OhioBob

I totally disagree with that. The current model is a bogus placeholder that was put in place in an alpha version of the game. There is no reason why anyone should expect it to remain. And I speak as a person who has only played the stock game. I am not an advocate for FAR, but I do think the game needs to be fixed to some degree. Right now the game is counter-intuitive, such as nose cones increasing drag, and that's just not right. Those things need to be fixed so the game makes sense to future players. Leaving a broken model in place to appease alpha testers who happened to get use to it is, in my opinion, foolish.

To better illustrate the point I made in my previous post, below is a table showing the temperature and average molecular weight resulting from the combustion of liquid oxygen and liquid hydrogen at different mixture ratios. Mixture ratio is the mass of the oxidizer divided by the mass of the fuel. A mixture ratio of 8 is stoichiometric, while lower mixture ratios are fuel rich. Also provided is (T/M)1/2, which gives us a comparison of how the different mixture ratios will perform in regard to specific impulse. These values have been computed for a combustion chamber pressure of 68 atmospheres (1000 PSI). [TABLE=class: grid, width: 500, align: center] [TR] [TD=align: center]Mixture Ratio[/TD] [TD=align: center]Temperature (Kelvin)[/TD] [TD=align: center]Molecular Weight (g/mol)[/TD] [TD=align: center](T/M)1/2[/TD] [/TR] [TR] [TD=align: center]8[/TD] [TD=align: center]3605[/TD] [TD=align: center]16.01[/TD] [TD=align: center]15.01[/TD] [/TR] [TR] [TD=align: center]7[/TD] [TD=align: center]3587[/TD] [TD=align: center]14.79[/TD] [TD=align: center]15.57[/TD] [/TR] [TR] [TD=align: center]6[/TD] [TD=align: center]3495[/TD] [TD=align: center]13.36[/TD] [TD=align: center]16.17[/TD] [/TR] [TR] [TD=align: center]5[/TD] [TD=align: center]3293[/TD] [TD=align: center]11.74[/TD] [TD=align: center]16.75[/TD] [/TR] [TR] [TD=align: center]4[/TD] [TD=align: center]2954[/TD] [TD=align: center]9.94[/TD] [TD=align: center]17.24[/TD] [/TR] [TR] [TD=align: center]3[/TD] [TD=align: center]2455[/TD] [TD=align: center]8.01[/TD] [TD=align: center]17.51[/TD] [/TR] [TR] [TD=align: center]2[/TD] [TD=align: center]1802[/TD] [TD=align: center]6.02[/TD] [TD=align: center]17.30[/TD] [/TR] [TR] [TD=align: center]1[/TD] [TD=align: center]986[/TD] [TD=align: center]4.02[/TD] [TD=align: center]15.66[/TD] [/TR] [/TABLE] Note that, as described, both temperature and molecular weight decrease as the mixture becomes more fuel rich. As Iskierka explained, the reason the temperature is lower is because some of the heat generated by the reacted particles must heat up the unreacted particles. As the mixture ratio decreases there are fewer reacted particles and more unreacted particles. The reason the molecular weight goes down is because, as the propellant becomes more fuel rich, there is a greater abundance of lightweight unreacted hydrogen molecules. Also note that at a mixture ratio of 8 you might expect the molecular weight to be 18, i.e. that of water. What is happening is the high heat of combustion is causing some of the large molecules to break down into smaller ones in a process known as dissociation. Although water is most abundant, there is also some H2, HO, H, O2 and O present in the mixture, thus lowering the average molecular weight. If we look at the value of (T/M)1/2 we see that it's greatest when the mixture ratio is 3. This means that the exhaust velocity, thereby the specific impulse, will be greatest at about this mixture ratio. (I say "about" because the specific heat ratio is also a factor in the exhaust velocity equation and has not been considered here.) In practice we see that no LOX/LH2 engines actually use mixture ratios that are this fuel rich. Most LOX/LH2 engines operate at mixture ratios in the range of 5 to 6. The reason for this is because of hydrogen's extremely low density. At a mixture ratio of 3, the combined propellant density is very low, resulting in the need for very large propellant tanks. Increasing the proportion of oxygen to hydrogen significantly increases the density and lowers the propellant volume. Having smaller propellant tanks provides benefits such as lower mass and less drag. These benefits offset the decrease in ISP that results from operating at a less than optimal mixture ratio. In engineering practice it is found that mixture ratios of 5 to 6 strike the optimum balance between ISP and propellant density.

