K^2

The tables kind of disagree on what the temperature is at a given altitude. The ones in K show about 300K at the same altitude as the other chart shows 0°C. I was going by the chart in Kelvin. Wind speeds seem to be pretty level across the relevant range of altitudes, so I don't think it will matter. Of course, if the ship is floating, absolute wind-speed is irrelevant. It's the turbulence and shears that are going to damage your structure. And I would expect upper cloud layer to have lower turbulence than lower cloud layer, but that's just going off from flight experience. Flight tends to be most shaky right before you hit cloud ceiling and starts to ease off as you raise higher through the clouds. Usually. But this might be specific to Earth or even specific climate zones. I don't know enough about weather in general to insist that this is always true. It'd be nice to see Doppler results from different altitude on Venus to make a conclusion.

I had that same thought for a few seconds. The problem is that the non-rigid tank would still have to be at least thick enough to contain H2 at some pressure, and it can't have a better tensile strength than a good composite tank. If you want to look at it another way, a non-rigid gas-filled balloon is at least as heavy as a rigid balloon of the same size. And it has the same exact mass before it is inflated. So in either scenario, you need to bring enough mass with you to contain gaseous H2. And that's way more extra mass than it'd take in cryo. Regarding the boil-off, for a long duration mission, it will probably make more sense to bring a refrigeration and condensation unit with you, making your cryo tanks closed circuit. We already do that at many facilities that make use of large quantities of liquid helium. (E.g. J-Lab.) Electricity is much cheaper in space than propellant is, and with large enough tanks, the extra weight of the condensation unit is going to be very small. (Boil-off rate scales as area, while total mass scales as volume. So with 2x larger tanks, condensation unit is going to be 2x smaller fraction of your weight.)

Again. It doesn't matter. You can take H2 at high pressure and small volume, and then you are hauling lots of tank because the walls are so thick. Or can take H2 at low pressure and huge volume, and then you carry exactly the same weight of tank because the walls have sugh a huge surface area. Either way, you need 1.8kg of tank per 1kg of H2 if you make the tank out of steel. You can do better with composites. But you will still have hard time beating 1:1 ratio. Compared to cryo H2, where in large volume, your tank weighs less than a tenth of your fuel. Now, do you want to bring 2kg of stuff for every 1kg of H2 you make use of, or 1.1kg of stuff for the same 1kg of H2? The choice is pretty simple. You bring cryo H2 at a fraction of a weight cost.

Venusian year is almost two Venusian solar days. I would also strongly suggest raising the altitude above 50km. 1 bar and 20°C might be perfect, but 0.5 bar and 0°C is better in many regards. You can maintain the same partial pressure of oxygen and use waste heat to keep comfortable temperature inside. (At 50km, you'll need heat pumps to keep interiors comfortable.) You'll also get less corosive atmosphere, and high wind speeds aren't as bad of a problem in lower density. Finally, the most important reason is that you get way more sunlight.

Exactly. The weight is the problem. Weight of the tanks you used to keep H2 compressed. And pressure doesn't even matter. Say you have a spherical tank, which is best geometry for weight efficiency. If you double the size of the tank, you can store the same amount of H2 gas at 1/8th of the pressure. But you've also increased cross-section area by factor of 4, while increasing cross-section circumference by factor of 2. So net decrease in thickness is factor of 4. But you've also increased surface area by the same factor, four. So the total mass of the tank remains exactly the same! Regardless of pressure, given amount of H2 gas at fixed temperature requires a certain mass of the tank. (This works for any gas, by the way.) And it's a lot. If your tank is made of steel, for every 1kg of H2 at 300K, you'll need 1.8kg of steel in the tank! So the only practical way to store H2 in a rocket of any kind, where weight matters, is to store it as LH2.

And would you care to explain why this is in disagreement with every crash test ever conducted? As for race cars, have you seen a typical race car collision? And a typical highway collision? These aren't the same thing. A highway collision involves driver continuing to move forward as the car very rapidly decelerates. Putting an airbag in the way helps. A typical race car collision involves car pirouetting through the air. So the best safety option is to have a very good harness that keeps you strapped to the sit and inside the roll cage. And yes, if passenger cars came with race car harnesses instead of the typical seat belt, you probably wouldn't need an air bag. But it certainly doesn't present an additional danger. Now, for the reason why passenger cars typically have a basic belt instead of a full racing harness, that probably has a lot to do with convenience. Though, safety at low speed collisions and ease of exiting are also factors. But with such a belt instead of a harness, you are much better off with an airbag.

