sevenperforce

Thanks, that's exactly what I needed to know!

Playing Demo and I have a few telemetry/control questions: Is there any way to determine the total mass of your craft at any given time during flight? Is there any way to determine how far you are from the ground at any given time? I'm tired of crashing at 120 meters above sea level. Is there any way to reset stages mid-flight, so that I can turn off my engines, throttle all the way up, and then press space to turn them all on at full throttle?

This is bullocks. I suppose everything is a "phenomenon" in some sense. But black holes are merely very dense heavy objects which happen to have fallen afoul of the curvature of spacetime. Supernovae may not get hot enough to synthesize the really heavy stuff. There's a fair chance that gold and heavier elements come not from supernovae, but from neutron star collisions.

Of course, if you don't know your time-to-impact, that gets tricky. A really quick way of doing it (for near-vertical approaches) would be to square your velocity, divide by 20, and add that number to your altitude. Then simply divide by your vehicle's TWR to get the altitude where you need to start your burn. The first step gives you the equivalent altitude with a potential energy corresponding to your kinetic energy, and then 1/TWR is the fraction of that altitude you would need to burn for in order for the work done by your engines to match the work done by gravity. For example, let's say you're at 640 meters, dropping at 85 m/s, and your T/W at full throttle is 3. 85^2/20 is 361. Add to your altitude and you get 1,001 meters. Thus, you can consider your total energy to be the same as if you had dropped from 1,001 meters at a starting velocity of zero. Since your engines produce three times the force of gravity, you only need to thrust for one third of that distance, or 334 meters. Should be simple enough to do mid-flight.

Those hypergolic engines are extremely reliable and deeply throttleable; they can lose any two and so have enough thrust to make a safe landing. So in a sense the answer is more rockets. But the backup parachutes (four in all) have enough area to slow the capsule down considerably. Splashdown with parachutes will be gentle enough to preserve the capsule for reuse as long as the saltwater corrosion isn't too bad. Parachuting to a landing on terra firma will crush the landing legs to splinters, but they will absorb enough of the shock to save the occupants from discomfort.

Flush-mounted solar panels in the upper aeroshell would not be a problem in re-entry. Not to beat a dead horse, but why do you keep saying that modifications "would make this a Dragon V3" when I quoted Elon specifically talking about modifying the V2 platform to use for the Moon or Europa or any other world? I mean, I love getting pushback because it helps me identify things I might not have considered; I'm just not sure why you're fixed on (relatively minor) retrofitting as the problem.

I'm playing in sandbox. Apparently I just don't know how to use them.

Unfortunately the demo doesn't have nodes. I was thinking, though...it's all about energy, right? If you have a near-vertical trajectory, then you can take the mass of your craft and multiply it by g*h + 0.5*v^2 to get the sum of your potential and kinetic energy. Then, divide by the thrust of your engines at the desired throttle level. Your answer will be in units of distance and will be the altitude at which you need to begin your constant-thrust burn, since force times distance equals energy. Of course if your trajectory is nowhere close to vertical then you'll need to use your trajectory arc length rather than altitude, which is nonlinear. But it should be close, and you can always do a boost back to kill horizontal velocity if you need to...the difference in efficiency won't be too bad. This avoids the rocket equation and mass change issues altogether since the Oberth effect is worked in implicitly.

They'd have to do an engineering feasibility analysis to determine whether it would be more mass- and cost-effective to install additional solar cells in the aeroshell, to add solid-state batteries, or to install a sealed fuel cell compartment with an external exchanger duct.

The tank for the MAKS concept was small because it was air-launched. And you don't need high thrust nearly as much on an air-launched spaceplane, so why bother with the complexity of a triprop? Anyway I also dislike LH2 because it eats through everything, making reusability hopeless. Ah, I didn't realize that both chambers had triple injectors. That's good. Even so, bi-oxidized ethylene holds a fair bit of promise. Not sure whether the single or the double turbopumps would be the better choice. H2O2 has a fantastically high heat capacity, so it might be worth it to run everything off a single staged-combustion peroxide turbopump. Then again, I have read that ethylene won't coke below 2000 psi, which would allow triple full-flow staged combustion. Now THAT would be something. Best of all, I'm pretty sure catalyzed hydrogen peroxide is hypergolic with most hydrocarbons, so you don't need any ignition system. Come to think of it: if low-pressure ethylene won't coke, you can do better. Two preburners: a fuel-rich one, powered by catalyzed fuel-rich ethyloxide, and a pure-oxidizer one, blending peroxide and LOX in the desired ratio and catalyzing peroxide decomposition to power it. FFSC with only two injectors, since the oxidizer ratio is varied in the preburner. That would be one sick rocket engine.

