sevenperforce

Specific impulse is the number of pounds of thrust per pound of propellant you burn each second. Or the number of tonnes of thrust per tonne of propellant. If you have an engine which produces 100 tonnes of thrust (9800 N) while burning 1 tonne of propellant per second, then you divide 100 tonnes by 1 tonne / second; tonnes cancel, and you get 100 seconds. If you have an engine which produces ten pounds of thrust while burning one ounce of propellant per second, you recognize that one ounce is 1/16th of a pound, so you fix the units and you get 160 seconds. If you have an engine which produces 60,000 pounds of thrust while burning 3 gallons of fuel per second, and you know your fuel weighs 6.68 pounds per gallon, then you can do the math and find out that your specific impulse is 3000 seconds. Of course, those numbers are only going to make sense on Earth, really, because you're using weight instead of mass. If you want to be a little more scientific about it, you can divide the thrust (in units of Newtons, preferably) by the mass flow (kg/sec) and you end up with a value measured in meters/second. For example, consider an engine which produces 800,000 N of force while consuming 262 kg of propellant every second. 800 kN / 252 kg/s works out to 3,050 m/s. As it turns out, 3,050 m/s happens to be the average exhaust velocity of the propellant coming out of the back end of a Merlin engine. That makes sense; the faster your exhaust is moving, the harder it is pushing against your nozzle as it leaves, and the more thrust you get out of it. Of course measuring specific impulse in seconds looks a little cleaner. So you can convert m/s to s by dividing by 9.8 m/s2, since that's how you convert between weight and mass. 3,050 m/s divided by 9.8 m/s2 is 311 seconds, which is the specific impulse of a Merlin engine.

Well, we may be talking past each other. Obviously, there are no reversible processes in real life. But I wouldn't say that reversibility is an approximation. Rather, reversibility is an attribute of isentropic processes. Isentropic processes describe the component of a thermodynamic system which performs work; if work is being performed, then there is a reversible, isentropic process involved, even if entropy is flying willy-nilly left and right. Reversibility and isentropy describe the underlying cycle which is available to perform work within a thermodynamic system. If you have a thermodynamic system which performs net work, then there is a reversible cycle somewhere in there, even if entropy would bleed off energy at each stage in the cycle. Reversibility exists even though the overall system is always going to have positive entropy. So, for any reactionless drive, we can ask, "Does it perform work?" If the answer is no, then it cannot accelerate anything and it is useless. If the answer is yes, then there is a reversible, isentropic cycle somewhere in the system, and so that reactionless drive can (in theory) also be used as a reactionless generator. A reactionless generator can be coupled to a reactionless drive to produce over-unity, violating conservation of energy (even if entropy would bleed some of that infinite energy from the cycle). IIRC, any violation of the laws of thermodynamics and kinematics can be exploited to produce infinite energy. For example, suppose you have a Brownian ratchet capable of extracting work from waste heat without a heat sink. This is a violation of the second law of thermodynamics. Couple that Brownian ratchet to a dynamo, and you can route the waste heat from the dynamo back into the Brownian ratchet, producing over-unity.

Cubing the sphere, actually. The cube and the sphere have equal volume. 4/3πr3 = x3.

You could have self-sustaining, replicating sentient plasma vortices populating the surface of the sun and we'd never know.

Dammit. Do not go gentle into that good night; rage, rage against the dying of the light.

In honour of pi day, I present the True Meaning of PiTM: Ten Science Points to the first person who knows why this represents Pi.

Can't you already do that with any engine?