The only thing I see with 1,600s for arcjet is H2 powered. Hydrazine will give you much better TWR, but at a cost of lower ISP. The highest specific impulse I could find for Hydrazine is 600s. That's still way, way better than just using it as a monoprop, which would give you up to about 240s.

The eye-brain-hand reaction is about .15s, and can be close to .1s for some people. The eye-brain-foot pathway is much slower. You will never be able to respond in less than .2-.3 seconds. As for 1s vs .5s for typical reaction time, 1s only makes my argument stronger, but feel free to run the numbers yourself. That's still two cars that can squeeze in by my count. Again, how you drive on an autobahn in low density traffic is a separate issue. Most accidents in these conditions are single vehicle anyways, and no braking system helps. We are discussing mostly dense traffic at high speeds. If this never happens where you live, I'm very happy for you. This isn't about my driving. I'm telling you what general traffic is like outside of many cities in US and Europe. I've personally driven in and near New York, Boston, Miami, and a number of smaller cities which are not quite as insane as these 3. LA is just as bad, but I've only been a passenger there. People don't slow down in these cities just because it's almost bumper-to-bumper traffic. And if you slam your brakes suddenly, you are almost guaranteed to be rear-ended. But the difference is being rear-ended by someone as you are slowing down vs being rear-ended by someone after you've come to a complete stop. The retro-rockets system brings you to a stop in about the same time as it takes for a car following close behind to catch up. Meaning it's a full-speed-on-stationary collision, which is absolutely horrible for people in both cars. By the time the crash becomes 100% unavoidable, it's too late to stop even with retro-rockets. That's what we're talking about. For an 18-wheeler it's different, because an unavoidable crash is determined much, much longer in advance. Personally, I would never drive a car with ABS or ESP, because these systems put me in more danger. But I go out and practice emergency braking on ice every winter, and I personally maintain my brakes and suspension, so that I know they work to their full potential. For a typical driver, ABS does better emergency braking. And as I've pointed out, even if the person behind you skids, taking his friction coefficient from .9 to .3, collision is nowhere near as bad as full-speed-on-stationary collision. So ABS never creates a more dangerous situation than it is designed to avoid. Same deal with ESC and roll protection. But that's not the case with retro-rockets. They don't just reduce your braking distance while allowing for a safe impact from behind. They stop you dead. If they don't, then the distance from which you detect "imminent collision" has to be such that no system could ever predict it in real traffic. There is no way around these two limitations. You either end up with the system that ends up triggering in normal traffic without any emergency, or you end up with a system that stops so rapidly that it gives no chance for a safe collision from behind. There are no alternatives.

Hydrazine is, basically, synthesized from amonia. So you're on the right track there. Typically, you'd have a few steps involving chlorine and some intermediate products, but the net reaction is amonia + oxygen -> hydrazine + water. Now, I don't know how practical it'd be to try and extract amonia from urine for this, but there are many asteroids and comets out there with amonia ice. You could probably mine these for amonia you need to turn into monoprop in deep space.

If you actually pay attention, you'll notice that I do account for collision that follows. I do use .5s reaction time, because that's a typical reaction time in an accident. Some people might take up to a second to react. Some people might freeze all together. An alert person responds in approximately .2 - .3 seconds. But .5s is a good average. Why don't you go back and read why it's not as bad to collide in this scenario. What sort of cars are you talking about? 15m is 3 car lengths. And if you think that's too small, you've obviously never driven on a freeway outside a major city. 2 car lengths is not atypical, and you see less than 1 frequently when somebody changes lanes. In some places, such as vicinity of Miami, you'd be driving on a freeway with 4-5 lanes of traffic, ranging in speed from about 60mph to over 90mph as you go from right to left. All driving at 1-2 car lengths and constantly shifting from lane to lane. Now, you might drive differently. But everyone else isn't going to suddenly change their driving style. The person behind you isn't going to know that your emergency braking system is going to stop you almost instantly, and he's still going to get behind you with less than 1 car length to spare in heavy traffic. The difference is that if you slam your brakes, you won't have time to slow much before he slams into you. And that means the impact won't be severe. If you use rockets, it gives you just enough time to stop before he hits you at full speed. Which I have already shown with numbers in my previous analysis which you haven't even bothered to look over, apparently. Do I really need to explain difference between operating an 18-wheeler and a passenger car? For starters, an 18-wheeler can't swerve in principle, because its trailer will keep going in the same direction, only helping the whole thing go into a flip.

