Rocket with turbine

[EcDarkSky]

HI guys i would like to start a project. It is a little rocket and for propulsion i would like to try EDF. So my question is if i got a thrust 3.8 kg would it be able to make a fly a 2.5kg rocket or no way

[The_Denks]

So you 'think' you have an engine with 3800 newton grams of thrust.

You have a payload of 2500 grams.

 

It will fly, albeit its a rather low TWR for a mdoel rocket.

I doubt it will get far, or be very stable

[bgibbz]

 

Yes. It should fly up at 5.1m/s^2 (I think, my only knowledge of physics is what I've  learned in the past month of high school physics). 

 

EDIT - This is wrong  

 

Spoiler

 

 

 

[EcDarkSky]

Just now, bgibbz said:

Yes. It should fly up at 5.1m/s^2 (I think, my only knowledge of physics is what I've  learned in the past month of high school physics). 

 

there is my math, some let me know if I'm wrong

could you give me a link to acces these equations pls thanks

[bgibbz]

Just now, EcDarkSky said:

could you give me a link to acces these equations pls thanks

I just learned them in physics class. fnet (net force) = m(mass) x a ( acceleration).

 

I did jut think about it though, and my answer is wrong. The force of gravity is correct, but my upwards force is incorrect. I'm not entirely sure how to calculate force in newtons going up simply given a thrust in terms of weight. All ive learned so far in school so far is how to solve it if im given an initial acceleration. 

[Factory Factory]

1 hour ago, bgibbz said:

I just learned them in physics class. fnet (net force) = m(mass) x a ( acceleration).

 

I did jut think about it though, and my answer is wrong. The force of gravity is correct, but my upwards force is incorrect. I'm not entirely sure how to calculate force in newtons going up simply given a thrust in terms of weight. All ive learned so far in school so far is how to solve it if im given an initial acceleration. 

You're already mostly there. f = ma, and f-net = m * a-net. 

 

For a 2.5 kg rocket, the force of gravity acting on it will be 2.5 kg * 9.81 m/s/s = ~24.53 N. If the engine could hold 3.8 kg against gravity, then it puts out a thrust of 37.28 N, resulting in ~12.75 N of net thrust, or a TWR of about 1.5, which is low but enough to lift the thing.

 

F = ma. 12.75 N = 2.5 kg * a, a = 5.1 m/s. So you had it right.

 

Part of the confusion is that "pounds" pulls double duty for weight (a force) and mass in the Imperial system, whereas SI uses the kilogram for mass and Newtons for all forces regardless of source. This distinction is not important on earth, where a pound-mass feels one pound-force due to gravity, but then people started going into space and landing on the moon and stuff, and imperial-units-English gets a convenient way to express lower gravity ("A 120 pound person only weighs 20 pounds!") that's correct but which totally glosses over the distinction being made in the units. Meanwhile, SI unit users is stuck talking about surface gravity in acceleration like chumps. This one benefit does not make the Imperial system better than SI on net, though.

[Hackentosher]

You have a 1.52 thrust to weight ratio, which is enough to get you going, but not that quickly. You'll need more, probably a higher cell count battery.

 

@straight_stewie, share your aerospace knowledge.

[straight_stewie]

On 11/7/2016 at 2:11 PM, Factory Factory said:

For a 2.5 kg rocket, the force of gravity acting on it will be 2.5 kg * 9.81 m/s/s = ~24.53 N. If the engine could hold 3.8 kg against gravity, then it puts out a thrust of 37.28 N, resulting in ~12.75 N of net thrust, or a TWR of about 1.5, which is low but enough to lift the thing.

 

F = ma. 12.75 N = 2.5 kg * a, a = 5.1 m/s. So you had it right.

That pretty much sums it up. I think that for his purposes (Which is really an airplane that looks like a rocket), a TWR of ~1.5 is perfectly fine. Even modern fighter aircraft capable of vertical acceleration only have TWR's of 1-1.25, so he's doing pretty good. An acceleration of 5.1 m/s²  is plenty, as that's roughly 11 MPH/s² . 

What I'm more worried about is the accuracy of these numbers and the stability of this aircraft:

On 11/7/2016 at 0:21 PM, EcDarkSky said:

HI guys i would like to start a project. It is a little rocket and for propulsion i would like to try EDF. So my question is if i got a thrust 3.8 kg would it be able to make a fly a 2.5kg rocket or no way

Here's a list of information that I would like to get, and an explanation as to why will follow:

• The dimensions of the nose cone of the rocket: Length and diameter.
• The location of the center of gravity of the assembled rocket.
• The location of the motor
• The location of the thrust outlet.
• The mass of the assembled rocket (battery and engine included)
• Or alternatively to the point above, the mass of every component. 

These data items will be used to determine if the aircraft will actually fly, and, if so, if it will actually be controllable. I can also go ahead and get the estimated top speed for you. My math will assume zero wing/body lift (which I assume is wrong). I will also be assuming standard atmosphere in gradient layer 1 only.