Larger Boosters...

[PistolPete]

Thanks BarryKirk. I was aware of what you and Propforce were talking about and I already knew about the rocket equation. However, one thing I was never sure of was exactly how thrust affects delta v. While I knew it was important, especially to first stages, I could never find a formula anywhere that quantified it. Do you have a formula or equation that I can use to determine this? It has been sort of a hobby of mine to design hypothetical LVs, however I don't have all of the formulas to take every factor into my equations (I'm basically operating off of just the aforementioned "rocket equation"). If you have any more formulas that could help me with things like atmospheric and gravitational drag they would be greatly appreciated.<br /><br />Peregrine LV<br /> <div class="Discussion_UserSignature"> <p> </p><p><em>So, again we are defeated. This victory belongs to the farmers, not us.</em></p><p><strong>-Kambei Shimada from the movie Seven Samurai</strong></p> </div>

 

[PistolPete]

This is what I was talking about in my first post. If you have a larger LV, then the scientists will want a larger probe, and the whole budget will have to be increased. Basically, a space probe's time on target has more to do with finances than it actually does with rocket science. We have most of the technology today to send manned missions to all of the inner planets, it's just that there isn't the money for it. <div class="Discussion_UserSignature"> <p> </p><p><em>So, again we are defeated. This victory belongs to the farmers, not us.</em></p><p><strong>-Kambei Shimada from the movie Seven Samurai</strong></p> </div>

 

[publiusr]

Look at the N-1 here:<br />http://www.rolfstabroth.de/ <br />http://www.ipms-phx.org/ <br /><br />Space elevator--a good payload for Direct <br />http://www.lulu.com/content/440980 <br /><br />Nice model--and I'm not talking about the rockets. <br />http://www.cjsaviation.com/

 

[bpfeifer]

“Until such time as someone orders a large number of boosters, the unit costs are going to remain high. Once we begin production on an assembly line basis, the unit costs will drop significantly. All we need is for NASA to admit that it will need 30 or 40 boosters of various Atlas configurations, say, and order them in advance, and the assembly line would be set up.”<br /><br />This is why I think SpaceX’s Falcon is a good interim solution. Instead of and assembly line of disposables, they will build a few reusable first stages. It should achieve some of the same cost benefits you would get from the assembly line. It is basically choosing to invest up front in the design and manufacture of a good first stage, rather than to invest in the design and manufacture of the assembly line. Both require significant up front investment to achieve cost efficiencies.<br /> <div class="Discussion_UserSignature"> Brian J. Pfeifer http://sabletower.wordpress.com<br /> The Dogsoldier Codex http://www.lulu.com/sabletower<br /> </div>

 

[spacester]

<p>Hi PistolPete. Gravity Losses and Atmospheric Drag Losses are difficult to determine analytically. <br /><br />The short answer is to just assume some numbers and then get on with the rest of your design task.<br /><br />Gravity Losses during Launch actually uses a simple formula, but it needs to be integrated (OMG Calculus!) along the flight path.<br /><br />The expression inside the integral over time (t) from launch to engine cutoff is<br /><br />g * (t * sin(theta))<br />where<br />g = 9.807 m/s^2<br />theta = thrust angle relative to horizontal<br /><br />Which would seem to be a relatively easy calc, but my research indicated there was more to it. I never got to the bottom of what more you need than the flight path: it would seem that if you know the exact flight path, then you can develop an expression for theta as a function of time, and then you simply perform a numerical integration and wholla you have your answer. My research clearly indicated there was more to it but that's kinda as far as I got.<br /><br />Iain McClatchie has a post on HobbySpace you might take a look at.<br /><br />There is a software program out there that the Pros use for this; in fact as of 10 years ago or so there were two competing programs and there was heated debate between the two camps on which is better. I cannot remember the names of either, and I'm not finding it in my archives. <br /><br />As an Engineer and not a Scientist, I am used to living with empirically derived answers when the pure analytic approach is unworkable.<br /><br />FWIW, I did a survey of total gravity losses for various launch vehicles in order to pick one single number to use for BDBs, and I came up with 1.35 km/s<br /><br />Atmospheric Drag is harder, and I'll just skip ahead to the number I came up with from a similar survey: 0.35 km/s<br /><br />hth<br /><br />edit: Also see</p> <div class="Discussion_UserSignature"> </div>

 

[PistolPete]

Thanks man, that helped a lot! It was a bit of an ego stroke to find out that a lot of my guesstimates turned out to be pretty accurate. One thing that I did find supprising is that it appears that there is a much higher loss of delta v due to gravity drag than from atmospheric drag. At first this seemed odd to me, but then I realised that gravity keeps working on the LV long after it has left the detectable atmosphere.<br /><br />BarryKirk, I think I understand how thrust effects delta v, at least with the first few stages, and as I understand it, it doesn't necessarily require any new calculations, but a better understanding of how thrust effects how much weight (and therefore fuel) can be lifted off of the earth. I have allways understood the concepts of T/W ratio and delta v, but I just neede to put them together.<br /><br />One thing I want to know is if I increase thrust, but do not do anything to the Isp or the weights of the rocket, does this do anything to the delta v? <div class="Discussion_UserSignature"> <p> </p><p><em>So, again we are defeated. This victory belongs to the farmers, not us.</em></p><p><strong>-Kambei Shimada from the movie Seven Samurai</strong></p> </div>

 

[barrykirk]

I was hoping to learn the answer to that myself.<br /><br />I seem to remember that their is a rocket simulator program available for download, but I don't know too much about it.

