a few specific questions

[wisnoskij]

Hello all, I have a few specific questions. I played a little 1.0 and watched a few videos (Scott Manley), and really want to get back into it but a few questions are really bugging me.

1. Solid Fuel Rockets. Are they strictly superior to liquid if you have no need to do anything but full throttle until empty? I am seeing a lot of designs using liquid fuel, even when it is just full burn. Am I missing something or do solid fuel rockets just have disadvantages without ant offset with advantages?

2. Speed in Atmosphere during takeoff. I have yet to see an explanation of how to optimize this. Say for example we have a rocket with 4 thumpers and a terrier. To Optimize performance, do you stage 2 jettison them, and start the last two, then go with the terrior after they burn out? Do you burst fire all four at a time, maybe even have the terror running? We want to get away from the gravity as fast as possible, but at a certain point the air resistance is a bigger concern? Is there a certain speed threshold you want to stay below?

3. Takeoff Angle. I never understood this. Everyone says start turning well in the atmosphere, but, ignoring the use of turbine jet engines for a second, isn't the entire point to get away from the air resistance and gravity as fast as possible, it is easy to add deltaV once we are at 70K+???

4. Spaceplanes. Once you have jet engine technology will that always be the most efficient first stage for escaping Kerbin?

5. Science Biomes. This always confused me. So not only do you want to do every experiment in every location but you want to do them multiple times. There is, for example. Ocean, flying above the ocean (both high and low in the atmosphere????), Orbiting above the sea (high and low orbit)?. Or is it less than that? Is there some setting I am missing that points out which biome you are in and tells you if you have science you are still supposed to do in that biome? Maybe a mod?

Thanks for any and all answers.

Edited May 11, 2015 by wisnoskij

[SRV Ron]

Hello all, I have a few specific questions. I played a little 1.0 and watched a few videos (Scott Manley), and really want to get back into it but a few questions are really bugging me.

1. Solid Fuel Rockets. Are they strictly superior to liquid if you have no need to do anything but full throttle until empty? I am seeing a lot of designs using liquid fuel, even when it is just full burn. Am I missing something or do solid fuel rockets just have disadvantages without ant offset with advantages?

2. Speed in Atmosphere during takeoff. I have yet to see an explanation of how to optimize this. Say for example we have a rocket with 4 thumpers and a terrier. To Optimize performance, do you stage 2 jettison them, and start the last two, then go with the terrior after they burn out? Do you burst fire all four at a time, maybe even have the terror running? We want to get away from the gravity as fast as possible, but at a certain point the air resistance is a bigger concern? Is there a certain speed threshold you want to stay below?

3. Takeoff Angle. I never understood this. Everyone says start turning well in the atmosphere, but, ignoring the use of turbine jet engines for a second, isn't the entire point to get away from the air resistance and gravity as fast as possible, it is easy to add deltaV once we are at 70K+???

4. Spaceplanes. Once you have jet engine technology will that always be the most efficient first stage for escaping Kerbin?

5. Science Biomes. This always confused me. So not only do you want to do every experiment in every location but you want to do them multiple times. There is, for example. Ocean, flying above the ocean (both high and low in the atmosphere????), Orbiting above the sea (high and low orbit)?. Or is it less than that? Is there some setting I am missing that points out which biome you are in and tells you if you have science you are still supposed to do in that biome? Maybe a mod?

Thanks for any and all answers.

1. SRBs. Useful in Career as they are cheap. Best used in the first stage due to their weight. You can tweak their thrust to control their acceleration before the rocket is placed on the pad. Disadvantage, no control of their thrust during flight.

2. You want to keep your speed well below the sound barrier in the lower atmosphere or you will have issues with drag on the front of your rocket that can cause you to swap ends. Still, you want it high enough to get it out of the lower atmosphere as quickly as possible. Experiment with different designs as well as changing throttle settings to control your flight.

3. Turn gradually as opposed to the old 10k-45* rule or you will swap ends or worse. Each design is different to where the turn works best. This is a learning process. Once above 70K there is no atmosphere effects that will affect your flight.

4. Yes and no.

5. There are several mods that will tell you which biome you are in. But there is a diminishing return for repeated experiments in the same biome. You do have Crew Report, EVA report, EVA landed, soil sample, Material bay, Goo Pod, and several other experiments that will yield Science for each Biome on Kerbal or off world and in space.

[DeMatt]

1. Solid fuel boosters tend to be heavier for a given delta-V than liquid fuel. Also, solid fuel boosters are limited to the sizes provided in-game (unless using a mod like Procedural Parts), which may not match the actual size required.

2. There is a threshold, yes; loosely speaking, it's where drag and gravity are equal, as increasing speed will increase drag loss more than it decreases gravity loss, and vice versa. This speed is also known as "terminal velocity", as when your rocket falls down, its acceleration due to gravity will be offset by drag and thus cap its speed. The exact value will depend on your rocket design - long and narrow has a higher terminal velocity than short and fat, for instance.

3. One assumes less-than-instantaneous vector changes. Imagine the extreme case: straight up to 70km altitude, then straight horizontal to orbital speed. While you're performing the horizontal acceleration, you'll have to keep your rocket in place against gravity, yes? Why spend the effort of lifting the horizontal fuel all the way to 70km, and holding it there, when you can burn it earlier?

