OhioBob

Fair enough. I'm not out to cause trouble.

I agree with all of that.

Exactly. That's my point. The ballistic coefficient is greater on the part with the nose cone. At a given velocity the acceleration produced by the drag force is less on the part with higher ballistic coefficient. Therefore the part with the nose cone will fall faster and have a higher terminal velocity. (ETA) Sorry, I miss read your comment to say the part with the nose cone would have the higher terminal velocity. So apparently we disagree on this point.

Yes. I acknowledged in my original post that adding the mass of the nose cone was going to decrease the launch vehicle delta-v. I never claimed that adding a nose cone was going to increase the performance of a launch vehicle. I claimed that adding a nose cone would decrease the drag loss, but this gain would likely be negated by the decrease in delta-v resulting from the added mass of the nose cone.

Yes, drag force is increased, I agree with that. However drag force is not the same thing as drag loss. Adding a nose cone increases the drag force, but the increase in drag force is proportionally less than the resulting increase in mass. This means that the negative acceleration produced by the drag force is less.

With all due respect, my posts were not about FAR vs. Stock. They were about whether it better to launch with a nose cone or not launch with a nose cone. Since the OP asked if fairing parts help during a launch, the discussion seemed to me to be exactly on topic. For the record, I've never used FAR and I know almost nothing about it, so I don't see how I could possibly be having a FAR vs. Stock debate.

My gut feeling is that you are almost certainly correct. However since drag is so ridiculously high in the stock game, I would want to test it to be 100% certain.

Wait a minute! I let you guys talk me out of my own analysis. I return to my original claim, however I did misspeak. Adding a nose cone does not reduce the drag force, it reduces the drag loss. The loss of delta-v from drag results from the acceleration produced by the drag force, which is FD/M. We've already established that FD ∠CdM, therefore mass cancels out and we find that drag loss ∠Cd. Going back to the numerical example, Starman4308 showed that the drag force with and without the nose cone is 7.84k and 7.8k respectively. To find the acceleration produced by this force we divide by mass, obtaining With nose cone: a = 7.84k / 39.4 = 0.199k Without nose cone: a = 7.8k / 39 = 0.200k The rocket is slowed less with the nose cone than without.

I see that now. Drag is proportional to CdA, but in the game A = 0.008M, therefore FD ∠CdM.

Technically that's not true. The only real fairing parts in the stock game are nose cones (though I suppose there are probably some mods that provide additional parts). While most parts have a drag coefficient of 0.2, nose cones have a drag coefficient of 0.1. This lowers the overall Cd of the part. For example, let's say we have a Jumbo-64 fuel tank (64 t) with a Skipper engine (3 t). Both of these parts have a Cd of 0.2. The way the game computes the Cd of an assemblage of parts is to use a weighted average based on mass. If all the parts have a Cd of 0.2 then the weighted Cd is also 0.2. Suppose we add a large nose cone (0.4 t) on top of the fuel tank. The Cd of the assembly now becomes, Cd = (36 * 0.2 + 3 * 0.2 + 0.4 * 0.1) / (36 + 3 + 0.4) = 0.199 The amount of drag produced is inversely proportional to the ballistic coefficient. In the stock game the ballistic coefficient is simply BC = 125/Cd. Therefore, without the nose cone the ballistic coefficient is, BC = 125 / 0.2 = 625 and with the nose cone the ballistic coefficient is, BC = 125 / 0.199 = 628 Adding the nose cone slightly lowers the drag. However, adding the nose cone also increases the mass of your vehicle, lowers its mass ratio, and decreases the amount of delta-V the vehicle can produce. The loss in dV from carrying the greater mass may offset any gain from lowering the drag. I haven't done the math to see if there is an overall advantage or disadvantage in adding nose cones. When I have some time to spare I'll have to put it through a simulation and see if I can find a definitive answer. Nose cones also add a little cost, so that's also a factor to consider in career games.

I definitely agree with those that say a gradual turn is most efficient. When sharp turns are made too much dV is lost in changing the direction of the velocity vector rather than in increasing the magnitude of the velocity vector. To be most effective you want to make sure your pitch is never far from the prograde marker on the NavBall. I like to start my turn shortly after passing 5000 m altitude. I then set my pitch a few degrees ahead of the velocity vector, i.e. near the edge of the circle on the prograde marker. As the prograde marker begins to fall towards the horizon, I continually increase pitch to stay a few degrees ahead of the center of the marker. If I have a well designed rocket and I've maintained good control of my attitude, my pitch should be horizontal at the time of engine cutoff with the prograde marker a few degree above the horizon. How fast the prograde marker falls toward the horizon depends on TWR. I tend to build all my rocket with similar TWR*, therefore controlling the rate of the turn becomes pretty standard and easy to do. However, if you have very differently designed rockets, the rate of turn can vary quite a bit. If the prograde marker begins to fall too quickly, or not quickly enough, you'll have to adjust your pitch to compensate. To increase the turn rate, increase the gap between your pitch and the prograde marker, and to slow down the turn rate decrease the gap (you may even have to change your pitch to the trailing side of the marker if the turn starts to get out of control and increases too quickly). Also make sure not to end your burn too low in the atmosphere. I usually like to be at an altitude of about 50 km when I end my first burn, with a subsequent circularization burn at my target apoapsis. The main points are (1) start your turn at about 5-6 km, (2) maintain your pitch close to the prograde marker and avoid sudden large changes, (3) end your burn with the prograde marker at about +3 degrees to the horizon, and (4) be at an altitude of about 50 km at engine cutoff. Stick to these rules and you should have pretty good success. I find that a couple launches with a new rocket is all it usually takes to find the right touch to have a pretty effective and well controlled ascent. BTW, I use the stock game. * Ideally I like my first stage to have a TWR of about 1.6-1.7 and my second stage about 1.3. Upper stages for orbital maneuvering or transfer orbit injection can be much less, generally <1 is adequate.