However, any thermodynamically consistent reactionless drive can be used to produce infinite energy. Just like any Maxwell's demon or Brownian ratchet capable of creating work from waste heat (without a heat sink) can also be used to produce infinite energy. The metric of spacetime balances this out, so conservation of energy is preserved. A lay explanation: https://www.quora.com/Does-dark-energy-violate-the-law-of-conservation-of-energy-mass OK, here is where I am going to disagree with you. You are ignoring entropy. If you take a liter of oxygen and a liter of nitrogen and mix them together, it takes a heck of a lot more work to separate them than it did to mix them. Mixing is not a reversible process. And try pumping exhaust gasses into a car engine and creating fuel and air. Good luck with that. Yes, you can synthesize hydrocarbons from CO2 and H2O, but not by "reverse burning" them. Bottom line is that reversibility is at best an approximation and at worse a very bad approximation, because entropy always has a look in. Yes, I was ignoring entropy, because I was talking about systems which work. Using work in its formal sense, of course: the product of force and distance. Apply a force of 10 Newtons to an object across a distance of 5 meters, and you have performed 50 Newton-meters of work. A Newton-meter is, of course, a Joule, and if you measure the change in kinetic energy of the object, you will find it has gained 50 Joules of kinetic energy. This is a reversible thermodynamic process, because there has been no increase in entropy and you can extract 100% of those 50 Joules of kinetic energy by allowing the object to impinge upon a spring or other energy storage mechanism. All thermodynamic processes which actually perform a net amount of work are reversible with respect to the work they perform. So even though entropy always has its lukewarm fingers in every pie, you can separate the positive-entropy component of a process from the work-producing component of a process. The former is irreversible; the latter is reversible. So if you have a process which actually produces any work, then there's a reversible cycle in there somewhere. Note that this is true even for something like a rocket engine, which seems like an example of a completely irreversible process. Is it really reversible? Well, yes, if you look only at where work is actually being performed. To start with, the turbopumps perform work by applying force to the propellants across the distance of the turbopump feed lines (we'll assume a separately-powered pump, like the one on the Rutherford, to avoid questions of combustion cycle). Then the propellants combust. Contrary to expectations, the combustion of propellants performs zero work. What performs work is the expansion of the exhaust against the bell nozzle of the engine. A perfectly-efficient nozzle would be infinitely long and all the exhaust would (very gradually) expand to zero degrees Kelvin and zero pressure. If this were the case, then your exhaust stream would be a perfectly-aligned flow of gas molecules, all moving in parallel. This would be reversible, because you could shut down the engines and fire retrorockets to arrest and reverse the direction of the rocket. The infinitely-long rocket bell would then collect all the molecules, forcing back into the combustion chamber and then through the turbopump feed lines. In fact, you could even use this to recharge your batteries, if you have a battery-operated turbopump. Of course no exhaust bell is 100% efficient, and so entropy would very quickly put a stop to this approach. But you can still identify the component of the process which performs work and is therefore reversible.

kg/m2 meaning tank dry mass divided by tank surface area? If so, that's not surprising. Centaur is a balloon tank that cannot support its own mass under compression unless it is pressurized; the ET was very much load-bearing. Then again, consider that the SLWT variant of the ET boasted a propellant fraction of 96.5% while Centaur only manages a propellant fraction 91.4% (subtracting engine weight). That's the square-cube law for you. But hydrogen is so fluffy that the Falcon 9 upper stage beats both Centaur AND the much larger SLWT hands-down, with a whopping propellant fraction of 96.9%. Or, rather, they end up with high-end hydrolox isp, but with kerolox-level density. Doesn't necessarily compensate for the tremendously high dry mass of the engine.

Using the STS ET (even the SLWT variety) isn't a good comparison, because a) the ET was a transverse-load-bearing member, and b) it had simply huge square-cube advantages. So it's not representative at all. Better to use something like Centaur. Compare to a methalox upper stage of equal mass and to one of equal volume and see how they fare.

THAT looks like a flight test article.

No, they have a minimum just like regular chutes.

The official rotation of Jupiter is the rotational period of its magnetosphere, because this represents the bulk of the mass of the planet. This was also the first rotational period to be accurately measured, since it could be measured via radiotelescopes. Parts of the surface orbit faster than this; parts orbit slower.

Happens. Every. Time. Though this is, in large part, why I play with Tweakscale whenever I can. I need to leave the upper stages fueled to calculate lower-stage dV, so knowing the total dry weight of my rocket wouldn't help. Only needing to defuel and refuel a single tank for each stage makes it easier. The console can still pause, right? I usually keep quantumg's dV calculator open on my phone. Right, this is absolutely correct. What if you have a single stage that has two different propellant types in parallel? The most efficient thing to do is burn your lower-ISP propellant up first, of course. But if you designed your stage with two different propellant types, you probably had a reason for doing so, one which involved using specific propellants at specific times, which will make your stage's propellant usage (and thus its total dV) lower than the most efficient case. The stock dV meter has no knowledge of when or how you will do this, of course. If it aims for the worst-case scenario, it'll be wildly wrong; if it aims for the best-case scenario, it'll be wildly wrong. Either way, it doesn't work. Or what if you have airbreathing engines and nukes on the same stage?

