Subortibal launches in new aerodynamics (and the ideal TWR for lift-off).

[Temeter]

Tumbling is mostly caused by aerodynamic instability. Press F12 to activate the aero arrows, then you'll see if the top of the rocket develops lift. If it develops to much lift, try using larger fins to create counter-lift.

Also, if you keep the rockets nose close to prograde you'll get less lift. A tw/ratio of 1.2 to 1.5 should be safe for easier launches, but you can go much higher.

[PTNLemay]

On 1/23/2016 at 9:58 PM, Waxing_Kibbous said:

Try it without the fairing http://forum.kerbalspaceprogram.com/index.php?/topic/130267-are-fairings-useless/

I've been performing a lot of tests using mech jeb (since it has a Max Q indicator) and I'm beginning to think that this bug was the root of all my problems.  I tried again with a 3-man capsule and it seems much more stable.  I've also been running repeated tests using the max-q limiting option (again, using mech jeb) and the notion that I should limit my TWR due to atmospheric drag is being put more and more into question.  There is a point, very early on in the flight in the thick atmo, where the Q peaks, but very quickly, around 20Km, the drag tappers off to nothing and I can crank the thust back up, as high as 8 Gs and the Q doesn't go above 20 KPa.  

Here are my numbers so far.  Using a small rocket with a mainsail, I set mechjeb to lift me up to a 600 km orbit, asking him to limit the Q to different peaks for each test.  I then checked how much DeltaV I would have upon reaching the circularization point.
 

Q must not surpass this value	DeltaV remaining at Apoapsis
20 KPa	462 m/s
22 KPa	501 m/s
24 KPa	530 m/s
26 KPa	549 m/s
28 KPa	561 m/s
30 KPa	565 m/s

Raising the Q limit higher than that led to diminishing returns, but it didn't hurt me, yet...  I'll keep going higher and higher and see at what point my rocket actually rips itself apart due to atmospheric sheer forces.

Edited January 28, 2016 by PTNLemay

[Nich]

gravity losses and oberth benefits both say go faster sooner.  With a TWR of 1.1 your loosing a ton of fuel to gravity and later you have to carry around that extra tankage further reducing your DV.  Otherwise everyone would start with a 1 and burn until you start moving.  everything I have read in real world states a max twr of 25 on unmanned craft (starting around 7) because the structure to support the craft become excessively heavy.  Air friction is not a major concern

 

Check out the challenges section they have everything from payload fraction, cost per ton, most tonage, least dv.

 

Only time I have seen a limit is below 6km you want to be less then 600m/s i think.  It was a standard rocket max height challange I believe the rocket had something like 2.5 twr at launch

Edited January 29, 2016 by Nich

[Temstar]

8 minutes ago, Nich said:

With a TWR of 1.1 your loosing a ton of fuel to gravity and later you have to carry around that extra tankage further reducing your DV.  Otherwise everyone would start with a 1 and burn until you start moving.

But that's what actually happened with Saturn V, it launched with a lift off TWR of 1.16. The thing is fuel is really cheap but engines are expensive, so to minimise cost it makes sense to carry a lot of fuel in the first stage to get as much use out of the big first stage engines as possible, even if marginal delta-V increase with more fuel is decreasing (but remains positive).

[Nich]

current leader in the low cost challenge has a 1.45 twr

http://forum.kerbalspaceprogram.com/index.php?/topic/130730-the-cheap-and-cheerful-rocket-payload-challenge-105/&do=findComment&comment=2384481

 

my entry which would be in second if not disqualified had 1.97 twr.  If tweaked it could easily take first

http://forum.kerbalspaceprogram.com/index.php?/topic/130730-the-cheap-and-cheerful-rocket-payload-challenge-105/&do=findComment&comment=2383129

You also have to remember ksp is tiny so comparing it to real world  doesn't work 2400 m/s vs 7700 m/s

I also though I remember hearing in ksp engines are too heavy and fuel is too light and ISPs are too low vs real world space shuttle has 345 atm 455 vac isp or something along those line that could be the cause of the discrepancy.  Also in the real world engines are much more expensive.  I believe the SLS just got 17 billion from congress and that has to be divided between 100ish launches to get RnD cost

I will try a 1.16 twr and see if I can get it any were as cost efficient

Edited January 29, 2016 by Nich

[Temstar]

15 minutes ago, Nich said:

I will try a 1.16 twr and see if I can get it any were as cost efficient

They didn't do it for cost so much. It's more that the Apollo spacecraft was overweight so they had to make up the difference somehow with the rocket. Most of the extra delta-V came from making the 2nd and 3rd stage have a common bulkhead and therefore lighter. But they did have this idea that small improvements could be made to the F-1 engine over time to boost the thrust further so they designed the first stage with huge fuel tanks that could barely lift off. The engines were improved over time hence why the last three flights were able to bring a rover with them.

