RocketEnthusiast101
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Hi. How much thrust would be required for a rocket reaching an altitude of 100km and also what set of formulas should I be using?
How much thrust would be required for a rocket reaching an altitude of 100km?
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DAllen
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How much does the rocket weigh? What is the diameter? How many fins?
Thrustcurve has a model rocket sim that will give you an idea as a starting point. Use OpenRocket or RockSim to give you a more accurate idea.
Thrustcurve has a model rocket sim that will give you an idea as a starting point. Use OpenRocket or RockSim to give you a more accurate idea.
RocketEnthusiast101
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Thanks for the response. So the altitude gained only depends upon fins, the diameter of tube and rocket weight (after everything has been loaded and the rocket is fully assembled?)
Thrust is used to move the rocket. It does not determine altitude. Thrust is the lifting force used to overcome gravity.
Thrust sustained over time is called impulse. Impulse is a major factor in how high a rocket will go. Total impulse is the average thrust of the motor over its burn time. Total impulse is the thrust multiplied by the length of the burn. So, 2 Newtons for 5 seconds yields a total impulse of 10 Newton-seconds; but 2 Newtons for 2 seconds yields 4 N-s.
Each motor, by its size and formula, has limits on how much thrust it can achieve and how long it will burn.
Then there is stability. A stable rocket flies straight up, whereas an unstable or overstable rocket will curve to some extent. straight up yields higher altitude.
So you see, the answers are more complex than the questions you are asking.
Thrust sustained over time is called impulse. Impulse is a major factor in how high a rocket will go. Total impulse is the average thrust of the motor over its burn time. Total impulse is the thrust multiplied by the length of the burn. So, 2 Newtons for 5 seconds yields a total impulse of 10 Newton-seconds; but 2 Newtons for 2 seconds yields 4 N-s.
Each motor, by its size and formula, has limits on how much thrust it can achieve and how long it will burn.
Then there is stability. A stable rocket flies straight up, whereas an unstable or overstable rocket will curve to some extent. straight up yields higher altitude.
So you see, the answers are more complex than the questions you are asking.
You really ought to learn how rockets work before talking about launching one 100km. There are many reference books out there.
Hello, that's sort of like asking "What vehicle should I buy to drive for the next two years?" Do you want to haul stuff? Camp? Drive fast? Want fuel economy? City or distance driving? How much money are you willing to spend?" There are a LOT of variables in that question, and even more in getting to 100 km.
If you can get a rocket to 100 km and recover it safely for less than six figures ($US) you'd be doing wonderfully. Because there's a huge learning curve that will cost quite a bit.
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boatgeek
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I'm going to answer a slightly different question that is more useful:
How do I figure out how to get to 100 km? You run some simulations. For starters, download OpenRocket and any OpenRocket file for a 4" rocket with a 54mm motor mount. You can get them from manufacturers. Don't worry that they're not exactly right, it doesn't matter for this exercise. Fool around with lots of different motors, and look at the differences in speed, acceleration, and altitude when you run the simulation with a high-thrust motor and a low thrust motor that have the same impulse. Look at the same factors for motors that have about the same average thrust but widely different impulses.
Once you've done that and you have some thoughts about how you might get to 100 km, build a sim yourself. See what you have to do to get it to 100 km. Again, don't worry if the sim is impractical, just add motors and reduce weight until you get there. Once you have that, try to get a realistic rocket that gets to 100 km. It'll either be a custom motor (like Go Fast) or multiple stages (I'd guess 6" to 6" to 4"). That will give you many of the answers you seek.
You don't need equations. Until you're a professional, the fine folks at OpenRocket, RockSim, and RASAero have already done the hard equations work for you. You just need to use the tools already available. Also, if/when you are a professional, unless you're actually on the team writing the professional grade simulator code, you won't be using massive equations either. You'll be running simulations and letting the computer do the work.