WHERE ARE MY LIKES DAMN YOU 1.4 DISCUSSION THREAD

A discussion of pseudoscience still involves science. I designed a great many perpetual motion machines at age 11-12, and it was only through the patient (and, at times, exasperated) explanations of my teachers that I was able to slowly get an intuitive grasp for how scientific inquiry works. I was a hardline creationist for 20 years, and it was only through the dedicated (and at time shouty) persistence of people willing to counter my bad arguments and point me to piece after piece of evidence that it all finally came crashing down. So I do my best to assume that the @Lordmaddogs of the world are inquisitive, if overly-excitable, minds who simply need to be pointed in the right direction...perhaps more than once. Hear, hear. Thats a rather unusual belief system. Aerodynamics doesnt "counter" gravity, nor is "aerodynamics" a "law", aerodynamics is a field of science developed by study of many things all in relation to the known laws of physics. The theory of gravitation did not change a single iota during the invention of aerodynamics. Aircraft dont even work without gravity... Smells like highly potent homeopathy.... Is there a Rule 34 for conspiracies? Because dentistry is a conspiracy.... Dint-ding-ding! THIS ^^ And see, this is what happens when I waste all my likes on discussions of 1.4.

This is the case for me as well. I know generally how much thrust and how much fuel I need to make orbit with a given payload.

I just do the math. But most players don't have degrees in physics, so...

Hmm. I just tested it with a standard Apollo-style flip-and-dock, no problem. Did it a few times in succession to be sure. Try keeping one craft's SAS off?

What is mesh switching? Just the reskins, or something else? Also, I just discovered the new advanced tab that allows you to reskin all your parts at once. Only a couple of options at present though.

I just tweakscale everything. Which works well with 1.4. Except for the new R-series tanks. They don't scale. Boo!

I tried to be nice about it. I was once like him. When I was, like, 11....

Sigh. Reactionless drives cannot work. Before I tell you specifically why your modified reactionless drive will not work, let me first attempt to explain in general why reactionless drives cannot work. All thermodynamic systems are reversible, at least in principle. An electrical motor which uses electrical energy to turn a wheel can be converted into a generator, which uses the motion of a wheel to generate electrical energy. Inward force can be applied to a piston to compress a gas, or a compressed gas can be expanded against a piston to produce outward force. This is why reactionless drives cannot work. Let us say you have a reactionless drive in a box. You don't know how it works, necessarily; you just put energy in, and it accelerates you forward. Great! Now, what happens if you run the cycle in reverse? Well, presumably, you would decelerate, and it would generate energy in response. But...deceleration is just acceleration in the opposite direction. So what's to stop you from "decelerating" continuously, for as long as you want, generating as much free energy as you want? Heck, I've got a better idea. Just point your reactionless drive in the opposite direction and run it in reverse cycle! You'll accelerate faster and faster in the direction you wanted to go all along, AND you'll generate unlimited energy! Obviously this is nonsense. And so are reactionless drives. Now, on to specifics: You may not realize it, but you seem to be assuming through this whole process that kinetic energy and momentum are proportional. They are not. Kinetic energy is a scalar that is proportional to the square of velocity; momentum is a vector which is proportional to the velocity. It is possible to change an object's momentum without changing its kinetic energy. First of all, where are you getting these numbers? Why would splitting the projectile in half reduce the total thrust by any appreciable amount? The "splitter" need only exert an infinitesimal force on the projectile to split it. Your second paragraph above is where you run into problems. Whatever force the purple tubes exert on the projectiles to turn them by 90 degrees, the projectiles also exert (in reverse) on the purple tubes...cancelling not 15-20% of their downward momentum, but 100% of their downward momentum. This seems strange, because the projectiles presumably strikes the plungers with momentum, right? Yes, each projectile has momentum, but the two projectiles together have zero net momentum, because the purple tubes canceled out the net momentum. They did this without eliminating kinetic energy, of course, so net kinetic energy is still present, but there is no net momentum and therefore no thrust. You have a great mind. Learn more math and invent something that will actually change the world.

You can build a pretty convincing R-7 with the new 1.4 fairings alone.

I would always just drop a fairing faceup on the bottom node of the tank I was going to be putting my engines on, shift it down with the offset tool, and then use cubic octagonal struts placed directly on the upper tank base to set up my engines before closing the fairing around the whole affair. But this is cool, I guess. Can it accommodate changing diameters?