Would this motor work for what I'm doing?

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rAiN_Twist

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Hey guys,
New to this forum, so sorry if this isn't in the correct section. Most people are going to say this isn't possible, and the few people that have done it have spent lots of money, blah blah. So say I wanted to shoot a "model rocket" to space. It would take roughly 340 model rocket engines to make a ~3 gram rocket reach "space" or right above the exosphere. So obviously that's not realistic to strap 340 single use rockets to a model rocket. So what if you make a small, but efficient jet engine that switches to a oxidizer at 40,000 feet (at 40,000 feet there is a scarcity of oxygen). I read somewhere that a jet engine wouldn't work for a real life rocket because it wouldn't be able to lift the payload. But a small model rocket (or maybe bigger than a model rocket) would that be plausible?


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Xrain

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Hey guys,
So what if you make a small, but efficient jet engine that switches to a oxidizer at 40,000 feet (at 40,000 feet there is a scarcity of oxygen). I read somewhere that a jet engine wouldn't work for a real life rocket because it wouldn't be able to lift the payload. But a small model rocket (or maybe bigger than a model rocket) would that be plausible?
So the short answer is it theortically could work. There is a company in the UK that is doing somthing somewhat similar. https://en.wikipedia.org/wiki/Skylon_(spacecraft)

Would it be a better option than normal rocket motors? No. Small turbine engines like RC ones are not very efficient, and pretty expensive. Also getting the turbine to run in a high oxygen environment would probibly be impossible without custom designing everything. A converted jet engine would make a very poor rocket engine, since the chamber in a jet engine runs at very low pressure compared to a rocket engine. (Higher pressure = higher efficiency in rocket engines)

The purpose of about 3/4 of a normal jet engine is solely for compressing atmospheric air enough to be useable. If you are injecting oxygen directly there is no reason to have that 3/4 of the jet engine. If you remove that part you basically just have a poorly designed liquid rocket engine. That is why all of the rockets that have involved jet engines typically used them in a carrier plane by themselves which carried a conventional rocket.
 
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rAiN_Twist

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So the short answer is it theortically could work. There is a company in the UK that is doing somthing somewhat similar. https://en.wikipedia.org/wiki/Skylon_(spacecraft)

Would it be a better option than normal rocket motors? No. Small turbine engines like RC ones are not very efficient, and pretty expensive. Also getting the turbine to run in a high oxygen environment would probibly be impossible without custom designing everything. A converted jet engine would make a very poor rocket engine, since the chamber in a jet engine runs at very low pressure compared to a rocket engine. (Higher pressure = higher efficiency in rocket engines)

The purpose of about 3/4 of a normal jet engine is solely for compressing atmospheric air enough to be useable. If you are injecting oxygen directly there is no reason to have that 3/4 of the jet engine. If you remove that part you basically just have a liquid rocket engine. That is why all of the rockets that have involved jet engines typically used them in a carrier plane which carried a conventional rocket.
So it would be more practical to build a rocket engine?


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Xrain

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So it would be more practical to build a rocket engine?
Yep there is a bit of a fallacy that rocket engines are inefficient. They are inefficient compared to a jet engine if you put both on a plane, since the jet engine didn't need to carry an oxidizer. If you make a jet engine carry it's oxidizer too the rocket engine wipes the floor compared the the jet engine.

We haven't been using rocket engines for the last 60 years to get to space for no reason. For the conditions that rockets need to operate they are about the most efficient method that we know of and that is politically acceptable. People seem to frown on things like when you set off thousands of nuclear weapons on your way to space.

You can get benefits of using other kinds of engines in the atmosphere where there are more efficient methods. Which is what skylon is doing.

If you had hundreds of millions of dollars you too could devise a novel launch system like that. For the normal people just do what I'm doing in my build log that I updated yesterday. Just use bigger motors and you would get above the karman line.
 

DavidMcCann

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The trouble is you're not accounting for the weight of the motors.
 

rAiN_Twist

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Yep there is a bit of a fallacy that rocket engines are inefficient. They are inefficient compared to a jet engine if you put both on a plane, since the jet engine didn't need to carry an oxidizer. If you make a jet engine carry it's oxidizer too the rocket engine wipes the floor compared the the jet engine.

We haven't been using rocket engines for the last 60 years to get to space for no reason. For the conditions that rockets need to operate they are about the most efficient method that we know of and that is politically acceptable. People seem to frown on things like when you set off thousands of nuclear weapons on your way to space.

You can get benefits of using other kinds of engines in the atmosphere where there are more efficient methods. Which is what skylon is doing.