There are several things that visually don't make a lot of sense in terms of realism. For instance, the LV-T30 and the LV-T45 are essential the same engine, yet, when compared side-by-side, the LV-T30 has a much larger nozzle. I could point out other things as well. I try not to let stuff like this bother me. I'm sure the artists had to make the engines look different enough that a player could easily distinguish between them. And as you say, there's a need to make some engines squat to fulfill certain roles. As much as some people want realism, KSP is a game first, thus some compromises have to be accepted for the sake of game play.

I too understand the tradeoffs. It looks like the developers gave us a nice assortment of engines with different thrusts that could be used in a wide range of applications. If they started modifying the ISPs to be more realistic, they'd have to start adding engines - a group for use on the lower stages and another group for use on the upper stages. Personally, I would be happy to see that, but I understand the reason for not overcomplicating it. The thrust/fuel flow issue is one that I've seen many people complain about. It bothered me at first but I just learned to live with it. All these issues would probably bother me a lot more if I were a FAR user and striving for maximum realism, but so far I've only played stock while waiting to see what Squad does with the update. If Squad does decide to make changes to ISP, I hope they do it across the board and fix everything. If they fix one thing and not something else, then the thing they didn't fix will probably bug more than if they did nothing at all.

Where is that? I've never seen it. Maybe I'm using an outdated version of KER (0.6.2.12).

If we really want ISP to be realistically modeled, there's a lot of rebalancing that needs to be done. To really do it right, there should be two classes of engines - small expansion ratio and large expansion ratio. Small expansion ratios are used at low altitude where the ambient pressure is high, and large expansion ratios are used at high-altitude or in space where the ambient pressure is at or near zero. A small expansion ratio engine might have a sea level/vacuum ISP balance of something like 260/300 s, while a large expansion ratio engine might be something like 100/350 s. These are basically real life values, in KSP specific impulses are a little higher than real life (assuming LOX/kerosene). In KSP engines are balanced in such a way that that we get the best of both worlds - both good sea level and good vacuum performance. Of course this is the theory behind aerospike engines. An aerospike performs like a variable expansion ratio engine, obtaining small expansion ratio like performance at low altitude and large expansion ratio like performance at high altitude. In this regard, all engines in KSP behave like aerospikes, except, of course, the toroidal aerospike rocket. The in-game aerospike gives a vacuum-like performance at all altitudes, which isn't analogous to any real life engine.

Running fuel rich DOES increase ISP, to a point, then it starts to decrease. There is an optimum mixture ratio at which ISP is maximized, and that optimum is on the fuel-rich side of a stoichiometric mixture. Generally speaking, exhaust velocity is proportional to SQRT(T/M). In other words, the higher the temperature and the lower the molecular weight, the higher the exhaust velocity. Maximum temperature occurs when the mixture is stoichiometric, that is, there is just enough oxygen present to react with all the fuel. Unfortunately this also results in a high molecular weight. Reducing the proportion of oxygen to fuel lowers both temperature and molecular weight. Initially the molecular weight decreases faster than temperature, so exhaust velocity goes up. Eventually the trend reverses and temperature decreases more rapidly than molecular weight, so exhaust velocity goes down. Maximizing exhaust velocity, thus ISP, involves finding the optimum point where there is the right balance between temperature and molecular weight.

We won't know until the update gets here, but if they reduce drag to a more realistic value, then it will surely make Eve easier. That is unless they change something else that increases the Eve difficulty (such as reducing ISP at pressures >1 atmosphere).

Agreed, I would like to have that as well.

I just observed something. I went to the map view and estimated the direction of zero celestial longitude to see if I could find any distinguishing feature to define it. It looked like a pretty bland part of space with nothing special in that direction. However, when I reversed my view and looked in the direction of 180o celestial longitude, I was looking right smack in the middle of the thickest and brightest part of the milky way. Perhaps it is the galactic center that defines the celestial coordinate system, with 0o being directed away from the center.

The longitude of your satellite in the surface display is Kerbin longitude, the latitude-longitude coordinate system on the surface of the planet (I suppose we can call it keographic coordinates). The longitude displayed in the contract is celestial longitude, which is a different coordinate system. In the real world, geographic coordinates use Earth's equator as zero latitude and Greenwich, England as zero longitude. Real world celestial coordinates use the ecliptic as zero latitude and the direction of the vernal equinox as zero longitude. In KSP, Kerbin's equator lies within the ecliptic plane, so keographic latitude and celestial latitude are one in the same. Longitude is different, however. Keographic longitude rotates with the planet, while celestial longitude is fixed in relation to the stars. The direction of zero celestial longitude is not clearly defined and, as far as I know, does not correspond to any identifiable feature. However, you can get a rough idea of its direction by going to the map view and zooming out until you see the orbit of Eeloo. The celestial longitude of Eeloo's apoapsis is 130 degrees and its periapsis is 310 degrees. From this you can estimate the direction of zero longitude (longitude increases going counter-clockwise).

