I need help calculating simple altitude records...

[Zeddah]

I have read (and seen Scott Manley's) instructions on how to begin the new career mode. So I figured I want to calculate trajectories first, and then fire my rocket to minimize the costs. Since I'm used to trial and error Also I liked the challenge. But I've hit some difficulties

So what I don't get is: for the first contracts (let's take example: set alt. record of 11km) Given the rocket is the RT-10 Solid Fuel Booster with a Mk1 pod and Mk16 parachute. And I want to calculate how much fuel I have to take, and what thrust I should be limiting to...

the Delta-v is:

ln(Mfull/Mempty)*Isp*9.81 = 2595.72 m/s/s

Mfull = 4.7

Mempty = 4.7-3.25 = 1.45

Isp = 225 in atmosphere

Now... I don't understand this number... 2595.72 m/s/s is like saying the maximum acceleration that rocket with that payload can get is 2595.72m/s per second... But the rocket can maximally burn for 29 seconds with max thrust (15liters/sec for 433 liters) and will only get around 500m/s when fuel is empty...

Also, how do you calculate from here on out what the optimum setup is to reach as close as say 11km? Or is it not possible to calculate and do you guys just do this by trial and error? I really feel I should be able to calculate this. I would really love it if I could. One small step for me

Please help me math wizards

Edited January 7, 2015 by Zeddah

[Taki117]

the issue with trying to calculate dV expendature while in atmosphere is you have atmospheric drag to consider, which makes the equation infinitely more complicated. Also, atmospheric Isp is usually less than vacuum Isp and as such will lower total dV.

[ghpstage]

As above, the real issue is atmospheric drag here, and I don't know how the game calculates that. Also the ISP isn't fixed, it slides towards the vacuum value as altitude increases.

Its not really worth trying to calculate in my opinion, trial and error all the way!

For a good example of the impact of drag try this, l

Launch an RT-10 with the default settings, and then launch another with the throttle set to 26%. The former will get to around 18k, the latter to 45k!

The fuel costs here are trivial by the way, unless you really want to challenge yourself to get the bare minimum you only have to worry about not killing off the next contract!

Now... I don't understand this number... 2595.72 m/s/s is like saying the maximum acceleration that rocket with that payload can get is 2595.72m/s per second... But the rocket can maximally burn for 29 seconds with max thrust (15liters/sec for 433 liters) and will only get around 500m/s when fuel is empty...

dV shouldn't be in m/s/s, thats an acceleration unit. It will be m/s.

What it means is that your craft can change its velocity by 2595.72m/s by burning all of its fuel when no other force is applied.

You aren't going to achieve an increase in velocity like that when launched from a planet, even one without an atmosphere, because gravity is acting against you with a force of m*g during your launch.

Edited January 6, 2015 by ghpstage

[Zeddah]

Thanks for the replies! I can move on now. I thought about the drag being in the mix, but didn't find anyone who added this parameter in calculations.

I will try experimenting with the thrust limiter in sandbox mode then before applying it to career mode

[DBT85]

I did all of these with no calcs. I used Engineer to ensure my lift off TWR was down at about 1.4 and then dropped the fuel accordingly. When my Ap reached the target I started banking over to not hit the next target.

For the 56k and escape the atmosphere you need a separator if you're using DRE as otherwise you'll burn the chute. I also use FAR btw.

[Zeddah]

For a good example of the impact of drag try this, l

Launch an RT-10 with the default settings, and then launch another with the throttle set to 26%. The former will get to around 18k, the latter to 45k!

I'm not sure what you mean by set throttle to 26%. This RT-10 engine cannot be shut down when launched and has no throttle control. So I guess you mean thrust limiter? Again, I tested several limits with full fuel (20%, 26%, 36, ...) And every time fuel runs out at 13km and the highest point I got was 28km. What am I doing wrong then? Is there a secret setting?

[Dr-Drunk]

I did all of these with no calcs. I used Engineer to ensure my lift off TWR was down at about 1.4 and then dropped the fuel accordingly. When my Ap reached the target I started banking over to not hit the next target.

For the 56k and escape the atmosphere you need a separator if you're using DRE as otherwise you'll burn the chute. I also use FAR btw.