Yes, I wrote and performed a series of computer simulations to try to optimize TWR and the gravity turn. For the simulation I used a simple two-stage launch vehicle. I found that the optimum lower stage TWR is 1.64 and the optimum upper stage TWR is 1.31. I found that a slow gradual gravity turn works best. For the given TWR I started the gravity turn at about 5300 m and maintained the pitch angle slightly ahead of the surface velocity vector by about 3 degrees throughout the turn. First burn cutoff ended at an altitude of about 50 km with the vehicle horizontal and the velocity vector about +3 degrees. This put me into a 75 km orbit with the highest possible payload fraction. Every other scenario/iteration I tried reduced the payload fraction.

That's not a unit of torque. The correct unit should be kN·m.

Sounds interesting, I'll have to try that. Is that comment directed at me? If so, can you explain further because I'm not sure I understand?

Wow, those are huge. I employed a different philosophy and designed the smallest, most stripped-down lander/launcher that I could come up with. Of course my mission is to simply land one Kerbonaut, plant a flag, take a sample, collect some science, and take off again. The following is what I came up with. With only two pairs of asparagus strap-ons I was able to get a dV of about 11,100 m/s*. The total mass as shown is 35.6 t. According to my simulations, it shouldn't have any problem getting to Eve orbit from a ground elevation of 3,000 m with, if everything goes perfectly, about 1000 m/s dV in reserve. (* This is actual simulated performance. Calculating dV based on vacuum ISP yields about 11,450 m/s.)

A few weeks ago I was planning my first return mission from Eve. To try to get my Eve launch vehicle right I created a computer simulation to test various designs. I adjusted many different factors, such as TWR, to try to optimize the design the best I could. My actual final design configuration ended up a little different than what I simulated, but the simulation provided a good basis to start from. I haven't actually flown my mission yet, so I can't vouch for how well it will work out in actual game conditions. The simulation is based on the stock game, so if you're using any mods the results will likely be different. My simulated design used a three-stage central core with three pairs of strap-on boosters. The strap-ons and the first stage were arranged to utilize asparagus staging. The two upper stages of the core were ignited in series after the strap-ons and first stage were jettisoned. The simulation assumed launching from am altitude of 3,000 m, and I was shooting for an orbital altitude of 105,000 m. I found that TWR is critical to a good design, thus I spent quite a bit of time going through multiple iterations until I hit on what appeared to be the optimum. Below is a graph of my simulated launch vehicle's TWR versus time. Each peak represents burnout of one of the stages. The first three peaks are the pairs of strap-ons and the final three peaks are the stages of the central core. The TWR is based on Eve gravity, where go = 16.677 m/s2. If your launch vehicle's TWR is significantly outside the range represented by the graph below, I'd recommend modifying it. I found that being significantly outside the optimum range yielded disastrous results in most cases. Controlling the rocket's attitude is also an important factor, though I didn't find the altitude of initial pitch-over to be all that critical. I started a slow gradual pitch-over immediately after jettisoning the last of the strap-ons stages, which for the simulation was at an altitude of about 18,000 m. I think it is important that we don't put too much emphasis on what the pitch needs to be, but rather on what the flight path angle should be. The flight path angle is the angle that the velocity vector makes with the local horizon. We alter the pitch simply to maintain the correct flight path angle. Below is a graph of the simulation's surface velocity flight path angle versus altitude. At some point during ascent the NAV ball will switch from surface velocity to orbital velocity, but by the time that happens the two vectors should be pretty close to each other. The result shown here was obtained by keeping the pitch vector just barely ahead of the velocity vector throughout the gravity turn (the pitch led the velocity vector by a constant 0.23o). In practice it's unlikely that a proper separation between the pitch and velocity vectors can be constantly maintained, thus frequent small adjustments will undoubtedly be necessary. Furthermore, your rocket configuration may require a different ascent profile than what I simulated. You're going to have to find out what works for you. I would definitely recommend trying to make a slow smooth transition from a vertical velocity vector at about 20 km to a horizontal velocity vector at about 100 km. For the simulation, engine cutoff occurred at an altitude of about 99.5 km with a flight path angle of +1 degree. This was followed by a coast up to 105 km and a small circularization burn.