The only improvements on the market currently that allow you to brake faster than the car behind you are spikes and chains. Everything else just removes the driver error. As well as both of these are useful only in limited situations, ones where people tend to drive very differently to begin with, and come with limitations and/or requirements to carry a clear sign identifying such a feature on your car in many jurisdictions. And this is for the precise reason that such cars can stop better than somebody behind them. Furthermore, even if we consider systems like ABS actual assists, despite the fact that a skilled driver can stop just as well or better, they are, generally, incremental improvements. You might say that going from .3g to .9g is a lot, but it's nothing like going from .9g to 5g. Not only is the increment higher, the reaction time is also reduced dramatically. Which means that it is plain impossible for a driver behind you to stop unless they are driving half a football field behind you. Finally, there is the question of velocity difference. Say I'm driving without ABS with someone who is driving with one. I drive only 2s behind at highway speeds. Say, 70mph. Person in front of me slams the brakes. He decelerates at .9g. I slam my brakes .5s later and decelerate at .3g. Car in front of me has time to stop, and I slam into the back going 43mph. Given that it's an impact into a car of similar mass, my car is trashed, but I'm completely unharmed, save for a bruise from the airbag. Or, lets say, I'm completely careless, and I ride the guy's bumper just .5s behind. In that scenario, assuming I'm a careless driver, collision happens just 2s after he began braking. But he doesn't have time to slow down all the way, and I still slam into him at only 31mph of relative velocity. No matter how things go from there on, I'm fine. And so are people further behind. Now, lets suppose that instead of braking at .9g, the car in front suddenly pulls a 5G maneuver. In the .5s scenario above, even braking at .9g, I slam into car in front still going 63mph. Ever seen a collision at 63mph? Or the aftermath. I'd be lucky to survive that, and even that, only if car in front of me is of similar mass. Passengers in that car would also be lucky to live through this collision. (If I'm driving a semi, they're all dead.) At 2s, lead, provided nothing goes wrong, I have time to stop. But given a 2s lead, the front car would also be able to stop safely, and everyone would be fine. To put it in simplest terms, it is generally going to be safer for the front car to crash then slow that abruptly if other cars on the road don't have similar capability. Finally, it is almost impossible to say what actually is an inevitable collision. If a car 1s in front of me slams into a concrete block and stops instantly, I can still swerve around it. In many cases, it's going to be way safer than stopping. At .5s I can still swerve around if I already began the maneuver. To come to a stop from 70mph in less than .5s, the car would have to undergo acceleration of over 6g. That's assuming that there is zero lag in firing the engines, which is not going to be true. So realistically, we'd be looking at what, 7-8g? That gives a stopping distance of about 12m. That's about the length of the impact absorbing barriers on the freeway. Ever seen a car ram into these? It's survivable. That's kind of the point. But the car's trashed. And that's with impact distributed over the entire front of the car. Now you want that load to be focused on just the rockets. So just what sort of a frame are you planning to attach these to? So we have to consider lower deceleration rates. Something like 4-5g should be doable. But this requires predicting inevitable collision more than a second ahead. How does the system know if I'm planning to swerve? If it's safe to swerve? If swerving or braking is more likely to get me killed? Am I even alert enough to react this fast? Guess wrong, and the system kills the driver who would have otherwise escaped the collision. Now, being useless is one thing. When the system designed to protect actually causes deaths, and fairly frequently, that's a very bad system. The only way to use retro-rockets as an actual safety feature is that all cars are operated by AI linked into a swarm that can coordinate traffic. Then, in the worst case scenario, such a system would be in position to decide if it's prudent to fire retro-rockets. But I suspect, with such a system in place, need for this would be very rare.

We really need a Latex parser on this board. Something like what they have on Physics Forums would do nicely.