If you had hundreds of millions of dollars you too could devise a novel launch system like that. For the normal people just do what I'm doing in my build log that I updated yesterday. Just use bigger motors and you would get above the karman line.
What motors are you using?


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rharshberger

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Umm did anyone read in the write up the following, "(For this design, I’m assuming something akin to Estes E9-4 engines, which are on the large end of the common model rocket engines. There are larger ones—the classes go all the way up to O and beyond—but at a certain point, they stop being model rocket engines, and just become rocket engines.)", there is absolutely no way you could get that rocket to fly very far with E9's..... BOOM! :roll:
 

rstaff3

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Umm did anyone read in the write up the following, "(For this design, I’m assuming something akin to Estes E9-4 engines, which are on the large end of the common model rocket engines. There are larger ones—the classes go all the way up to O and beyond—but at a certain point, they stop being model rocket engines, and just become rocket engines.)", there is absolutely no way you could get that rocket to fly very far with E9's..... BOOM! :roll:
If one at the bottom of huge stack of them went, well, the results would be 'really cool'.
 

rAiN_Twist

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Umm did anyone read in the write up the following, "(For this design, I’m assuming something akin to Estes E9-4 engines, which are on the large end of the common model rocket engines. There are larger ones—the classes go all the way up to O and beyond—but at a certain point, they stop being model rocket engines, and just become rocket engines.)", there is absolutely no way you could get that rocket to fly very far with E9's..... BOOM! :roll:
Very, Very, Very good point. Can someone tell me how high the G80-7 will go, and the weight that it will go with that height? Because we might be able to do it with as little as 50 if they are really as big as they say.


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rAiN_Twist

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Umm did anyone read in the write up the following, "(For this design, I’m assuming something akin to Estes E9-4 engines, which are on the large end of the common model rocket engines. There are larger ones—the classes go all the way up to O and beyond—but at a certain point, they stop being model rocket engines, and just become rocket engines.)", there is absolutely no way you could get that rocket to fly very far with E9's..... BOOM! :roll:
Oh, and btw. It only goes to G I haven't found any "O or beyond" engines.


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Xrain

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Very, Very, Very good point. Can someone tell me how high the G80-7 will go, and the weight that it will go with that height? Because we might be able to do it with as little as 50 if they are really as big as they say.
Let me give you some perspective. The G80 has 136 N*s of impulse. The two motors I'm using together have 28,361 N*s, which is 208 G80s. My rocket also even optimistically wont get very close to space, despite being far more optimized for aero and weight than any XKCD esq rocket would be. I'd estimate you would need at least around 40-60,000 N*s to get close even in a crazily optimized rocket.

The XKCD rocket is really, really optimistic in many areas. In reality I don't think you could get a rocket running on E9's or G80s to space even if they all magically lit perfectly.

Oh, and btw. It only goes to G I haven't found any "O or beyond" engines.
That is because G is the largest motor you can buy as a general consumer without a certification. If you are seriously interested in this stuff, go do your NAR or Tripoli certifications. Doing so will answer many of these questions you have, and you will then know what the important questions you should be asking are.
 
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rAiN_Twist

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Let me give you some perspective. The G80 has 136 N*s of impulse. The two motors I'm using together have 28,361 N*s, which is 208 G80s. My rocket also even optimistically wont get very close to space, despite being far more optimized for aero and weight than any XKCD esq rocket would be. I'd estimate you would need at least around 40-60,000 N*s to get close even in a crazily optimized rocket.

The XKCD rocket is really, really optimistic in many areas. In reality I don't think you could get a rocket running on E9's or G80s to space even if they all magically lit perfectly.



That is because G is the largest motor you can buy as a general consumer without a certification. If you are seriously interested in this stuff, go do your NAR or Tripoli certifications. Doing so will answer many of these questions you have, and you will then know what the important questions you should be asking are.
Oh ok! Thanks! What motors do you use? Those are crazy!


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DavidMcCann

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Very, Very, Very good point. Can someone tell me how high the G80-7 will go, and the weight that it will go with that height? Because we might be able to do it with as little as 50 if they are really as big as they say.
Well 2 G80's sent my 4" rocket to 1200', so totally possible

 

DavidMcCann

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Lok I'm going to step aside here for a second from the hailstorm of crap you're about to get.


If you're interested in rocketry, thats awesome. Design and build some small stuff so you have an actual gauge and idea of what you're talking about. Because right now you're talking about trying to fly one of these to the moon.

 
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