Need contract help- Test BACC "Thumper" Solid Fuel Booster in flight over Kerbin

[cmallow]

I'm having trouble meeting the following conditions with the thumper:

alt: 52,000m to 57,000m

Spd: 20.0m/s to 150.0m/s

Can someone help? I can't seem to slow down the space craft to less than about 300m/s

[Snark]

15 minutes ago, cmallow said:

I'm having trouble meeting the following conditions with the thumper:

alt: 52,000m to 57,000m

Spd: 20.0m/s to 150.0m/s

Can someone help? I can't seem to slow down the space craft to less than about 300m/s

Welcome to the forums!

If you have at least a level-1 pilot, or a HECS-or-better probe core, then you have "hold prograde" and "hold retrograde" as options for your SAS, which makes "slow down" easier.  Just launch straight up, and watch your Ap climb in the map view.  When your Ap reaches around 57 km, cut throttle and coast.

When you get up in the relevant altitude band, just look at your speed.  If you're going too fast, choose "hold retrograde" on your SAS, and then thrust until you're within the appropriate speed range.

It helps if you keep the contract window open while you're doing this, because that way you can watch the little green check marks appear as you satisfy the various conditions.  When all the check marks are checked, then stage the booster to complete the contract.

[FullMetalMachinist]

Hi @cmallow, welcome to the forums! 

34 minutes ago, cmallow said:

I'm having trouble meeting the following conditions with the thumper:

alt: 52,000m to 57,000m

Spd: 20.0m/s to 150.0m/s

My first suggestion for that would be "that's a dumb contract, don't take it." But, seeing as how it's too late for that, let's see what we can do. 

34 minutes ago, cmallow said:

I can't seem to slow down the space craft to less than about 300m/s

By this I assume that you are going either too fast on the way up, or maybe going past your target altitude and then coming back down too fast? 

The way I would tackle the requirements would be to build a big, slow first stage using a liquid fuel engine, and keep your speed down as you climb. It should have enough fuel to make your launch pad TWR around 1.2 or so. This might see a bit low, but that's what we want, because otherwise it would be really easy to go zooming past your speed target.

Going that slow is going to really waste a lot of fuel, so you'll probably need a second liquid fuel stage to get to altitude. Then put the solid booster to be tested in the third stage. 

Flight profile would be something like "take off going fairly slow and straight up, then use the throttle to keep you going up and slowly gaining speed." Keep going like that until the altitude/speed requirements are met, and quickly kill the LF-O stage engine, and start the solid booster.

When you do that, immediately turn towards the horizon. The reason being that going straight up and then back down is a good way to end up as a fireball when you cash into the ground.

Basically when you're that high up, gravity pulls you down so fast when the air is thin that the thick air close to the ground can't slow you down enough to use parachutes. You can get around this by prolonging the time you spend in the thicker air. The way you do that is by flying sideways. So when you fire the booster to complete the contract, point sideways and a little bit up, that should work pretty well. 

In all honesty though, that's a really narrow window to complete the contract. It'll probably take lots of trial and error to get it. If you find that you still can't get it to work, post a screenshot of the rocket you're using a give a detailed description of what your flight profile is like. From that you'll hopefully get some more specific and helpful advice. 

[FancyMouse]

My take is always go straight up and adjust appropriately. Then just make it into space and go back KSC and let the (unmanned) test ship die.

Remember to empty out solid fuel of SRB.

[Kryxal]

Isn't the contract's velocity requirement to be met in surface mode?  Navball switches to orbit mode at something over 30k, that probably makes things more difficult...

[Hodari]

I'd say the only reason why you would ever want to take a contract like that would be so that you can NOT complete it and have the part available to use before you've actually unlocked the node for it...

It can be easy to miss sometimes, but this is why before you accept a test part contract, you should always look at ALL of the details carefully and ask whether or not they're realistic.  Would you ever actually use that part under those conditions?  A solid booster should normally only be used as the first stage, so a contract that requires you to use it at high altitude like this(or worse yet, when you're already in space) is probably better to avoid. 

Since you already have taken it with the intention of actually completing it though, the next question would be:  Does the contract say you need to actually TEST the part under those conditions or just HAUL it into flight under those conditions? 

[Chakat Firepaw]

An important formula for velocity/altitude windows is:

h = v²/20

That is, if you are going straight up and turn your engine off, you will continue to ascend for the square of your velocity divided by 20, (on Kerbin, neglecting air resistance).

Do that calculation for the max speed of the contract requirements, add the width of your altitude window, (e.g. 5000 for your contract), and back-calculate your entry speed.  With your numbers, you can safely enter the window at up to 350m/s and still slow down enough without thrust or aerodynamic braking.

You can also calculate numbers for earlier benchmarks, (e.g. at 40km you want to be under 600m/s).
 

Now, this does have serious problems for low altitudes and high velocities but with those it's more likely to be an issue of "how do I get that fast that soon?"

 

(Disclaimer:  This involves flight profiles that can be hazardous to the survival of any portion of your craft, as such you may want to avoid using a biological control system.)