Launcher design and wieght calculations?

[hoax]

Hi!<br /><br />I'm new to this site but want to ask a question on how to model a launcher design. I'm intrested in how to estimate weight for a launch system and looking for some formulas, good sites/URLs or other hints. Structual design of launcher stages and other related issues are also of intrest.<br /><br />Any pointers are welcome. Thank you all in advance... <img src="/images/icons/smile.gif" />

 

[jurgens]

model rockets? Or real rockets?

 

[hoax]

Real rockets. I working to build a model of an "old style" 2-stage RP1+LOX launcher and would like some understanding on how the weight of a launcher is distibuted and would like to gain formulas to calculate weight of the launcher tanks and structure to support it during flight.

 

[propforce]

Launcher? as in the rocket itself, or the launch "pad" ? <div class="Discussion_UserSignature"> </div>

 

[jurgens]

Its not that hard really, F=ma, K= 1/2mv^2, E = mgh, E = integral of F dx.<br /><br />Those are classical physics equations, and almost are rocket equations are derived from those equations. :-D

 

[nacnud]

pV=nRT ?

 

[propforce]

<font color="yellow"><i>Its not that hard really, F=ma, K= 1/2mv^2, E = mgh, E = integral of F dx. </i></font><br /><br /><br />Well... yeah, it's the "m" term that he wants to know how to calculate. <div class="Discussion_UserSignature"> </div>

 

[propforce]

<font color="yellow"><i> pV=nRT ? </i></font><br /><br />Very good, now derive from the above equation to the following:<br /><br />P/Pt = [1 + 0.5*(gamma - 1)*M^2]^(-gamma/(gamma -1)) <div class="Discussion_UserSignature"> </div>

 

[nacnud]

sorry I never got the hang of ASCII maths.<br />

 

[propforce]

LOL... no worry. It's only a compressible flow equation. <div class="Discussion_UserSignature"> </div>

 

[nacnud]

fluid dynamics passed me by I'm afraid, I was across the hall learning nuclear astrophysics, so much easier <img src="/images/icons/smile.gif" />

 

[frodo1008]

I don't believe it is quite that simple. The only general configuration where you could even begin to calculate a direct thrust to payload situation would be single stage to orbit, and even there there would be some dependency upon the shape of the rocket due to air resistance.<br /><br />However, when you get to muliple stage rockets starting at two stage to orbit it becomes considerably more complicated. Your payload now depends not only upon the thrust of your first stage engines, but also upon the thrust of your second stage engines. And so on...<br /><br />Perhaps some kind of idea could be obtained by a list of the current rocket launchers and their various configurations. Perhaps somebody knows of such a list on the net, I am sorry but I don't (and would be grateful to anybody who did myself). I do know that the Delta IV Heavy with three RS68 engines at 665,000 lbs each is capable of placing some 50,000 lbs into LEO.<br /><br />It would indeed be very interesting to find out what the capabilities of all the current launchers and their various configurations (extra solids, etc) are...

 

[frodo1008]

I don't believe it is quite that simple. The only general configuration where you could even begin to calculate a direct thrust to payload situation would be single stage to orbit, and even then there would be some dependency upon the shape of the rocket due to air resistance.<br /><br />However, when you get to muliple stage rockets starting at two stage to orbit it becomes considerably more complicated. Your payload now depends not only upon the thrust of your first stage engines, but also upon the thrust of your second stage engines. And so on...<br /><br />Perhaps some kind of idea could be obtained by a list of the current rocket launchers and their various configurations. Perhaps somebody knows of such a list on the net, I am sorry but I don't (and would be grateful to anybody who did myself). I do know that the Delta IV Heavy with three RS68 engines at 665,000 lbs each is capable of placing some 50,000 lbs into LEO.<br /><br />It would indeed be very interesting to find out what the capabilities of all the current launchers and their various configurations (extra solids, etc) are...

 

[propforce]

<font color="yellow"><i>I don't believe it is quite that simple. </i></font><br /><br />Yes Frodo, it's not that simple <img src="/images/icons/smile.gif" /><br /><br />For one thing, the mass of vehicle is continuing to decrease as it rise from ground to altitude (due to the burning of propellant), so assuming the thrust of the engine(s) such as the RS-68 is constant (which is not, due to decreasing back pressure therefore an increasing vacuum thrust as altitude increase), the the vehicle acceleration is continue to increase to the point that one must throttle the engine thrust level down or it will exceed the vehicle structural design limit.<br /><br />Since F = m * a<br /><br />So, a = F/m<br /><br />As m gets smaller as a function of altitude, then a gets bigger until a /> g-limit<br /><br />BTW, F = F-net = F-engine - F-drag<br /><br />Where <br /><br />F-drag = F-gravity loss + F-aerodynamics drag loss + F-base pressure loss + F-turning loss+ F-thrust vector loss + .... etc. <br /><br />and<br /><br />F-engine = F-vacuum - P-exit * A-exit<br /><br />P-exit = function of altitude <div class="Discussion_UserSignature"> </div>

 

[arobie]

propforce,<br /><br />Ok, the total net force is the force of the engine minus the drag force. Thats simple and makes sense.<br /><br />What I don't understand/know...<br /><br />What is 'F-base pressure loss'?<br /><br />Does 'F-turning loss' only occur while in the atmosphere, or does it occur in space also?<br /><br />Isn't F-thrust vector loss accounted for in solving for gravity losses?<br /><br />And could you explain the equation:<br /><br />F-engine = F-vacuum - P-exit * A-exit <br /><br />What are the different parts? What do they stand for and how do you find them?

