How do you design a rocket to fulfill the contracts for testing an engine?

[noahtech]

I keep seeing contracts asking me to test X engine at height Y while going at a speed of Z and I can't for the life of me figure out how to do the math to design a rocket that will meet those requirements. I'm trying to design one that will launch straight up, meet the conditions at some point in its trajectory, then fall back down.

Edited December 18, 2014 by noahtech

[Superfluous J]

Two ways:

1) Some engines have the option to right click and "test" them. Check them on the launchpad though because not all engines do and there isn't a pattern.

2) For others, you MUST do it in staging, meaning you MUST bring a different engine to use to take your engine wherever you want to test it.

And regarding how to get there, you have 2 more options

1) Use, overuse, and blatantly abuse "Revert to VAB" to keep redesigning your rocket.

2) Make a testing plane. Even the lowest tier plane should be able to test pretty much any part at pretty much any altitude up to about 22km, at pretty much any speed under say 600m/s. If you add an emergency SRB you can go higher and/or faster if necessary.

Edited December 16, 2014 by 5thHorseman

[noahtech]

Is there a way to do it that doesn't rely so much on trial and error?

[SpeedDaemon]

Two ways:

2) For others, you MUST do it in staging, meaning you MUST bring a different engine to use to take your engine wherever you want to test it.

You don't necessarily have to use another engine (unless something's changed since I did this). I was able to complete a "test by staging in a suborbital trajectory" mission by using the test subject engine for ascent to a suborbital trajectory, disabling it via right click, and fiddling with the staging so I could stage it back on again. (This was in 23.5)

[WanderingKid]

Unfortunately, no. You either have to fiddle a bunch with it or do the math up front. Some helpful pointers:

- Before accepting the contract, look at the Terminal Velocity information on Kerbin on the wiki. This will help you get an idea of what ranges are possible at what altitudes.

- If you're planning on testing on the way up, expect to be a little lower than that unless you've got really high TWR.

- If you're planning on testing on the way back DOWN, expect to be a little higher unless you're running drogue chutes.

- The engine doesn't have to be fueled. The engine doesn't even have to have thrust! You can basically disable the engine in the VAB/SPH and ignite it via staging. Empty SRBs are MUCH easier to test if you empty them out first.

- Check the part prices! I realize this is tough on experimental items but many of these contracts aren't worth the price of the part, especially at higher difficulty levels. If you're not bringing it back with you make sure you're profiting on the launch.

[noahtech]

Unfortunately, no. You either have to fiddle a bunch with it or do the math up front. Some helpful pointers:

- Before accepting the contract, look at the Terminal Velocity information on Kerbin on the wiki. This will help you get an idea of what ranges are possible at what altitudes.

- If you're planning on testing on the way up, expect to be a little lower than that unless you've got really high TWR.

- If you're planning on testing on the way back DOWN, expect to be a little higher unless you're running drogue chutes.

- The engine doesn't have to be fueled. The engine doesn't even have to have thrust! You can basically disable the engine in the VAB/SPH and ignite it via staging. Empty SRBs are MUCH easier to test if you empty them out first.

- Check the part prices! I realize this is tough on experimental items but many of these contracts aren't worth the price of the part, especially at higher difficulty levels. If you're not bringing it back with you make sure you're profiting on the launch.

What do you mean "do the math up front"? I'm trying to calculate how much delta v I'll need for the test. before all I was doing was building one that could get to orbit and hoping that I could pilot it to do the test instead of orbiting.

[Sirrobert]

Personally, I take a tiny probe core, put the engine ontop of that.

And then build a rocket under that to take the entire monstrosity up to the required hight. Then hit space to fulfill contract, and let the whole thing crash

[noahtech]

Personally, I take a tiny probe core, put the engine ontop of that.

And then build a rocket under that to take the entire monstrosity up to the required hight. Then hit space to fulfill contract, and let the whole thing crash

That's what I'm doing except that I'm attaching parachutes to all the parts so I can get them back.

[WanderingKid]

What do you mean "do the math up front"? I'm trying to calculate how much delta v I'll need for the test. before all I was doing was building one that could get to orbit and hoping that I could pilot it to do the test instead of orbiting.

Apologies if that came off wrong. I wasn't trying to be insulting. The problem with a lot of engine tests is they're deadweight until you get to orbit. IE: The LV-T45 is a 1.5t engine. The pod is only .84tons. So, it's the equivalent of lifting 3 pods instead of one.

So, either you end up fiddling with what you've got, a lot, or you have to sit there and work out the entire dV equation from scratch until you know you can get where you need to be. The majority of your dV is spent getting up to 20-25km, and another burst for circularization. Right now, in the 18t limitation, you really don't have enough room to deadlift 3 pods to orbit. Okay, someone may have figured out a way, but as far as I've figured out you can't.