LAN is measured in reference to some defined point in space. In real life it is the direction of the vernal equinox. I don't know what it is in KSP. I generally don't worry about the value of the longitude of ascending node and argument of periapsis when I'm completing a contract. I just visually match the shape and inclination of the orbit display and make sure I'm traveling in the right direction. The contracts are forgiving enough that that does the job.

Although we don't know the exact composition of the atmosphere, we know its molecular weight. We are given following: Sea level density = 1.2230948554874 kg/m3 Sea level pressure = 101325 Pa Sea level temperature = 293.15 K From this we can calculate Rspecific, the specific gas constant, which is 282.6 J/kg-K. And from Rspecific we calculate the molecular weight of the air, which is 29.42 kg/kmol. We know oxygen is present (mol. wt. = 32) so, given the above molecular weight, the bulk of the remaining atmosphere is almost certainly nitrogen (mol. wt. = 28). The only additional piece of information we need to calculate the speed of sound is the specific heat ratio γ. Given the known composition and temperature, γ = 1.40. The speed of sound is, therefore, about 340.6 m/s at sea level. Although the temperature on Kerbin changes with altitude (as recorded by a thermometer), this is ignored in the thermodynamic model. Pressure and density are computed as if there is a homogeneous atmosphere at constant temperature. Because of this, the speed of sound does change and is constant everywhere. (ETA) If we assume the atmosphere consists only of oxygen and nitrogen and no other gases, then, given the average molecular weight, the atmosphere must consist of 35% oxygen and 65% nitrogen by volume.

I've had fun playing stock. One of the reasons being that I already know how things work in real life. Playing stock was a challenge because I had to forget some of what I already knew, discover how stock worked, and adapt to it. That being said, I'm looking forward to the upcoming changes. While stock was fun, the game needs more realism (streamlining should actually mean something). I'm in the camp that wants realism up to the point that the game does not become over complicated and difficult. Hopefully SQUAD will find the right balance.

Agreed. True. But if I follow all three* of my rules of thumb, my first stage will burn out at such time that 1.3 is the ideal TWR for my second stage. The rules only work collectively, not in isolation. * I should say four rules of thumb because the ascent trajectory is an integral part of it. The rocket won't perform optimally if you don't fly it right.

I agree with the 1.65, but I use about 1.3 for the upper atmosphere. I arrived at these numbers using a computer simulation. I kept changing and tweaking the numbers until I couldn't improve the rocket performance any further. I figured at that point I had pretty much hit on the optimum design. When I tried out my theoretical design in the game, it performed exactly like the simulations. Here are my basic rules of thumb: Stage 1 TWR = 1.65 Stage 2 TWR = 1.30 Ratio of Stage 2 thrust to Stage 1 thrust = 0.35 Of course it's not always possible to hit those numbers exactly, but I just try to get as close as possible. I've been happy with the results. Using a 2-stage rocket I can routinely get a payload fraction of about 0.16, and sometimes better. Just as important as the rocket design is the ascent trajectory. I usually start my turn at an altitude of 5000 m. I then turn gradually while keeping the nose of my rocket within a few degrees of the prograde marker on the NAVball. I accelerate up to about 2300 m/s at an altitude of about 50 km, with my rocket horizontal at that point. I cut the engine once my apoapsis reaches my intended altitude (it's actually necessary to overshoot the target altitude a bit because drag will lower the apoapsis by the time I get there). I typically require a 50-100 m/s burn at apoapsis to circularize the orbit. Using this technique I routinely get to orbit using no more than 4550 m/s delta-v. Note that this is based on using stock aerodynamics. It's my understanding that when using NEAR/FAR, lower TWRs are ideal.

It is derived here: http://www.braeunig.us/space/propuls.htm#impulse

I like Bob because, well, I'm Bob too.