Thing is though, not considdering the rocket cartwheel maneuver, getting higher altitudes with rockets in FAR is easier. FAR can be more difficult to fly in if you don't know what you're doing but the reworked aerodynamics model actually makes it so that you need far less Dv to get to higher altitudes or an orbit for that matter.

I believe in stock KSP the Dv required is about 4500 with 4600 suggested for margin for error. In FAR the number is closer to 3900 as long as the rocket has no flat surfaces and all.

As for OP's question though. I suggest you just do this by trial and error. Adjust the thrust limmiter and fuel level of the booster and just note down the results to apply them in carreer. As others have already stated, doing the maths on the atmospheric drag is nearly impossible.

[CorBlimey]

11km ~= 1 mk1 pod + 1 chute + small SRB at about 20% thrust with about 1/3 fuel. Just guessing .

Some random musings for the next stage after altitude records:

I have found that the top 4 factors in order of importance for an efficient ascent to orbit (not your question but still rubs in the factor of air resistance):

1) Keeping speed as close to terminal speed as possible (about 80 m/s at 0m to about 230 m/s at 10km, approximately linearly, then very roughly doubling every 5km - see http://wiki.kerbalspaceprogram.com/wiki/Kerbin#Terminal_Velocity_Table) .

2) Not skimping on Thrust to weight ratio so as to be able to maintain number 1 - just because something has higher dV on paper doesn't mean it can get to orbit as efficiently. You will generally want a variable thrust main engine (you can use boosters but dial down your main engine as the boosters burn off their fuel). Approx. thrust to weight ratio of 1.5 minimum at launch which should climb to 2+ by the first 10km of ascent. Higher TWR (2.5-3 ish) is useful above 10km until about 30km altitude to keep accelerating up to terminal speed, which rapidly increases with altitude.

3) A smooth ascent profile (the simplest to learn first is the classic 10km vertical then 45 degrees until apoapsis is 75km, then coast to apoapsis for circularisation. I personally (unless I have seriously too much TWR) start turning slowly at 8km then keep reducing heading until dipping below the artificial ground horizon so the gravity turn continues at about 50-55km until apoapsis hits 75km)

4) Adequate torque (for a light craft), or a gimballed main engine (for heavier craft) and/or control wingtips to maintain this smooth ascent profile. The T30 engine may look better than the T45 engine due to lower mass and more thrust but for any early-career typical multi-stage tall and wobbly rocket, the T45 will give you more consistent dV to orbit due to a superior ascent profile. T30's work ok if you have lots of torque or wingtips and a symmetrical design.

The best launch vehicles at early game include SRB's for cheap launches. Tweak the thrust downwards in the VAB to prevent it blasting above terminal speed. Boost initially with your main engines till 100 m/s then let the SRB's carry you up to 10km or so.

Being able to ascend efficiently makes a vast difference to the tonnage and dV you have to play with once in orbit for your payload. Ascending too fast or too vertically can easily cost upwards of 500 m/s which is equivalent to the dV needed for a Munar landing descent.

[ghpstage]

I'm not sure what you mean by set throttle to 26%. This RT-10 engine cannot be shut down when launched and has no throttle control. So I guess you mean thrust limiter? Again, I tested several limits with full fuel (20%, 26%, 36, ...) And every time fuel runs out at 13km and the highest point I got was 28km. What am I doing wrong then? Is there a secret setting?

Yes the thrust limiter is what I meant

Are you playing any mods? Anything that touches the aero will make a mess of this.

Also, this only works for the OPs rocket setup, an RT-10 with a mk1 command pod and a mk16 parachute, with no other weight.

Edited January 6, 2015 by ghpstage

[ElWanderer]

I have been working on a spreadsheet that models a simple, vertical launch at split second intervals. For ages it was wildly overestimating how high an RT-10, pod (monoprop removed) and parachute should be able to go. Eventually I discovered SRBs have a drag factor of 0.3 rather than 0.2, no idea why, but it has a massive effect! With that extra drag taken into account, it still overestimates, but not by much (RT-10 with 30% fuel and 19% limiter actually went to 7040m rather than the predicted 7140m). I've just checked the mk16 parachute and it seems that has a drag factor of 0.22 not 0.2, which might cause the difference. I would try to correct it here and now, but loading the spreadsheet almost crashed my tablet. Will have to wait until tomorrow when I've access to a PC.

