The Quest For The Holy Trajectory

[LN400]

6 hours ago, Norcalplanner said:

No big surprises with the 2.5m rocket....

 

6 hours ago, Norcalplanner said:

1.25m rockets are a whole other kettle of fish.

This is a surprise to me. I rarely use 2.5 and above. Great find! The curious me wonder if there is a great difference in the thrust (or Isp or mass)/cross section ratio, generally speaking, between the two sizes. Time for me to fire up KSP and punch in some numbers.

 

EDIT. Looking at the data for the T30 compared to the T45

           mass     thrust (atm/vac)    Isp (atm/vac)     TWR (atm/vac)

T30    1.25     200.667/215          280/300             160.5/172

T45     1.5      168.75/200            270/320             112.5/133.3

I wonder if there is any gain in having less fuel in the first stage of my rocket (T30) to reach between 10000 and 15000 instead of 20000-35000, and more fuel in the second stage with a "vaccuum engine" seeing the Isp and thrust get pretty high towards max at around 15km.

The way I figure, what I want is velocity and that comes down to TWR and burn time vs drag and gravity.

Burn time is a function of thrust setting and Isp. Given identical tanks (stock 4.5t) and capping the T30 thrust to the closest I can get to the T45 (vaccuum values), the burn time for

T30    58.9s      for 199.95kN, Isp 300 TWR 3.54/11.65

T45   1m  2.8s  for 200 kN, Isp 320 TWR 3.40/10.19

The question then is at what point does the longer burn time of one engine with a lower TWR give a higher velocity than a higher TWR, shorter burn time engine or rather where would the 2 graphs meet. This is going to be a long study.

Edited August 19, 2016 by LN400

[Norcalplanner]

If you look at the tutorial linked in my sig, you'll see that I'm not a big fan of the Reliant due to its inability to gimbal. I also have a rule of thumb that LFO lifter engines should be run as long as possible, generally at least two minutes, to maximize return on the investment in the engine. So I guess I would recommend adding fuel to your bottom stage, at least until the initial pad TWR gets down to 1.5 or so.

I'll be unavailable most of today, so it may be awhile before I post again.

[foamyesque]

13 hours ago, Norcalplanner said:

25+ runs complete, including some with 1.25m rockets.  Spent more time varying initial velocity for the gravity turn this time.

Initial impressions:

No big surprises with the 2.5m rocket.  Very possible to get to orbit for less than 3,100 m/s with an initial TWR of 1.57.  Not worth cranking it over more than 10 degrees under any reasonable scenario.

1.25m rockets are a whole other kettle of fish.  Really difficult to get below 3,300 m/s to get to orbit with a serially-staged rocket.  A bit easier using 0.625m SRBs from SpaceY, making a smaller version of the 2.5m rocket configuration.  One thing that does appear to make a difference- you can save a bit of delta V by moving those two solar panels off of the capsule and on to the top fuel tank.  Less likely to explode this way too.

It's definitely going to be tomorrow before I put together an imgur album and tally the numbers.

 

That's entirely expected. As you decrease rocket diameter, drag losses are going to increase dramatically.

 

EDIT:

It's also true that these numbers are going to vary depending on your payload. A heavier payload in the same diameter rocket will tend to require less dV to put it into orbit.

Edited August 19, 2016 by foamyesque

[Norcalplanner]

The new thread with stock Kerbin results is now up, although imgur is being cantankerous again.