sevenperforce

You could pack positive and negative charge on two separated wires. Difficulty with electromagnetic propulsion in the Jovian system is power; solar power is in short supply out there. Venus has no meaningful magnetic field.

If you hadn't noticed, there is an unending dispute over whether these are, in fact, the actual conditions of the experiment. As @K^2 has helpfully derived, it is not enough for the belt to reach a high speed; the belt must continually accelerate in order to exert constant force on the plane. Since all known formulations of the problem talk about the speed of the belt, not some constant acceleration, this interpretation is...unlikely. "match the speed of the wheels" is an idea you still have yet to define. Depends on how you define it. Landing gear is going to remain firmly in the static friction regime. It's what it's designed to do. A slipping wheel cannot provide lateral forces allowing any sort of control. And while rudder does assist at all in early stages of landing or late stages of takeoff, you rely on wheels for much of steering during runup. If wheels began to even just noticeably slip during takeoff before you are ready to rotate, you would not be able to keep the plane on the runway. This I can tell you from first-hand experience. Well, yes, a slipping wheel is useless for steering, but what does that have to do with this experiment? The sorts of friction that increase as a function of RPMs will reduce traction as the wheel-belt interface skips between static friction and kinetic friction. This means the belt must work harder and harder to add momentum to the wheel. All I'm suggesting is that a continuously-accelerating belt arrangement will rapidly reach RPMs at which slippage will become non-negligible. I didn't say this was necessarily below the failure point for real airplane wheels. You're the one who posited the continuously-accelerating belt that was never part of the original formulation of the problem. Yes, if the failure speed of the belt is greater than the failure speed of the wheels, for any reason, then trivially the wheels will fail first.

The reason that drag works the way it does is this: in order to push air out of the way, a rocket must (notionally) accelerate that air to the same speed it is going. By Newton's second law, acceleration is force divided by mass. By Newton's third law, the force applied to the air is equal to the force the air applies to the rocket. So the coefficient of friction is defined for a shape with the reference area being the cross-sectional area of the entire rocket at its widest point.

Exactly. Wouldn't that be a lot simpler than a parafoil boat catching scheme? Waterproofing would be easier said than done. It's probably a lot easier to waterproof a cellphone than it is to waterproof bespoke fairings. I thought Elon already said the latter. We know Merlin can still run non-densified propellants. Please don't ask this! Now that Block 5 is flying, how soon do you expect we will see more than one reflight of a booster? Is the upper-stage recovery plan intended for research or for actual reuse? Has any development been done on the auxiliary systems for BFR, like the RCS thrusters?

Right. I haven't read the paper either, but from Manley's video it sounds like the effect is a coupling between the circuit powering the resonance chamber and the magnetic field of the Earth. If the effect was actually located inside the resonance chamber, then we'd have a path to a more compact tether design. But so far there has been no indication of this. Current tether designs (no pun intended) use electrical current. I wonder if a physically rotating object with an electrostatic charge (think two loops of wire forming a capacitor) could be more compact/efficient.

But still presents a possibility. (I'd prefer without the numbers, you can't barely tell anyway.) The point is that if a statistically-generated answer has error bars on the order of the answer* it generates, then the statistically-generated answer is not useful for anything. It would be like the German tank problem, but instead of predicting a production run of 270 tanks, they said "German has made 300 tanks, plus or minus 290 tanks". If a statistical estimation produces this sort of variance then that's a clue that statistical estimation will not provide a meaningful answer. And that's what we already knew: intuitively, estimating the timing of a doomsday event using the number of humans alive today is clearly not meaningful. Examining the error bars gives us the mathematical reason why. *Technically, the breaking point is where the error bars are on the order of the range of the dependent variable in the estimation, but when the output range is a series of natural numbers, like a number of people or a number of tanks, you can approximate the range as the answer generated.

The drag coefficient of any solid body is a fancy way of putting a number on the question, "How easily does this shape push through air?" Unfortunately, this is not an easy question to answer. A shape which pushes through air very easily at low speeds may push through the air much more poorly at high speeds. A shape which has a boat tail at the back will have a lower drag coefficient than a shape which terminates as a cylinder. A shape which has a rocket exhaust plume at the back will have a different drag coefficient than a shape which has no plume. In actual practice, every rocket design is placed inside a supersonic wind tunnel and tested from Mach 0 to Mach 5 to determine drag coefficient. There is no reliable way to calculate drag coefficient other than by using a wind tunnel. In theory, you can compare the shape of your rocket to the shapes of objects with known drag coefficients and work accordingly. If you really want to skimp on effort, you can just estimate drag coefficient at 0.21. Go with 0.23 if your rocket has fins.