N1 also had the same spacecraft overweight problem, they "fixed" it by superchilling the fuel to make it denser and adding six more engines much to the rocket's detriment.

[Nich]

So I did nearly perfect gravity turns and got everything out of both configs that I could.  Vehicle 1 had a TWR on the pad of 1.14 and made orbit with 12088 units of fuel to be converted to payload.  Vehicle 2 was only able to get  11106 units.  I forget how to convert to tons (?/9000 accounts for fuel and tankage?)  if so that is 1.343t and 1.234 or a 8.833% decrease in payload.  Cost of rocket 1 was 107150 cost of rocket 2 was 94150 or a 13.807% decrease

edit

ops forgot to add the payload which was the same for both rockets of .9t giving the 1.47 rocket even better results of only decreasing 5.11%

Edited January 29, 2016 by Nich

[wumpus]

Oddly enough, my experiences with adjusting rockets left me with the amounts of delta-v left [note these TWRs are limited in the VAB and flown pegged at the new limited "max"].

TWR - deltaV leftover in orbit.

1.25 - 123 m/s

1.51 - 287 m/s

1.67 - 294 m/s

1.75 - 304 m/s

2.00 - 308 m/s

2.27 - 305 m/s

One thing that must be noticed is that this involves a rocket *capable* of leaving the pad at 2.27g.  It obviously makes more sense to use a lighter engine that will only produce ~1.75g (or less), if such an engine is available.  It just makes no sense to throttle down a rocket that makes less than 2.0g for any reason other than ease of rocket control: Throttling down will simply cause higher aero losses.  Note that I've had "kicker" SRBs explode at roughly this [initial] acceleration (>2.1g), so be careful.  It also isn't terribly significant as you are burning ~3000m/s to get there, so these vary just a few tenths of a percent of efficiency (while the control issues can be significant: I couldn't use mechjeb for launches to get this data).

[PTNLemay]

6 hours ago, wumpus said:

One thing that must be noticed is that this involves a rocket *capable* of leaving the pad at 2.27g.  It obviously makes more sense to use a lighter engine that will only produce ~1.75g (or less), if such an engine is available.

True, in the above tests I was using a mainsail and the delta-V remaining at orbit just kept going higher and higher the more TWR I gave it.  But I tried to switch out the mainsail with a skipper, the TWR dropped down to 1.3, and yet I made it into orbit with 100 m/s more than any of the mainsail tests.

So the Oberth effect wins over aerodynamic drag, but making your rocket lighter wins out over everything else.  And using lower thrust engines will always reduce overall rocket mass (nuclear rockets not counting).  I just wish there was a nice middle-ground rocket with more thrust than the skipper, but less mass than the mainsail.

Edited January 30, 2016 by PTNLemay

[Nich]

 I believe that mod is called Tweek scale

But most people consider it cheaty for challenges

Edited January 30, 2016 by Nich

[suicidejunkie]

On 1/30/2016 at 8:28 PM, PTNLemay said:

True, in the above tests I was using a mainsail and the delta-V remaining at orbit just kept going higher and higher the more TWR I gave it.  But I tried to switch out the mainsail with a skipper, the TWR dropped down to 1.3, and yet I made it into orbit with 100 m/s more than any of the mainsail tests.

So the Oberth effect wins over aerodynamic drag, but making your rocket lighter wins out over everything else.  And using lower thrust engines will always reduce overall rocket mass (nuclear rockets not counting).  I just wish there was a nice middle-ground rocket with more thrust than the skipper, but less mass than the mainsail.

I dare say that the aerodynamics of your craft should play a large part in determining an optimum setting as well.  A more streamlined craft would benefit more from the increasing TWR, and an ugly blob of rovers on a stick would benefit more from the lower speeds in the heavy atmo using a lower TWR.

Varying the throttle to get off the ground fast, then cruise subsonic until the air is thin enough to go faster sounds like the best of both to me.

 

PS:

If the mainsail is too big and thrusty, try adding more payload