There are a range of problems. First, if you go with a retro rocket, you don't go with SRB. You go with a hybrid. Much safer, and you can control burn duration. That said, rockets are just bad for this. A typical car is about 1.5T. At highway speeds you'd want to kill at least 55mph of forward speed. (That would bring you to safe collision speeds even if brakes completely fail. Otherwise, with brakes you might be able to stop fully.) Your hybrid's ISP is going to be around 300s. That means your rocket is going to have more than 12kg of fuel. This is a) Expensive, Presents huge risk in case you can't avoid collision. This part can be solved. You don't actually need high ISP here, so you can bring a bunch of inert reaction mass with you. Say, water. Water can be sprayed from the front in a way that makes it relatively safe at moderate distance. You'd still need a rocket to push the water out, but it'd be a much smaller rocket. Say, you have 50L of water in your retro jets. Now you can get away with less than 1kg of propellant to push it out. This can be contained even if it decides to explode. The whole system would probably add about 100kg to your car's total mass, but if it would make it significantly safer, that would be acceptable. On to the next problem. Your tires can decelerate you at about 1g. If you are flying at 70mph, that gives you braking time of 3.2 seconds. Basically, by the time you realize that you need to brake and you can't make it, it's really too late. "Lets just fire these automatically!" Alright, say I'm driving on a freeway. There is a slow moving vehicle in front of me, so I'm going to shift lanes. Just as I start, retro rockets fire, spin me out of control, and put me in the middle of the lane at zero speed. That's a great situation to be in on a freeway. Now, you could probably come up with some way to make this work, like use the radar, make sure closing speed is too fast to stop, and the driver is applying full brakes. But even that might create new hazards. So it's a question of whether this can even help you in more cases than it would hurt you. Which brings me to my next point. You are in traffic. You need to stop. You do, thanks to the retrorockets. But what about the guy behind you? Unless he has a similar system installed, he's still plowing ahead at 70mph, and probably just starting to apply brakes. And suddenly, he has a stationary car in front of him. Now he slams into you at probably higher speed than you would have slammed into somebody, drags you forward, and quite possibly pushes you into something as well as getting rear-ended himself. You've just turned what could have been a two car collision into a pileup. Is that always going to happen? No. But the odds of such outcome increase with speed, and therefore, it's more likely than not in all of the situations where it would have a chance of saving your life, rather than just your car. And don't forget. The main purpose of all safety features is to save lives of people, not reduce damage to cars. If we worried about cars, we could just make their frames tougher. They'd survive minor collisions much better. They'd kill driver at higher speed collision, but even then a lot of the spare parts would be recoverable. That seems much easier than retro rockets. But again, that's just not the point.

I used to drive a '98 Tiburon that did that. If it sensed engine brake, it would cut fuel and open up a bypass valve. The downside is that you couldn't actually engine brake with that car, because the engine did not have to work against the manifold pressure, but on the plus side, it didn't use up any fuel when coasting downhill on high gear. I wouldn't be surprised if a lot of cars cut fuel supply when engine braking. This is a challenge with a carburetor engine, but when you have a computer-driven injector, it's a few extra line of code that significantly improve mileage.

Metals are going to be very hard to come by, and they are required by many processes. Chlorophyll, for example, relies on a trapped magnesium ion. And, of course, there is iron's roll in Hemoglobin. Of course, you don't need much, so you might be able to bring all of this stuff down form orbit. Also, keep in mind that most of sulfur in atmosphere is going to be in form of sulfur dioxide. There is also some water vapor, so you will have some sulfuric acid, but you might as well just grab water vapor from atmosphere. That seems more efficient. But yeah, other than getting some necessary metals and halogens, which you can bring with you or get a shipment of every once in a while, you can have a pretty good thing going. You'll have all the carbon you could use. You can trap enough water to get by. So you can maintain an Earth-like environment for human habitation and some farming. And you can use either plant byproducts or synthesize polymers from scratch to use for construction.

R1 and R2 include radius of the parent body. So if your apoapsis is 200km, and periapsis is 100km, but you are around Kerbin, your R1 and R2 are 600km + 200km and 600km + 100km respectively. Giving you semi-major axis of 750km. Things get a bit more complicated with a real body, like Earth, since it's not exactly a sphere. But if you don't know the actual orbital parameters, and you just know the altitude of apses, you can use Earth's mean radius for a decent enough estimate.