It's hard to pinpoint why it's wrong, but once you pinpoint it, it makes perfect sense. The answer? It's not wrong; it's imprecise. Given the available sample and the question you're asking, then yes -- the expected value of the total human population across all history is 1.2 trillion. But the precision of that value is the problem. The prediction depends on the conjecture that choosing a human being alive today is a random selection, but we do not know how random that selection is. This introduces a variance on the order of the uncertainty in the randomness of the selection. There are 7 billion people alive today out of 107 billion who have ever lived. What are the odds that picking someone at random out of today's population is, in fact, a random selection relative to the entire historical human population? Well, 6.5%. So your actual answer is "the expected value of the total human population across all history is 1.2 trillion, plus or minus 1.12 trillion. Since the prediction (1.2 trillion) is smaller than the variance (2.24 trillion), the whole exercise is hogwash.

A compact resonance chamber that can produce thrust effects equivalent to an electromagnetic tether would be hella useful, even if it wasn't truly reactionless. LEO-to-GEO-and-back space tugs that never have to be refueled? Sign me up!

The nearest star is a good deal closer than all that.

All of these factors make it easier to pull the plane back. Think what happens if you apply brakes on a plane that's already sitting on top of a moving treadmill. Does the plane go forward or backward as a result? You're conflating static friction and kinetic friction. A wheel which is slipping while rolling has less purchase than a wheel which is not slipping while rolling. Naively, one might imagine that an increase in rolling resistance in the wheel would make it easier for the belt to exert a force on the body of the airplane. However, the sort of rolling resistance which increases with respect to velocity (hysteresis and oscillation/vibration) will produce slippage, which will reduce traction, which will reduce the belt's ability to transfer energy to the wheels via rotation. You're the one advancing a reference frame under constant acceleration. If the wheels are frictionless and both the wheels and the treadmill components have infinite structural integrity, then yes. But if the wheels are frictionless and the wheels and treadmill components have finite structural integrity, then the treadmill components will fail before the wheel components and the plane will take off. Remember that your model requires the belt to be under constant acceleration, so the airplane need only hold its engines at a constant thrust in order to force the belt to accelerate straight up to relativistic speeds. If the wheels have ordinary friction, then yes, the acceleration rate of the treadmill can initially be lower than with frictionless wheels. However, if the wheels have friction, then second-order effects like vibration will rapidly reduce traction to the point that the treadmill will be unable to act on the wheels. But this begs the question. Your construction requires the belt to accelerate continually while the engines need only maintain static thrust. The engines can maintain full thrust indefinitely. Thus, one of two things will happen: A) The belt will accelerate until the speeds involved exceed the structural limitations of its components. Assuming comparable structural integrity between the belt and the wheels, the belt will fail first. The plane will then proceed to a normal rolling takeoff. B) The belt will accelerate until second-order effects induce slippage and reduce traction. The belt will no longer be able to continue accelerating the wheels, and the plane will take off.

So, I was under the impression that most antitank missiles are just penetrators; they punch through the armor and fill the inside of the tank with molten metal. There will be a chunk missing from the side of the tank and the insides will be shredded, but it's more or less intact. Then I watched this: Holy crap. That missile doesn't even impact; it's designed to do that much damage simply by exploding directly over the target. It produced a shockwave so severe that it ripped the turret off the tank and sent it bouncing onto the ground in a ball of flames. It's not entirely out of the question to imagine a miniaturized version, equipped with a depleted uranium or even osmium tip at the front to improve penetration. Even an explosion 1/10th as strong would absolutely obliterate a tank if it exploded partly inside. (For reference, I was previously thinking of a HEAT missile, which would look more like this:) A shaped charge missile could probably be made smaller than a TOW.

All the answers: https://what-if.xkcd.com/89/

At low speeds, yes. But that simply isn't true. Rolling resistance includes hysteresis, wheel bearing resistance, vibration/oscillation losses, and wheel slippage. Of these, only wheel bearing resistance is not a function of RPM. Wheel slippage increases as a second-order effect of velocity due to the increase in hysteresis and vibration/oscillation. The faster the wheels (and the belt) are spinning, the more total rolling resistance you will have. But there is a qualitative difference between the vehicles placed on the belt. If you have a car on a belt, then the car's engine is using the transmission and wheels to exert a force on the belt in order to push the car forward against the rearward travel of the belt. Traction losses and rolling resistance reduce the engine's ability to fight the belt. If you have a plane on a belt, then the belt is using the plane's wheel inertia to exert a force on the plane in order to push the plane backward against the forward travel produced by its engines. Traction losses and rolling resistance reduce the belt's ability to fight the engines. That is the qualitative difference.