[wumpus]

I'm not sure what you mean by set throttle to 26%. This RT-10 engine cannot be shut down when launched and has no throttle control. So I guess you mean thrust limiter? Again, I tested several limits with full fuel (20%, 26%, 36, ...) And every time fuel runs out at 13km and the highest point I got was 28km. What am I doing wrong then? Is there a secret setting?

Right underneath the thrust limiter should be "amount of fuel". You need to reduce that one. Also do all the tricks in the Scott Manley video to grab all the science you can. For later runs you probably want to use liquid fuel rockets (or some combination of a liquid stage on top of a solid stage), if only to have a throttle control. Once you are well on your way you can use the map mode (hit "m" to see where your rocket is going) and look for the apoapsis mark (mouse over it to find out your eventual height). Throttle down your engines (the "x" key) once your apoapsis is above the contract point (but add some extra, it will come down a bit due to air resistance).

I'd really suggest the science mode for new players. The career mode seems designed to let current players experience KSP all over again with a hand or two tied behind thier back* (science mode locks parts. Career mode locks parts, weight restrictions, manuever nodes, and a few others (number of parts for one).

* I'm not abysall lurker.

(oddly enough, this would have helped in my learning to hit the Mun without manuever nodes).

[SRV Ron]

Check out my tutorial for First Time Career KSP 0.90 Beta http://forum.kerbalspaceprogram.com/threads/106183-An-easy-guide-for-first-time-Career-in-KSP-0-90-Beta

The suggested tweaked values for the RT-10 for the first four altitude missions are listed.

[Zeddah]

Yes the thrust limiter is what I meant

Are you playing any mods? Anything that touches the aero will make a mess of this.

Also, this only works for the OPs rocket setup, an RT-10 with a mk1 command pod and a mk16 parachute, with no other weight.

Wow, indeed. I added the 2 side parachutes to recover the RT-10. It's hard to believe that such a small addon can increase the drag by so much. After removing those I can reach 45k.

[MarvinKitFox]

There are basically two methods you could use:

1) Apply post-doctoral theoretical physics and mathematics to calculate the needed multidimensional integrals and nonlinear vector equations to actually predict accurately what will happen.

or (my approach)

2) You are using just the one RT-10 booster, with just two variables: Fuel load and burn tempo. Both of these settings are quite granular, so you only have 8-10 settings possible each.

set up a simple testbed, try the various combinations and note them down.

For the actual flight, select from your spreadsheet the combo that works for you, and profit.

Actually, you will very soon see the trend. I find no more than 3-4 tweaks are needed to arrive at the exact optimal solution.

[SRV Ron]

Wow, indeed. I added the 2 side parachutes to recover the RT-10. It's hard to believe that such a small addon can increase the drag by so much. After removing those I can reach 45k.

At the tweaked thrust levels being used, the added weight and drag of the two radical parachutes, (MK-16 plus braces,) will greatly affect performance. So long as you land in the water, the MK-16 parachute will recover the MK-1 pod and spent RT-10 booster safely.

[Kesa]

Wow, indeed. I added the 2 side parachutes to recover the RT-10. It's hard to believe that such a small addon can increase the drag by so much. After removing those I can reach 45k.

The additionnal 300 kg is quite a lot compared to the dry mass of the craft (1.3t, 1.6t with them). When you add mass, you always should consider that you add this mass to the dry mass of the stage the part is. An one man EVE lander won't be able to carry 2 kerbals, despite the facts that it only adds 144 kg to a several tons monster. The playload is twice heavier, hence twice difficult to lift.

3 Grider Segments as legs can also be used for land recovery. They are heavier, but also cheaper than radial chutes, don't need tech unlock and may be more mass efficient for heavier designs.

Landing on water will also save your RT-10

[numerobis]

I guess at the fuel amount, then I set the thrust limiter in order to get a TWR around 1.8 to 2.

Then I launch. To get to 5 km you really don't need much fuel. After that, I boost the fuel about linearly (double the height, double the fuel).

A launch costs only a bit more than $1k, and you get to recover almost all of it. I launch with just one parachute, even though the SRB gets crushed when I do that and land on land.