That's what I meant by the math up front. The weight of the part is crucial to where you can test it at the beginning of career. However, if you fiddle with it (and can get the LV-T45 to orbit still attached), you can setup a new staging group, move the engine into it, and re-stage it to 'test' it in orbit. S

Add to that you might need a TWR of 3:1 to get up to the speeds requested for some of the contracts I've seen... or be under drogue chutes to slow down enough to be in the range. Sooo... fiddle with it.

Just as a reminder, a lot of contracts literally aren't worth doing. They become a lot more obvious once you've done the research against them a few times, though I still occasionally pull out one that I go back to with "What the hell was I thinking?!" (AKA: Test chutes at 400m/s at 21km+)...

[noahtech]

Apologies if that came off wrong. I wasn't trying to be insulting. The problem with a lot of engine tests is they're deadweight until you get to orbit. IE: The LV-T45 is a 1.5t engine. The pod is only .84tons. So, it's the equivalent of lifting 3 pods instead of one.

So, either you end up fiddling with what you've got, a lot, or you have to sit there and work out the entire dV equation from scratch until you know you can get where you need to be. The majority of your dV is spent getting up to 20-25km, and another burst for circularization. Right now, in the 18t limitation, you really don't have enough room to deadlift 3 pods to orbit. Okay, someone may have figured out a way, but as far as I've figured out you can't.

That's what I meant by the math up front. The weight of the part is crucial to where you can test it at the beginning of career. However, if you fiddle with it (and can get the LV-T45 to orbit still attached), you can setup a new staging group, move the engine into it, and re-stage it to 'test' it in orbit. S

Add to that you might need a TWR of 3:1 to get up to the speeds requested for some of the contracts I've seen... or be under drogue chutes to slow down enough to be in the range. Sooo... fiddle with it.

Just as a reminder, a lot of contracts literally aren't worth doing. They become a lot more obvious once you've done the research against them a few times, though I still occasionally pull out one that I go back to with "What the hell was I thinking?!" (AKA: Test chutes at 400m/s at 21km+)...

So do you normally get into orbit in order to test the engines? I'm not really talking so much about engines in orbit (those I can figure out). I'm talking about the ones that require me to be going at a suborbital speed at a suborbital height.

Also what if I attached parachutes to all the stages so that I can recover them?

[WanderingKid]

Can't recover stages without mods, unfortunately. Once they're more than 2.5k away, they disappear/self-destruct.

I normally don't take any testing in the atmo. Suborbital can be easier, as you just have to ghet to that height, test, and fall. I don't usually try to recover the parts, so if the contract won't cover the part cost, I won't take the contract. Exceptions are when I'd want to use the part, or it's equivalent, anyway where they're asking for a test.

[noahtech]

Can't recover stages without mods, unfortunately. Once they're more than 2.5k away, they disappear/self-destruct.

I normally don't take any testing in the atmo. Suborbital can be easier, as you just have to ghet to that height, test, and fall. I don't usually try to recover the parts, so if the contract won't cover the part cost, I won't take the contract. Exceptions are when I'd want to use the part, or it's equivalent, anyway where they're asking for a test.

So there's no simple way to calculate how much delta v I'd need for one of these contracts? :/

[Superfluous J]

So there's no simple way to calculate how much delta v I'd need for one of these contracts? :/

No, not really. You need the dV to get up to whatever speed, plus the dV to get up to whatever altitude in the atmosphere, which is very dependant on flight path. A vertical flight path is easier to calculate, estimate, and is cheaper to boot, but then you have to hit the exact right speed at the exact right time (while you're screaming upward through the goldilocks zone at the target velocity). A horizontal, leveling off flight path is easier to achieve, but you need more fuel and thrust and drag and gravity losses are more fluid depending on your flight path.

That's why I suggest, if you're going to do a lot of in-atmo tests, to build a plane. Unlock the basic plane parts and make one. You won't get landing gear (which flummoxes me as well) but just start it on its tail and land with parachutes and you'll be fine. 90 science well spent IMO.

If you don't want to make a plane, I would suggest instead just ignoring the in-atmo tests.

[noahtech]

No, not really. You need the dV to get up to whatever speed, plus the dV to get up to whatever altitude in the atmosphere, which is very dependant on flight path. A vertical flight path is easier to calculate, estimate, and is cheaper to boot, but then you have to hit the exact right speed at the exact right time (while you're screaming upward through the goldilocks zone at the target velocity). A horizontal, leveling off flight path is easier to achieve, but you need more fuel and thrust and drag and gravity losses are more fluid depending on your flight path.

That's why I suggest, if you're going to do a lot of in-atmo tests, to build a plane. Unlock the basic plane parts and make one. You won't get landing gear (which flummoxes me as well) but just start it on its tail and land with parachutes and you'll be fine. 90 science well spent IMO.

If you don't want to make a plane, I would suggest instead just ignoring the in-atmo tests.

Alright thanks.