53T is Falcon Heavy.

No, even having multiple time dimensions isn't the same thing as having multiple timelines. Usually, alternative timelines show up in either the context of "parallel dimensions" or Many Worlds. The former is a bit of a misnomer, and is actually closer to the more modern concept of having multiple branes in the bulk. There, you do have extra dimension(s) which you can associate with alternate time lines, but it's a little bit more complicated overall. And these extra dimensions certainly aren't just extra time dimensions. Many Worlds, in contrast, doesn't rely on extra dimensions at all. All of the time lines share the time dimension via superposition.

There is no simple formula. You have to integrate the trajectory, and you need to know a lot of additional information about the way the jump is performed.

If you have a smartphone, you might want to get yourself Google Sky Map for the future. It uses GPS, compass, and accelerometers to figure out where you are pointing your phone and where from, and draws the sky map that corresponds to that direction. So you literally point your phone at the star and read off the name on the screen. Now, it's far from perfect in determining the orientation, but it can put you close enough to figure out the rest. And it is certainly way more precise than trying to guestimate azimuth and altitude by eye.

It's not that. For steady ascent, you need to apply a force to counter weight (or its component). Force applied over distance is work, and force applied over time is impulse. Now, work is pretty much fixed. There is the weight of the vehicle, and there is the height you need to ascent. Can't do anything about that. But impulse depends on duration. Now, the question is how "expensive" the impulse is. Car pushes off form Earth. That's almost infinitely heavy. Impulse from "infinitely" heavy object is free, because no matter how much force you apply to Earth, you aren't accelerating it, so you aren't doing any work. But if you have a very light reaction mass, like rocket's exhaust, suddenly, impulse is very expensive. So total energy you need depends on time more than altitude you need to gain. As a result, energy requirement for a climbing car is independent of velocity, until we factor in drag. Similarly, until we factor in drag, rocket's climb is optimal when it is as rapid as possible. Once we factor in drag, you end up with optimal climb speed for a rocket being terminal velocity, and for car, as slow as possible. Of course, that's not exactly true. It takes fuel just to idle the engine. So with a real car, there is optimal speed as well. On level ground, between idle losses and drag, it works out to be something like 50mph (depends a LOT on the car, but that's the general ballpark figure.) As you climb, that figure drops fast. So for any reasonably steep hill "as slow as possible" might not be far from truth.

From Tampa? Unless it simply hasn't gone up yet, you'd see Sirius way better than Betelgeuse. Of course, it all depends on how clouds/haze are distributed. You could have so much more haze near horizon that Sirius is obstructed. Betelgeuse is also hard to confuse with something because it has a reddish tint that's very persistent even when the star is difficult to see. What was the time? That can help a lot.

Yes, head-diving into a black hole is a known way to break the light barrier. Unfortunately, that is not a very practical thing to do even if you have one sitting around.

In short, ÃŽâ€KE ≠½m ÃŽâ€v². That's because ÃŽâ€KE = KEf - KEi = ½m (vf² - vi²), and (vf² - vi²) ≠(vf - vi)². In terms of an infinitesimal increment in velocity, dv, one can write dKE = mv dv. So as v increases, dKE per dv increases as well. Edit: Fixed a typo in the derivative.

Forget about increasing acceleration for a moment. Take a rocket just starting to take off. It hasn't burned through enough fuel yet to make a big difference in mass. To go from 0m/s to 1m/s will take 0.5J of energy per kg of mass. To go from 1m/s to 2m/s takes 1.5J of energy per kg. And this keeps increasing. There is a much simpler way to look at it. Power is equal to thrust times velocity. Rocket's thrust remains pretty much constant. It doesn't matter that the rocket gets lighter. But its velocity increases. So the rocket's power increases as its velocity does. Note that we've taken mass out of it completely now. So even with all of that taken into account, as the rocket goes faster, the rate at which its kinetic energy increases grows faster. And again, it has been explained in this thread. And yes, it's Obereth effect, because it's not about altitude. It's about velocity. And the reason it works here is exactly the same as the reason it works in Obereth effect, and that has also been explained in this thread.