Call me crazy but I enjoy the math. I calculate all my interplanetary trajectories using the method described in the following web page: http://www.braeunig.us/space/interpl.htm#gauss I put all the formulas into a spreadsheet. I input a launch date, a time of flight, and I then compute the trajectory that connects the departure and arrival points. It takes several iterations to find the energy optimum trajectory. I know that there is a online tool that does the same thing, but that almost seems like cheating. I don't use any modes or tools created by somebody else. To me the fun comes in figuring it all out myself. I also like to compute my own delta-V. I find it very satisfying to plan a mission, compute the required delta-V budget, design an spacecraft to meet that requirement, and then successfully fly the mission as planned.

Thanks, guys. I thought those green bars where just indicators, didn't realize they were sliders. I'll consider some of the other suggestions as well.

Is there a way to launch a payload with the propellant tanks empty? Instead of building an enormous launch vehicle to lift my payload fully fueled, I'd rather launch it unfueled on a smaller rocket. I could then launch a couple follow-on missions to rendezvous with it and fuel it in orbit. If anybody knows how to do this, your help is appreciated? (I suspect I might have to edit the ".craft" file.)

Yes, I did. I just figured it was a typo.

I just changed this thread from "Unanswered" to "Answered". I think we've had some pretty good discussion. It seems that, for the most part, people agree that asparagus staging is not the most cost effective method for reaching Kerbin orbit. Of course cost has only recently become a important consideration, and then only in career mode. Everyone seems also to agree that asparagus staging is one of the most mass efficient configurations. When cost is not a factor, such as in sandbox mode, or in some special cases, such as launching from Eve, asparagus staging is still one of the best ways to go.

It is clear from the OP that I wasn't talking about Eve. The overwhelming majority of anyone's launch cost is going to come in getting off of Kerbin. It is launching from Kerbin surface to low orbit where cost efficiency is a primary driver, and it is this that is the context of the discussion. Launching from Eve is a special case that requires a rebalancing of priorities. In the Eve scenario mass efficiency becomes much more important than cost efficiency.

That's what I thought. With the design philosophy I've used in the past I though I was doing pretty good to get a 0.12 payload fraction. I'm curious to see in practice how close I can get to 0.162. It's surely unrealistic to expect to reach the theoretical limit, but it should be possible to get much closer by implementing some of my findings.

Providing the code wouldn't be very practical, but I do provide a description of the basic method in an article I wrote about a my Saturn V simulation. If you're interested you can read it here: Saturn V Launch Simulation. For the KSP simulation I simply had to modify everything to conform to the Kerbal universe. As I mentioned previously, the mass of all the rocket parts are based on typical ratios found within game. I primarily based it on a "large" sized rocket (Rockomax). There are five parameters that I attempted to optimize: Stage 1 TWR, Stage 2 TWR, propellant mass distribution, pitch-to-velocity vector separation angle, and flight path angle at engine cutoff. The pitch-to-velocity separation angle determines how rapidly the rocket pitches over. It is assumed constant throughout the pitch over phase. This angle, along with the initial pitch over altitude, determines the rocket attitude and flight path angle at cutoff. I started out by assuming all five parameters and then determined the pitch over altitude and cutoff time needed to attain the correct cutoff conditions to reach my target 75 km orbit. It would take several iterations to find the payload mass that exactly exhausted all of the propellant. The next step would be to change one of the assumed parameters and find out in what direction I had to go to increase the payload fraction. I would continue to change the parameter until I reached a maximum on the payload fraction. I would then move to the next parameter and adjust it until I again reached a maximum. Then the next parameter and so. I would then return to the first parameter and go through another iteration of adjustments. I eventually reached a point where changing the numbers resulted in no further increase in payload fraction. The resulting numbers are those that I give in the opening post.

Yes Correct. A second burn would have to be made at apoapsis to circularize the orbit. I forgot to mention in my original post that I'm assuming a 75 km orbit. Correct again. In practice I'm rarely able to get it to work out as perfectly as the theory. I'm usually going slower than optimal at first burn cutoff and then have to compensate with a fairly high delta-v burn at apoapsis. However, I find that, in theory, performance is optimized by nearly reaching orbital velocity at first burn cutoff and then making a low delta-v second burn (about 60 m/s). If performed correctly, I don't think there is any need to throttle back. For the particular scenario that I simulated, cutoff would occur at an altitude of about 50 km and, with such a high horizontal velocity, you'd in a long arcing trajectory a good distance away from apoapsis. According to the simulation, first burn cutoff would occur 6.5 minutes before reaching apoapsis. I agree that's low for the real world. I was expecting it to be higher, like about 3:1. In the KSP universe, however, it didn't work out that way.