicks in but it is usually "a good thing".

You obviously need 1+ TWR to get off the ground vertically and having a decent head of steam gets you out of the thicker atmosphere quicker. Once you are into your gravity turn though then a lower TWR will indicate you aren't carrying too much heavy engine unnecessarily and you won't go so fast that you lose a lot of dV to drag losses or risk heatplosion.  

For Eve it can be essential to either have a lower TWR upper stage or throttle back a lot to avoid high drag losses and/or heatplosion. 

Edited March 12, 2019 by Foxster

[villetuomas]

Interesting! I guess I have hauled excessively heavy engines in my crafts all along!

My concern was mainly because I had the low TWR at around 30k (this is a guess, so give or take) which I consider still pretty low. I assume one can also throttle down the further the the turn goes, right? The more horizontal you are the less you need to go up and the more you need to gather horizontal speed. Assuming, of course, that your AP is high enough. 

[Xd the great]

6 hours ago, villetuomas said:

Interesting! I guess I have hauled excessively heavy engines in my crafts all along!

My concern was mainly because I had the low TWR at around 30k (this is a guess, so give or take) which I consider still pretty low. I assume one can also throttle down the further the the turn goes, right? The more horizontal you are the less you need to go up and the more you need to gather horizontal speed. Assuming, of course, that your AP is high enough. 

A low TWR at 30k, with Ap at 60k, is good enough for many purposes. And a low twr means smaller and lighter engines means more fuel and payload.

And a smaller lower stage.

[Zhetaan]

6 hours ago, villetuomas said:

My concern was mainly because I had the low TWR at around 30k (this is a guess, so give or take) which I consider still pretty low.

30 km is still mostly out of the atmosphere.  The pressure there is about 400 Pascals, or less than half of a percent of sea level.  There is still drag (it's certainly enough air for aerobraking, for example) but it's not going to materially interfere with your ascent.

6 hours ago, villetuomas said:

I assume one can also throttle down the further the the turn goes, right? The more horizontal you are the less you need to go up and the more you need to gather horizontal speed. Assuming, of course, that your AP is high enough. 

Yes, you can.  Real-life rockets and the Space Shuttle didn't have the throttle range that KSP rockets have, so they would actually pitch down (meaning add a little radial in) to circularise.  That wastes fuel but the difference in cost between adding more fuel capacity versus designing a deep-throttle-capable SSME is such that anyone who approved that line of development would have earned all of Senator Proxmire's Golden Fleece awards.  I understand that the SSME of itself is capable of throttling down to something under twenty percent, but the pumps have trouble ... suffice it to say that while the capability would be nice, sometimes, the real-life answer really is, 'More boosters!'

In any event, you have the basic idea:  horizontal velocity is yours to keep, but vertical velocity is not.  The entire point of a gravity turn is to trade vertical velocity for horizontal as your speed and altitude increase, within the limits of your thrust-to-weight ratio.  A perfect gravity turn is one that exchanges vertical for horizontal velocity as soon as you no longer need it, and does so continuously throughout the ascent.

This can result in some interesting-looking gravity turns.  For example, Minmus gravity is such that many landers have a surface TWR of around eight or ten, or even more.  With such extreme thrust and no atmosphere, the perfect gravity turn on Minmus is to thrust up just long enough to clear the ground and then turn hard over to horizontal:  you'll ascend to orbit easily because you have too much engine power to do otherwise.

On the other hand, for your Kerbin ascent, you are seeing the opposite end of the spectrum.  You need a TWR greater than one to get off the ground; that's non-negotiable.  However, staying off the ground is a new problem, and once you begin to ascend, the situation begins to change.  As your trajectory begins to look more and more like an orbit, your TWR needs begin to reflect that.  Since orbital thrust is only required to be nonzero (although greater thrust means shorter burns), it means that the required TWR begins to drop.

Practically, that means that you need enough thrust to counter whatever drag you still experience, and you need enough thrust over that to keep the surface of Kerbin receding faster than you fall, which simultaneously counters gravity losses and gains altitude.  Especially when you're in the upper atmosphere where drag is low, and doubly especially when you have a lot of horizontal speed such that the gravity losses are low, that required TWR will be less than one.

If you ever decide to try spaceplanes, you'll start to see this much more clearly.  Wings to provide lift alleviate the need to thrust vertically, which means that the engines can focus on providing horizontal thrust alone.  The main challenges are to reduce drag, and to balance the interests of building up enough horizontal speed while low enough in the atmosphere for the wings and jets to work but also high enough that you won't burn up from the increased drag of the lower atmosphere, but meeting these challenges does not require a TWR of greater than one.

Edited March 12, 2019 by Zhetaan

[villetuomas]

@Zhetaan Thank you for a very concise reply! I get it a lot more clearly now. I think this one launch and subsequent thought process has been the highlight of my KSP career this year. This is such an amazing game. It just forces you to think and learn new things on a regular basis. Payloads, here I come!