4. Most efficient in terms of fuel expended, yes. Simplest? Cheapest? Maybe not.

5. There are mods that do this - [x] Science! and ScienceAlert come to mind. Stock, you can look at the Science Archives tab in the R&D building to see what experiments you've done where.

[FancyMouse]

To add

1. SRB also has no gimbal, making it hard to control (need both fins and reaction wheels)

3. Gravity turn means you want the gravity to help you turn your vertical speed into horizontal - keep in mind your orbital speed is horizontal, not vertical. First go to 70km+ then turn means you'll need to build all horizontal speed yourself. That's much less efficient.

5. Depending on the situation and experiment, it could be biome-specific or just global (i.e. biome doesn't matter). This gives you a good picture what possible combinations are.

[cybersol]

1. SRB were nerfed if 1.0, so their niche is much smaller now

2. Speed is much less of a concern than ever before in 1.0+. You do pay more in atmospheric drag, but you also leave the thicker atmosphere faster. Practically speaking in 1.0+ if you don't burn up first you will likely save fuel by going faster. This is new, so I don't think I have fully adjusted to this, but empirical test show it is true.

3. Once you are in orbit, all you speed is parallel to the ground. So all the time you spend burning perpendicular to the ground is wasted energy -- known as gravity losses.

4. Basic jets aren't that great, and as DeMatt points out airbreathing stages are expensive if you are not making a reusable craft.

5. I found the Science Alert mod to be great for helping me learn what I was missing.

[The Yellow Dart]

To add a little more...

1. SRBs, while cheaper than the alternative, are very heavy compared to how much you get out of them. In early career, when you are limited pretty severely by weight via the tier 1 launchpad (18 tons), you can get more dV out of a rocket using only liquid-fueled rocket engines because they are more efficient. Using multiple SRBs will add a lot of weight without adding as much dV.

[Eric S]

To go into a little more detail:

1) SRBs have a lower ISP than liquid lifter engines. This means that it takes more fuel mass to produce the same amount of thrust. Because of this, SRBs generally should be used as early as possible so that you can get rid of the less effective mass ASAP. In early career mode, I might use SRBs to get a rocket up to a high apoapsis, then circularize using liquid fuel engines. As I transition into having more variety of liquid fuel engines, I tend to use SRBs less often outside of the first stage, and then mostly to get the rocket out of the lower atmosphere so that the liquid engines have a higher ISP. Also, as rockets get heavier, I find SRBs to become less and less useful.

2) it used to be that you wanted to follow terminal velocity for efficiency, but that's not the case so much with the reduced atmospheric drag in 1.0 and later. I still find myself not exceeding terminal velocity below 25-30km altitude, but this is now because of rocket stability. Going transsonic alters the way drag affects the craft, and quite often crafts that are stable below terminal velocity may not be stable above it.

3) First, understand that the atmosphere drops off rapidly, enough so that other factors become more important than aerodynamic drag. The two major factors are gravity losses and steering losses.

For gravity losses, you're looking at two factors. First, if you thrust straight up with a 2.0 TWR, 1 G of gravity cuts your acceleration to 1G of thrust upward. If you're at 30 degrees above the horizon, however, your thrust equals 1.0 upward and 1.73 toward the horizon. 1G of gravity negates the upward thrust, but now you're accelerating towards the horizon considerably faster than you were accelerating upward when thrusting straight up. Second, as your horizontal velocity increases, the more your forward momentum cancels out gravity (the phantom centrifugal force) meaning you spend even less of your thrust fighting gravity.

For steering losses, if we ignore gravity for a moment, if you burn in one direction for 1000 m/s of acceleration then turn 90 degrees and burn another 1000 m/s of acceleration, you've only given yourself 1414 m/s of acceleration. If you had turned 45 degrees in the same direction then burned for 1414 m/s, you'd have the same velocity, though not necessarily the same position.

So basically, a gravity turn is all about balancing all of these. Turn too aggressively, and you're fighting too much aerodynamic drag, don't turn aggressively enough, and your gravity and steering losses increase enough that they increase than the aerodynamic drag decreases.

4) I haven't tried doing spaceplanes in 1.0 or later, but from what I've heard, turbojets aren't as good for spaceplanes as they were previously. Many players have switched over to either strictly using RAPIER engines or a mix of RAPIER and turbojet engines. In either case, in 1.0, spaceplanes are more about reusability than low fuel cost. Prior to 1.0, I had a no-cargo spaceplane that would come so close to orbital velocity that it only took 50-80 m/s of delta-v to circularize. That's not happening anymore.

5) Different experiments respect biomes at different altitudes. The negative gravioli detector, for example, is the only science part that pays attention to biomes at the "high above" altitude range, all other science experiments will get you one shot at a full-value report. Most, if not all, science experiments respect biomes at ground level.

EDIT: SRBs SHOULD be used ASAP, not shouldn't.

Edited May 11, 2015 by Eric S

[wisnoskij]

That helps a lot everyone. Thanks for all the explanations.