 

[barrykirk]

PistolPete,<br /><br />with more thrust, but the same ISP and same amount<br />of fuel. You will go through your burn faster.<br /><br />So your total delta V is unchanged.... but....<br /><br />If your starting from the ground the gravity losses will<br />be reduced because your burn takes less time.<br /><br />but.....<br /><br />Your aero losses will increase because your going<br />faster lower in the atmosphere. And your Max Q will<br />occur lower in the atmosphere and at a higher pressure.<br /><br />If you watch a space shuttle launch it's a delicate<br />balance of trying to minimize gravity losses while trying<br />to hold down aero losses and Max Q pressure. That is<br />why the shuttle is throttling up and down during the<br />first 2 to 3 minutes of flight.<br /><br />Hope this helps.

 

[PistolPete]

Actually it does, it confirms what I was thinking but I wasn't quite sure, thanks. <div class="Discussion_UserSignature"> <p> </p><p><em>So, again we are defeated. This victory belongs to the farmers, not us.</em></p><p><strong>-Kambei Shimada from the movie Seven Samurai</strong></p> </div>

 

[propforce]

<blockquote><font class="small">In reply to:</font><hr /><p>... However, one thing I was never sure of was exactly how thrust affects delta v. While I knew it was important, especially to first stages, I could never find a formula anywhere that quantified it. Do you have a formula or equation that I can use to determine this? It has been sort of a hobby of mine to design hypothetical LVs, however I don't have all of the formulas to take every factor into my equations (I'm basically operating off of just the aforementioned "rocket equation"). If you have any more formulas that could help me with things like atmospheric and gravitational drag they would be greatly appreciated. <p><hr /></p></p></blockquote><br /><br />So what's thrust gotta to do with it ? <br /><br />This reminds me the song by Tina Turner <img src="/images/icons/smile.gif" /><br /><br />The effect of thrust on the ideal delta-vee, especially for the first stage, is that a lower thrust would require the vehicle to stay in the gravity well longer, hence increase the ideal delta-vee required. However; you don't want the 1st stage thrust too high neither, as this will increase the vehicle acceleration above your axial G limit of your vehicle. It will also introduce max-Q sooner, increasing the aerodynamic drag of the vehicle and again, increase the overall delta-vee required. The common practice, e.g., industry "rule of thumb" is to have the initial take-off T/W between 1.2 to 1.4, and the max G limit for unmanned (EELV) vehicles at 6, or for manned (shuttle) vehicle at 3. For upper stage where aerodynamics and gravity is not much of a problem, a typical initial T/W is around 0.15~0.25 (Arianne and EELVs).<br /><br />On your Peregrine vehicle, your pick of the RD-270 engine with its thrust of ~ 1.5M lbf. This engine will give you an initial take off thrust-to-weight ratio of 1.45, pretty ideal for a launch vehicle, a T/W of 4.8 near the 1st stage burn out, or a burn-out T/W of 5.2 for GTO mission due to a lower GLOW <div class="Discussion_UserSignature"> </div>

 

[PistolPete]

The Briz M was something I just threw in. I didn't feel like designing another stage, and seeing as how it was an N2O4/UDMH fueled stage, I thought it just fit right in.<br /><br />I looked up the KVRB upper stage on Astronautix.com. It is an H2/LOX fueled upper stage, essentially the Russian version of the Centaur upper stage that was designed for use on the Angara 5A launcher. I plugged it into the Peregrine LV in place of the Briz M and this is what I got:<br /><br />KVRB: <br /><br />Gross Weight : 23,300<br />Empty Weight : 3,500<br />Engine: x1 RD-56M<br />Isp (vac) : 461<br />thrust : 73.5 kN <br />T/W ratio : 0.17 <br />Mass ratio : 0.15 <br />Fuel wt. : 19,800 kg <br /> <br />LEO Payload : 21,000 kg<br /><br />GTO Payload : 8,400 kg<br /><br />Definatly a performance jump. A 3,200 kg to GTO and 5,000 kg to LEO payload increase. While the T/W ratio is not as ideal as your previous post indicated, it is definaly an improvement. <div class="Discussion_UserSignature"> <p> </p><p><em>So, again we are defeated. This victory belongs to the farmers, not us.</em></p><p><strong>-Kambei Shimada from the movie Seven Samurai</strong></p> </div>

 

[barrykirk]

"However; your 3rd stage starts with an initial T/W of only 0.05 and ends with a T/W of 0.11. It looks like you can use a more powerful engine, at least a 20,000 lbf engine, to bring that initial T/W up."<br /><br />From what I understand, most third stages start with the vehicle already in LEO. At that point, a very low T/W ratio is a good thing. Your gravity losses are fairly minimal and if the thrust vector is correct, non-existant.<br /><br />A low T/W ratio means that you have very little engine and a lot of fuel. This is a good thing. Your not paying a weight penalty for more engine than you need.