Essentially yes. What I did first was to settle on a basic design configuration - in this case it was the three central stages with the three pairs of asparagus strap-ons. I choose this arrangement because it gave me the ÃŽâ€V I needed. I then ran a series of simulations in which I could alter two key parameters: (1) the starting TWR of each stage, and (2) the distribution of propellant between the stages. I went through repeated iterations, adjusting one parameter, then the next, and then back to the beginning for another series of iterations. I just kept making adjustments until I could obtain no further improvement in performance. I figured at that point I had pretty much optimized the TWR because any further change I would make would lessen the performance. In the end my basic conclusions were the following: Propellant should be distributed between the stages in roughly to the following ratios: * Stage 3 = 1 (uppermost stage) * Stage 2 = 5 * Stage 1 = 14 (core stage onto which the strap-ons are mounted) * Strap-ons = 7 x 6 each Liftoff TWR, all strap-ons + Stage 1: 1.60 Stage 1 TWR after jettison of the last strap-on pair (propellant tanks full): 1.40 Stage 2 TWR at ignition: 1.235 Stage 3 TWR at ignition: 0.80 Of course this is only theoretical and hasn't been proven in practice. I should also note that this is based on an ISP of 320 s sea level and 370 s vacuum for all engines. Results may vary with different engines, such as the aerospike. Let me also specify that my definition of 'optimum performance' is 'maximum payload fraction'. For the simulation that I've been talking about, the payload fraction is about 0.00565. That's miniscule, but then Eve is a particularly tough place to launch from. The ÃŽâ€V of the simulated launch was about 10,770 m/s (from 3000 m elevation). Although I haven't tested the above recommendations within the game, I have used a the same simulation techniques to try to optimize my Kerbin launch vehicles. I found that when I put those results to the test in gameplay, the launch vehicles performed identical to the simulations. (edit to add) Note that during the simulation I assumed that all engines are operating at 100% throttle all the time. I just took a look at my simulation results and I see that toward the end of Stage 1 burn (about 124-149 s elapsed time) the launch vehicle exceeds terminal velocity. In practice the engine should probably be throttle back during this time just enough to keep at or below terminal velocity. This would truncate that first tall peak in the TWR graph. By the time of the second tall peak at Stage 2 burnout, terminal velocity is large enough that there is no need to throttle back. This change should provide a small improvement in performance.

I haven't lifted off from Eve yet so take this advice with caution. A short time ago I was planning a manned mission to Eve that I haven't flown yet. In preparation for that I did some computer simulations to see if I could figure out the best TWR. I used three core stages stacked is series and three pairs of asparagus strap-ons. The following is what I came up with, in graphical form. TWR is based on Eve gravity. The first three peaks represent burnout of the strap-on pairs, while the last three peaks represent burnout of the core stages. Note that my initial plan was to land at a high elevation somewhere, so the above is based on taking off from an elevation of 3000 meters. One of the reasons I never flew the mission was because I decided to change my design to land near sea level. I'm not sure how landing at the lower elevation will change the TWR. I was planning to rerun the simulations but I haven't gotten around to it yet.

I feel your pain. I experience many of the same problems that you do. I've just learned to live with it and managed to get better with practice. One trick that I discovered that helps is that I can use scroll wheel on my mouse to add/deduct velocity. If it's too difficult to actually click on and drag the correct arrow, I can just point at it with the cursor and then use the scroll wheel, which has the same effect as dragging the arrow. This also helps with fine adjustments. I usually just nudge my ship using the WASD keys as I'm beginning a maneuver to get myself oriented as to what my ship will do. I'll then roll my ship or rotate the view as necessary so that the movements I'm seeing on the screen mimic what I'm doing with the keys. This makes rotating the ship more intuitive and natural. After you've gotten an encounter with your target planet (you see a "Pe" on the map screen at the encounter location), click on and focus your view on the target planet. If you zoom in on the planet you will see the path your spacecraft will take through the planet's sphere of influence. While zoomed in on the planet, rotate your view until you can see your maneuver node in the background. You can now adjust your maneuver node and see the effect your changes have on the encounter trajectory. Tweak it until you get the encounter you want. When far away the encounter trajectory is extremely sensitive to very small changes in velocity. Initially just try to get close. You can perform a course correction latter on to fine tune it.

Argument of periapsis is the angular distance between the ascending node and periapsis. The orbital elements are explained here: http://www.braeunig.us/space/orbmech.htm#elements

And hoping that you don't land on and tumble down a mountain side.

The contract should give the inclination of the orbit. If it less than 90 degrees then the orbit is prograde. If the inclination is greater than 90 degrees then it is retrograde.

One additional clarification on the part above. Note that escape velocity is the special case where the burnout velocity is exactly that needed for the body to reach infinity but with zero residual velocity. In other words, escape velocity is the value of v1 where v2 is equal to zero. Thus, v12 - 2GM/r1 = 0 Vesc = (2GM/r)1/2 - - - Updated - - - True, but for those who don't mind the math it can be helpful. Some people aren't satisfied just to know that an equation works, they want to know why it works. Those who don't like math can simply skip over it. We can aim to please both the math geeks and those who are math adverse.