How to make an efficient payload rocket and get it into orbit?

[OhioBob]

Ferram Aerospace Research is a mod that adds a realistic aerodynamics model to the game.

Thanks. I'll have to check that out eventually. I haven't tried any mods yet because I'm a little afraid of messing something up.

The current model is to add up all the drag values of all parts on the ship and create an artificial drag value. Of course if the part is behind another part, it's still counted where in the real world it would be aerodynamically shielded. This is kind of stupid. Welcome to the forums!

I'm aware of how the stock game handles drag. It is this drag model that I use in my simulations. I don't doubt that a more realistic aerodynamic model would likely result in different ideal TWR than what I computed.

[Alshain]

Thanks. I'll have to check that out eventually. I haven't tried any mods yet because I'm a little afraid of messing something up.

I'm aware of how the stock game handles drag. It is this drag model that I use in my simulations. I don't doubt that a more realistic aerodynamic model would likely result in different ideal TWR than what I computed.

Ah, well I didn't know what you knew Yes, real aerodynamics use a lot less fuel, but it also adds difficulty too. The OP stated they were using it so that's why our advice is geared for it.

If you want to try it, I recommend NEAR first, then FAR after you've mastered NEAR. The main difference is that FAR simulates aerodynamic failures that rip your craft apart if you put too much pressure on them and mach effects on the Center of Lift. NEAR does not simulate those, so it's sort of a stepping stone that lets you ease into it.

[kStrout]

My rockets usually have a TWR of 1.5 on the launchpad, but at around 10km when you drop the boosters It should be just above 1. then turn sideways always pointing the nose high enough that your time to apoapsis increases.

[OhioBob]

My rockets usually have a TWR of 1.5 on the launchpad, but at around 10km when you drop the boosters It should be just above 1. then turn sideways always pointing the nose high enough that your time to apoapsis increases.

I generally shoot for a TWR of about 2 for both my first and second stages. I'll ascend vertically to about 5000 m and then start to slowly pitch over. I try to keep my pitch axis about 5-10 degrees ahead of the surface velocity vector. If all goes well I should be pitched over about 75 degrees (15 degrees above the horizon) by the end of my first burn. In this way I can get into a 75,000 m circular orbit using only about 4,400 m/s delta-v, of course that depends on how you calculate it.

Delta-v depends on the specific impulse, however specific impulse has two values - sea level and vacuum. I generally like to average those values for any of my stages that ignite on the ground, while I use the vacuum value for any of my stages that are ignited in flight. I think this does a better job of estimating the expected performance of a launcher vs. computing delta-v based on the vacuum specific impulse alone. Is there any kind of standard convention used around here when expressing delta-v?

[Pecan]

I generally shoot for a TWR of about 2 ... Is there any kind of standard convention used around here when expressing delta-v?

... and whatever works for you is good too :-) Launch requirements through an atmosphere are notoriously hard to optimise since there are so many variables - each stages' start and end TWR and burn time, gravity-turn start altitude, rotation speed, throttle settings, inherent drag, etc. etc. [Goddard problem].

Generally I think most of us just use the vacuum deltaV figures because by the time you reach 2km the atmosphere is thinning-out considerably and it's just too much bother to try to interpolate. Ultimately, vacuum deltaV at launch- vacuum deltaV in orbit = how much it took, in retrospect.

[Jouni]

Obviously do what you think is right, but I get slightly higher losses at 1.5-1.7. I just ran a couple simulations for a sample rocket with all things being equal except for the throttle setting and the pitch rate. With the throttle set to give me a pad TWR of 1.6 my losses were: drag 857 m/s, gravity 1234 m/s, and total 2091 m/s. With the throttle set to give me a pad TWR of 2.0 my losses were: drag 1029 m/s, gravity 1048 m/s, and total 2077 m/s. That's not a big difference, but TWR 2.0 was a little better.

You're optimizing delta-v usage here, which is different from optimizing the payload fraction. If your rocket has less dry mass due to having fewer or smaller engines, it gets more delta-v from the same amount of fuel. You can then use some of the saved mass to increase the payload, while still having enough delta-v to compensate for the higher losses due to having lower initial TWR. In traditional asparagus-staged rockets, the optimal TWR is usually 1.6-1.7 at launch.

[Yakky]

With that much thrust in FAR, you want to tilt a little to the east very soon after launch. As a rule of thumb in FAR, I start my eastward tilt at about 150 m/s and then slowly increase it from there. By the time you are going 300 m/s or more, it's too late to start tilting. And I would also suggest reducing your throttle once you reach terminal velocity. Remember it is inefficient to go faster than terminal velocity because you are wasting thrust on pushing too much air around. Also, the high dynamic loads create a lot of aerodynamic stresses around stage separation events.

BTW the problem with boosters crashing into the sides of the main tank is well known. I'm not sure if it's a bug or a legitimate aerodynamic effect, but it seems to be worse under high aerodynamic loads and any asymmetric situation (anytime your ship is not pointed exactly in prograde direction at separation). With all your power (3+ TWR), you are likely running some enormous aerodynamic loads by the time your boosters burn out. The separatrons might not be strong enough to overcome the force. FAR will tell you how high your aerodynamic forces are. It the "Q" value is getting above 40,000 and it says "High Dynamic Pressure", watch out.

Also, those special boosters you are using might make it worse because their asymmetric nose cones are going to aerodynamically rotate them into the center after you separate them. As an experiment, try rotating them so the nose cones face away from the core and see if the problem is any better. (It will look ugly but it will help you figure out if aerodynamics are the problem.) Then at least you will know what's going on.

Edited November 8, 2014 by Yakky

[OhioBob]

You're optimizing delta-v usage here, which is different from optimizing the payload fraction. If your rocket has less dry mass due to having fewer or smaller engines, it gets more delta-v from the same amount of fuel. You can then use some of the saved mass to increase the payload, while still having enough delta-v to compensate for the higher losses due to having lower initial TWR. In traditional asparagus-staged rockets, the optimal TWR is usually 1.6-1.7 at launch.

Interesting that you say that because I was just in the process of designing some tests to determine the optimum configuration for maximizing payload fraction. I'm curious to see if I come up with numbers similar to yours.

Theoretically your point is valid, but it assumes there is a smaller engine available that will suit the particular need. That's not always going to be the case. For instance, the particular scenario that I cited was for a simple two-stage rocket using a Mainsail engine on the first stage. The configuration gave me a liftoff TWR of about 2.1 at full throttle. To hit your sweet spot I'd need an engine with a thrust of about 1200 kN, however, there are no engines of that size in the game. I could downsize my core stage and add some combination of strap-ons to get me to the desired TWR, but that's way more trouble than its worth. I'd rather just keep things simple and fly with the Mainsail engine. In this case my tests show that it would be better to throttle up to about 1.8-2.0 rather than 1.5-1.7. At the higher TWR my overall losses would be less, thus I'd reach orbit with more propellant left in the tanks. The extra propellant would provide a little more on-orbit maneuver capability.

I think the moral of the story is... TWR 1.5-1.7 may be optimum if you're using small engines operating at full thrust, but it is not ideal to throttle back a bigger engine just to get into the 1.5-1.7 range.

[Jouni]

Theoretically your point is valid, but it assumes there is a smaller engine available that will suit the particular need. That's not always going to be the case. For instance, the particular scenario that I cited was for a simple two-stage rocket using a Mainsail engine on the first stage.

The traditional assumption has been that if you optimize payload fraction, you're using asparagus staging with at least three pairs of boosters. Two-stage rockets work in a different way, and I'd guess that their optimal TWR would be much lower.