How do I figure out how to get to 100 km? You run some simulations. For starters, download OpenRocket and any OpenRocket file for a 4" rocket with a 54mm motor mount. You can get them from manufacturers. Don't worry that they're not exactly right, it doesn't matter for this exercise. Fool around with lots of different motors, and look at the differences in speed, acceleration, and altitude when you run the simulation with a high-thrust motor and a low thrust motor that have the same impulse. Look at the same factors for motors that have about the same average thrust but widely different impulses.
Once you've done that and you have some thoughts about how you might get to 100 km, build a sim yourself. See what you have to do to get it to 100 km. Again, don't worry if the sim is impractical, just add motors and reduce weight until you get there. Once you have that, try to get a realistic rocket that gets to 100 km. It'll either be a custom motor (like Go Fast) or multiple stages (I'd guess 6" to 6" to 4"). That will give you many of the answers you seek.
You don't need equations. Until you're a professional, the fine folks at OpenRocket, RockSim, and RASAero have already done the hard equations work for you. You just need to use the tools already available. Also, if/when you are a professional, unless you're actually on the team writing the professional grade simulator code, you won't be using massive equations either. You'll be running simulations and letting the computer do the work.
jlabrasca
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To answer your question I will use a process called Fermi analysis. Basically very rough order of magnitude.
First the work required to get 100,000m is going to be mgh + work against drag. The work must equal the energy output of the motor.
Assuming a 200N rocket. The mgh portion will be 20,000,000 N-m or Joules. I will just plain out guess that the drag work will be 25% of that. So your motor must generate 25,000,000 Joules of Kinetic energy. Lets just round up to 30 Million Joules.
Now rocket motors are specified in impulse (J). Impulse causes the change in momentum of your rocket. Since your rocket starts with 0 momentum, a rocket motor will have a momentum of J = mv after the motor burns out assuming no drag. So to calculate how much impulse is required to get 30 million joules of energy into your rocket we can estimate:
J^2 = m^2V^2 = 2m * mV^2/2
30,000,000 = mv^2/2
so J^2 = 2m * 30,000,000, for a 200N rocket m=20kg
so J^2 = 40*30,000,000
J ~ 35000 N-s if you can generate that much energy a 20kg rocket (very unlikely). But I will leave it up to the student to do the calculations with more realistic heavier rockets.
Remember this method is very rough order of magnitude estimation. Pick your weight, run these equations, double the result and there's you rocket motor size estimator.
First the work required to get 100,000m is going to be mgh + work against drag. The work must equal the energy output of the motor.
Assuming a 200N rocket. The mgh portion will be 20,000,000 N-m or Joules. I will just plain out guess that the drag work will be 25% of that. So your motor must generate 25,000,000 Joules of Kinetic energy. Lets just round up to 30 Million Joules.
Now rocket motors are specified in impulse (J). Impulse causes the change in momentum of your rocket. Since your rocket starts with 0 momentum, a rocket motor will have a momentum of J = mv after the motor burns out assuming no drag. So to calculate how much impulse is required to get 30 million joules of energy into your rocket we can estimate:
J^2 = m^2V^2 = 2m * mV^2/2
30,000,000 = mv^2/2
so J^2 = 2m * 30,000,000, for a 200N rocket m=20kg
so J^2 = 40*30,000,000
J ~ 35000 N-s if you can generate that much energy a 20kg rocket (very unlikely). But I will leave it up to the student to do the calculations with more realistic heavier rockets.
Remember this method is very rough order of magnitude estimation. Pick your weight, run these equations, double the result and there's you rocket motor size estimator.
jlabrasca
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Rough indeed.
I get something like 32,000 N•s -- but close enough for a Fermi estimate.
That's an O motor, yes? The OP would only need a to certify L3 for their space-shot >smile<
If the OP hasn't been scared off -- take a look at this
https://openrocket.info/documentation.html
for an insight into why the answer to the question isn't a plug-and-chug computation.
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The mass assumption is wrong. Plug in the mass of q motors, double the result like I said and it will converge to a realistic motor size.
