Propforce...rocket help...

[arobie]

On another thread, you posted some equations for rockets. I asked some questions, and you answered them for me, and I appreciate that. I did not continue after that because I did not want to hijak hoax's thread. <br /><br />I would like to now ask if you would mind me continuing to ask questions on this thread. I would like to deeper understanding what you put out: calculating engine force, velocity loses, and subtracting the latter from the former to find the net engine force.<br /><br />So...Would you mind helping me out. If it's not too much trouble, would you mind edumacating me?

 

[propforce]

Arobie,<br /><br />I'd be happy to help. Others may chime in with answers as well.<br /><br />Ask away....<br /><br />Prop- <div class="Discussion_UserSignature"> </div>

 

[a_lost_packet_]

WOW! I can already tell I am really going to enjoy this thread! Seriously! I hope you won't mind me asking a few questions of my own. <img src="/images/icons/smile.gif" /><br /><br /><watching /><br /> <div class="Discussion_UserSignature"> <font size="1">I put on my robe and wizard hat...</font> </div>

 

[arobie]

Propforce,<br /><br />Thank you.<br /><br />Ok...so you posted:<br /><br /><font color="yellow">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-ambient * A-exit<br /><br />P-exit = function of altitude</font><br /><br />First, I'de like to make sure I have this clear...<br /><br />F-net = F-engine - F-drag <br />is the same as saying:<br />the net-dV = engine-dV capability - lost-dV (due to various items)<br /><br />Correct? Just starting off simple, making sure I'm on the same page as you.

 

[arobie]

a_lost_packet,<br /><br />Welcome to the thread, and enjoy it. Feel free to add in any questions you would like to.

 

[jurgens]

Im not a rocket scientist, just a student but I think for a rocket the three most important things you have to model are<br /><br />1) The Non-Constant(Unless you have a nice engine) thrust produced by the engine, for this I have no clue how to do.<br /><br />2) Non-Constant Drag force which changes as a function of distance(Drag Coeffecient changes) This shouldn't be too hard.<br /><br />3) The force provided by gravity on the rocket as a function of distance from the surface(gravity isn't constant) This one isn't too hard either.<br /><br />and those three forces sum up to equal the net force experiened on the rocket... Then you need to do Fnet = ma, but in this case m isn't constant so you have to create a differential equation which is dependent on the rate that fuel is consumed(mass lossed) by the rocket engine.<br /><br />If you have multiple stages on the rocket, your going to probablly have to create an equation for each stage of the rocket and also account for conservation of momentum inbetween each stage.

 

[arobie]

<font color="yellow">Im not a rocket scientist, just a student...</font><br /><br />Well then there's something we have in common, I'm also a student. I'm currently in high school, what about you?

 

[jurgens]

Im a Sophomore Physics Major :-D

 

[tap_sa]

<font color="yellow">"1) The Non-Constant(Unless you have a nice engine) thrust produced by the engine, for this I have no clue how to do."</font><br /><br />I'm assuming this means the change in thrust by altitude. Propforce already gave the basic equation and pointed to Sutton's book, since I just got my copy I'll attempt to chime in little more details from it.<br /><br />Total engine thrust comes from two part, momentum thrust and pressure thrust. First part is the basic Newton action-reaction 101, engine throws propellant backwards so the rocket gets pushed forwards. This thrust remains the same despite altitude. Pressure thrust is the result of difference between ambient pressure and exhaust gas pressure at nozzle exit. If exhaust gas pressure if bigger than ambient then you get positive thrust from it. To sum this into equation:<br /><br />F = <i>d</i>m*v_exit + (p_exit - p_ambient)*A_exit<br /><br /> <i>d</i>m rate of propellant consumtion, v_exit exhaust gas speed at nozzle exit , A_exit nozzle exit area<br /><br />I recommend getting the Sutton book, bit pricey but it's worth the every cent! <br /><br />(Especially when Amazon UK sent along with the books a voucher to a <i>lingerie</i> shop, 30% rebate from 100£ garments ... I guess they thought the rocket interest has a Freudian meaning!?)

 

[a_lost_packet_]

<font color="orange">"1) The Non-Constant(Unless you have a nice engine) thrust produced by the engine, for this I have no clue how to do." </font><br /><br /><font color="yellow">Tap_sa - I'm assuming this means the change in thrust by altitude. Propforce already gave the basic equation and pointed to Sutton's book, since I just got my copy I'll attempt to chime in little more details from it. </font><br /><br />Maybe he is referring to the possibility of variance in thrust that one might think would happen with, for instance, a model rocket engine. ie: sputtering as solid fuel ignites unevenly.<br /><br />I know that modern solid rocket boosters, such as found on the shuttle, have carefully calculated burn rates and have channels cut into the material to vary surface area to modify burn rate vs expected thrust required.<br /><br />I believe that, usually, solid rocket boosters also have what amounts to "fire extinguishers" and blow out panels that are supposed to stop combustion in case of an emergency. I don't know if re-ignition is possible if that was done.<br /><br />Liquid fueled engines should be easier to control as material is mixed and pumped into the combustion chamber.<br /><br />Is there any variance normally taken into account based upon fuel used for calculating uneven burn rates? Could that be more along the line of his questioning?<br /><br /> <div class="Discussion_UserSignature"> <font size="1">I put on my robe and wizard hat...</font> </div>

 

[tap_sa]

<font color="yellow">"solid rocket boosters also have what amounts to "fire extinguishers" and blow out panels that are supposed to stop combustion in case of an emergency."</font><br /><br />Yes, shaped charges to blow out throat&nozzle and/or additional exhaust ports to the front end of the motor case. This causes rapid decompression which extinguishes the burn process too. Procedure is used in normal flight too, during solid rocket staging. Blowing out the forward ports causes a negative thrust for a moment so stage separation is safer. Sometimes in ground tests plain water is sprayed into the motor to stop it without breaking it, to inspect partially burned grain.<br /><br /><font color="yellow">"Is there any variance normally taken into account based upon fuel used for calculating uneven burn rates?</font><br /><br />I'd guess the general approach is to assume the burn rate to be even over relative long time, pressure changes caused by uneven burn rates lasting a few milliseconds cancel each other over a timescale of one second or more. If they don't, well, that usually means pressing the big red button.<br /><br />

 

[jurgens]

a_lost_packet, yeah that is what I was referring to, but Tap_Sa is right, you also need to take into account that the ISP of the engine changes as the altitude changes.

 

[arobie]

Starting simple to make sure I'm clear...<br /><br />When I say:<br /><br />F-net = F-engine - F-drag<br /><br />Is that the same as saying:<br /><br />the net-dV = engine-dV capability - lost-dV (due to various items)??