 

[propforce]

Arobie,<br /><br />F-base pressure loss has to do with flow separation loss. When air flow over/ around an object, such as a car for example, the flow separate at the back end and generate recirculation. That's because most vehicle's back ends with a shape like ] as opposed to like />. The sudden discontinuity of the shape caused flow separation. This flow separation is a form of "drag" and gets worse as speed increase (such as race cars, airplanes, and launch vehicles). <br /><br />F-turning loss occur in both the atmosphere and in space, though in space we often relate it to "plane change" or "orbit change". The most obvious for a ground launch is when it turn from a vertical direction to somewhat horizontal direction.<br /><br />F-thrust vector loss, or angle-of-attack loss, has to do with the different thrust vector between the flight path angle of where vehicle is going and the angle of where the vehicle needs to go, e.g., engine gimble angle, since the effective thrust is a sine function of the thrust vector.<br /><br />F-engine = F-vacuum - P-exit * A-exit <br /><br />This has to do with doing a control-volume around the engine for thrust accounting. It also explains why a vacuum thrust out of an engine is always higher than it's sea level thrust. As the vehicle climb with altitude, the ambient pressure decreases therefore the engine is subjected to less resistance due to the atmospheric air until P-ambient becomes zero in vacuum. Sutton's book can explain this better.<br /><br />Hope this help.<br /> <div class="Discussion_UserSignature"> </div>

 

[arobie]

Propforce,<br /><br />Thank you very much. That helps alot. <img src="/images/icons/laugh.gif" />

 

[propforce]

Actually I've just notice a mistake I made <img src="/images/icons/blush.gif" /><br /><br />The P-exit on the F-engine equation should be P-ambient instead, so the equation should be<br /><br />F-engine = F-vacuum - P-ambient * A-exit <br /><br />Now that makes sense <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>

 

[arobie]

<font color="orange"><b>Hoax,</b></font><br /><br />About how big of a rocket, and how high do you want it to go? After you tell me those two things, then I have some equations that can help you.<br /><br />BTW:<br /><br />Welcome to the boards. I'm sure we can help. <img src="/images/icons/smile.gif" />

 

[arobie]

Propforce,<br /><br />Ok, thank you. I had figured that is what you meant, but the confirmation removes the doubt...makes it clearer.

 

[hoax]

Thanks to all for their input but this not really what I'm looking for... I want to calculate the mass of my rocket! <br /><br />Say I have a 2000 kg payload I want to put into orbit. My 2nd stage need to hold that mass ontop and carry fuel in its tanks plus additional equipment. Same for the 1st stage, it will have a huge load with the 2nd stage ontop. How do I model the mass of this rocket? <br /><br />- The mass of the tanks to hold the pressurised fuel.. I think I found formulas for that on one site. (bookmark somewhere)<br />- The structure of the rocket body to be able to carry the payload ontop and handle the stress during launch.<br />- Weight of additional equipment...<br /><br />How do the weight of the rocket change with different solutions of the rocket structure? Eg, Atlas type where pressurised tanks is part of the building structure. How much lighter can such a design be?<br /><br />If we want it to carry a heavier payload, how much heavier must the rocket be build to not breakdown because of the increased load? <br /><br />Any input or hints are welcome.. <img src="/images/icons/smile.gif" />

 

[arobie]

It looks like the next thing you need to find is the mass of your propellant...<br /><br />The three things one needs to know about a rocket one's designing is:<br /><br />1) Delta Velocity (dV) - the change in velocity your rocket needs to attain to get where you want it to go. (Includes the loss of velocity from gravity, drag, etc...)<br /><br />2) Specific Impulse (Isp) - the efficiency of the engine, measured in seconds. The highest theoretical max for chemical engines is 525 seconds. The higher, the more efficient the engine. Rocket engine designers will make it known what the Isp of their engine is. You can find it on any stat sheet of their engine.<br /><br />3) Mass of the rocket - specifically the mass of the rocket with no fuel, the mass of the payload, structure, tanks, etc...<br /><br />Once you know these three, you use them in a nifty little equation, well two. This first equation solves for a propellant fraction (pf). You then use the pf in a second equation to solve for your propellant mass, how much propellant you need to achieve your goal dV. It's really quite simple, you solve one equation, then use that solution to solve for propellant mass. <br /><br />The first equation:<br /><br />pf = 1 - [ 1 / (e ^ dV / Ve)]<br /><br />If your not traumatized by the formula like I was when I first saw it, you'll notice that it has Ve in it, and that I have not explained that yet. The other thing is e, but you probably know what that it already. It's a mathematical constant. I couldn't tell you much more about it. I have not learned it yet from school, so I just know it's value and use it for this stuff.<br /><br />Ve is 'exhaust velocity'. To find it, all you do is mutiply the Isp by gravity (9.807 m/s^2)<br /><br />Ve = Isp * 9.807<br /><br />e = 2.718<br /><br />After you find the propellant fraction (pf), you can then plug that into this equation:<br /><br />Wp = [(Wi * pf) + (WL * pf)] / (1 - pf)<br /><br />Wp is your propellant mass.<br /><br />Wi is the mass of your ship without th