Adrian Adamson didca sim awhile back showing an N5800 to n5800 could get you there. So yes,an O is possible....
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beeblebrox
8 C6-0, 12 D11-9, 20 D20-0, 20 E5-0, 3 Cinerocs
Just curious, has anyone done a direct comparison of Openrocket, vs Rocksim with the exact same models. I have openrocket...
blackjack2564
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About 400,000.00 worth.
Plus a few million worth of expertise from volunteer team mates !
oslersorbits
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I really like this response. Thank you for taking the time to respond properly.
Imitation chemist here: osmium perchlorate would have molar mass that's way too high for decent Isp. Highly concentrated perchloric acid can be thought of as hydronium perchlorate -- (H3O)ClO4 -- but being as it is a liquid, and decomposes violently when it contacts organic matter of any sort, probably there are better choices.
Best regards -- Terry
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jlabrasca
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Well, color me chagrined. I only got as far in my thinking as the notion that more mass per particle would mean more impulse for a given exhaust velocity. I didn't take my back-of-the-envelope algebra far enough (or even do any algebra on the back of anything).
The mass flow rate is, indeed, on the bottom of the Isp ratio. Oh well, fortunately its not my job to be mathematically rigorous about this kind of thing ... wait ... damn.
I've been thinking...the properties of AP are really useful, compared to all the other oxidizers. It's not significantly hygroscopic, unlike almost every other perchlorate. Useful burn rate in propellants. No metal ions so molar mass of exhaust is low. At room temperature it doesn't decompose readily in contact with the fuels used. Quite inexpensive compared to other oxidizers of comparable properties.
Ammonium nitrate is cheap but highly hygroscopic and has very low burn rates unless augmented with magnesium or a really good burn rate catalyst, and Isp will never match AP. Nitronium perchlorate...the less said about that one, the better. Potassium nitrate is okay for amateurs but not so hot (no pun intended) for high-performance space vehicles. Potassium perchlorate: rather high burn rate exponent, and again, it won't give performance quite up to that of AP compositions. Newer exotic oxidizers have great performance but often decompose unpredictably in contact with fuels, or are in and of themselves high explosives, and/or have costs that run into $1K per kg or more. ("I've got a J395 reload here, only run ya $750... or $700 cuz I like you...")
I would suggest the Bitcoin mining set of formulas, because 100km is going to cost you some serious coin.
Since this is a model or sport rocket forum, perhaps you should have asked how many Estes C6 motors would be required to reach 100km?
I commend you for your interest, but as Bat-Mite explains in post #4 your question is not valid. Thrust is the instantaneous force that acts upon an object, but to reach a specific altitude requires enough thrust to lift the object against gravity AND that the thrust be applied long enough to reach that altitude. And as Bat-Mite said, thrust delivered for a period of time is called impulse.
It’s possible to have a very low lifting force that just exceeds the downward force of gravity, such as lift from a balloon, and reach 100 km over a period of hours, days, or longer. It’s also possible to have an extremely high force, such as being fired from a gun, and reach that altitude in minutes. Even in that case the thrust is delivered over a time period. During that time period the mass being thrusted is accelerating. That’s the second of Newton’s Laws:
Force = mass * acceleration
So, a better question would be “How much impulse would it take to reach 100 km?” It would still depend on the weight of the rocket and making sure to keep it flying straight up, but with some assumptions at least we could come up with a numerical answer.
5km is about 16000 feet. It seems that most clubs have a around 10-15K waiver so your estimate makes sense. But let's take a more modest goal. How common is it for people to get to say 50k and above at events with high altitude waivers? What's a more realistic upper limit for a hobbyist?
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There are about a half dozen 50k waivers AFAIK: KS, MT, AZ, NV IIRC. 45k is the highest most folks can plan for, and even that isn't exactly common.
I thought there were a few events with higher waivers such as balls? They go to 100k right?
They go as high as you can go.
Charles_McG
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Depends on if it's a Black Friday sale or not :-D
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