That's called making stuff up. There is absolutely nothing in the physics of a tire that allows for a significant reduction of traction at high speeds, until you start to experience equally significant deformations, which leads to failure under load pretty much instantly. If anything, heating up the tires will slightly improve traction. If you're driving a car on a treadmill, rolling resistance losses sap from the engine and require a greater power output. Rolling resistance losses for free-rolling wheels, as with an airplane, are going to contribute to slippage, which robs the belt of its ability to slow the airplane. Obviously.

Dammit.

Come to think of it, once the wheels spin up high enough, they're going to have trouble maintaining traction. I bet traction fails before the wheels do, and at a high spin rate they'll have a different coefficient of friction than if they were braked and skidding.

The tracking sim is really cool at the pole.

Separation and MVac start! Fairing away!

Sure. With a car on a treadmill, it is a battle between the torque in the belt gearing and the torque in the vehicle transmission. The wheels are geared to the vehicle transmission; if the vehicle transmission cannot keep up with the rotation rate of the belt. With a plane on a treadmill, the wheels are decoupled and so it is a battle between wheel inertia and the belt gearing. If the materials and components in the belt are anywhere close to the materials and components in the wheels, then the belt will fail before the wheels do.

Craziest sort of error...but makes total sense.

Not while people were actively getting on board. But there have been numerous instances of rockets blowing up while fully fueled, on the pad. Numerous ground support injuries and fatalities as well. Damascus Titan, Soyuz 7K-OK No.1, Soyuz T-10a, Atlas Agena, the Brazilian VLS-3, Nedelin...there are quite a few. More people have been killed by fully-fueled rockets on the pad than have been killed in actual launch failures. Granted, these were all fairly nascent launch programs. But the precedent is there.

So what is the programming? 1 Get(VehiclePosition); 2 IF (VehiclePosition > 0) BeltTorque++; 3 ELSE IF (VehiclePosition < 0) BeltTorque--; 4 GOTO 1

Well, no, I am arguing from the "bias" that if I'm the ground support personnel helping Behnken, Boe, Hurley, and Williams into the capsule, I damn well would prefer that the rocket I'm standing inches away from NOT be fueled during the process. And if I'm Behnken, Boe, Hurley, or Williams, I would damn well prefer to take my chances with LES (which is no more violent than a typical Soyuz EDL, something I would have experienced multiple times already) than climb into a fully-fueled rocket with no safety net. Better the devil you know. That's your opinion. If load-and-go was the only option, and SpaceX was fighting to avoid a complete redesign of their vehicle, then perhaps my analysis would be subject to suspicions of bias. But that's not the case. SpaceX can easily throw Dragon 2 to the ISS without using densified propellants. This also would fail to certify safety, due to non-destructive testing fatigue. I used to be part of the US oil and gas pipeline regulatory agency. One concept in high-pressure pipeline safety is the hydrostatic pressure test: you purge the lines of their product, then fill it up with water, cap it at both ends, and pump it up to several times greater than its operating pressure. The theory is that if the pipeline has any defects, flaws, cracks, or leaks, they will rupture during the hydrotest rather than bursting during oil pumping operations. However, hydrostatic pressure testing has a flaw. Metal pipelines are ductile. If a pipe has small cracks, the hydrostatic pressure test could conceivably cause those cracks to grow via ductile expansion without actually rupturing them. The result is an intact pipeline that will fail the next time at a lower pressure than it otherwise would have. So they declare it safe, pump it full of oil again, and the next time someone switches pressure heads upstream and a transient pressure wave races down the pipeline at the speed of sound, those cracks burst and dump 20,000 gallons of dilbit into someone's backyard. The same concern exists with any wet dress rehearsal or static fire. All things considered, it is probably safer to perform a WDR or a static fire than it would be to forgo it. But an operator cannot ignore the possibility that a test induces stress fatigues that will cause the next engine or structure to fail under subsequent loads. For example, if you did a WDR, drained the tanks, loaded the crew, and then loaded again, the first WDR could cause thermal contraction in a COPV flaw which would open up cracks, allowing LOX encroachment during the second fuel load.

LES has only been activated in a live launch once. LES systems have been tested dozens if not hundreds of times, and every single man-rated vehicle launch (other than the Shuttle) has been a test of the back end of an LES event. You seem to be setting R2 equal to 0, which isn't at all warranted. IIRC, rockets have blown up on the pad while fully fueled before, even if it was only in the early stages of orbital rocketry. The equation is quite simple: if the risk of pad RUD while fully fueled is greater than the risk of injury during LES activation times the risk of pad RUD while fueling, you are better off with load-and-go. Load-and-go provides 100% protection to ingress assist personnel.