I don't like LH2. It's the death knell for virtually any chance at full reusability. I mean, if you're building an expendable upper stage with a really wide body, then by all means use LH2. But not for an RLV. Here's a thought -- instead of using two fuels, why not try using two oxidizers with a single fuel? The fuel would need to be dense if possible -- ethylene would be a good choice. It cokes, so it can't be used for a fuel-rich preburn, but it is denser and higher-impulse than liquid methane. The Soyuz rockets use HTP to run their turbopumps, so there's already a lot of research in that area. HTP can be run in staged combustion mode without needing a reducer, and its density is comparable to that of LOX. At launch, use HTP/C2H4 with just a trickle of LOX, then gradually reduce the HTP flow and increase the LOX flow to decrease thrust and increase specific impulse. You can either use the HTP to run the turbopumps the entire time, or you can stage-combust the LOX as well. So you end up with just a single combustion chamber (unlike the dual combustion chambers of the RD-71) and a highly variable thrust vs specific impulse profile.

Yeah, this is what it looked like when the SRBs hit the water. Impact speed: 52 mph. The chute system weighs three tons, actually. If you want to cut the speed to 26 mph (which, I'll point out, still won't allow a touchdown on land), you'd have to increase the chute area by a factor of 4, increasing their mass to well over 12 tonnes.

Trying to get the hang of powered vertical landings. Playing Demo. I built a small test craft with eight pre-extended, suspension-locked landing legs and a couple of liquid-fueled engines, put RCS thrusters with plenty of propellant on the capsule up top, and stuck Jeb in it. I launch, navigate to a smooth landing area, and throttle the engines down until I start to drop, then tell Jeb to use RCS to maintain retrograde alignment. Then I just play with the throttle to try and stick the landing. I've gotten a couple of landings, but it's hit or miss (well, I never miss; I just hit too hard). I know how to sit down and calculate out exactly what I need for a true suicide burn, but for landings on the fly, what's the best way to eyeball my altitude and speed to pull it off smoothly?

While your considerations regarding horizontal speed are entirely valid, I'd also point out that reducing vertical speed becomes prohibitively difficult below a certain point, because terminal velocity is proportional to the square root of your parachute cross-sectional area.

Yeah, either a fuel cell or aeroshell-mounted solar panels would be necessary, as I mention above. No service module or external cargo bay; thus, the legs it already has are fine. The article I linked above demonstrates that Elon expects modifications to the V2 platform as a matter of course.

Gas embolisms are not the only type of embolisms. And high pressure will still rupture your ear drums and make it impossible for you to breathe. You mentioned 0.1 bar, but you said "we would be ok no matter how thin or thick would be our atmosphere" without limiting it to the range; it seemed like 0.1 bar and 1000 bar were just examples. If you really meant "we could survive a range of pressures between 0.1 bar and 1000 bar" then that is a different claim.

At low pressure you have extreme embolisms and the moisture in your mucus membranes starts to boil away. At high pressure your eardrums rupture and it becomes impossible to expel air from your lungs.

Been playing the demo, and it is pretty limited, but I wanted to try and do a lunar landing with a rover. Only, there are no wheels in demo. So I had to get creative. First rover test: So far so good... Uh oh. Okay this was a bad idea. Back to the drawing board. Let's make it a bit less explosive. These reaction wheels don't have quite the jazz of jet engines, but they get the job done! I use roll control for forward and reverse. Turning is tricky; you have to use W and A, but since the rover is constantly spinning, pressing W turns different ways based on where you are in the roll. Thankfully, it's pretty slow, so you have time. Seems quite capable. You can even recharge it: Oh, wait, these panels aren't retractable. So once you start to move, they snap off. Guess I'll need to mount them in a better place next time. Figured I'd head for the water to see whether it would float. You can get a bit more speed if you're going downhill. Will it sink or swim? It swims! Not very fast, but it does. It can even recharge in the water without the solar panel snapping off! Though I need to fix the configuration anyway. Off we go! Now to mount this bad boy on a lunar lander. In Demo. This will be fun. EDIT: Fixed those solar panels. They work much better now. Should even give me a little extra boost if I go back in the water.

Ah, "perceived gravity". I see what you're asking. Gravity pulls us down. Atmospheric pressure, on the other hand, pushes against us from all directions, including up. If you're crushed by atmospheric pressure, you implode; if you're crushed by gravity, you splat.

Putting chutes on the Falcon first stage would not reduce its speed enough to let it touch down gently.

Oh, my bad -- I thought you were suggesting that New Horizons would actually intercept and sample the ejecta.

That's okay, it's not a stupid question. I happen to have a soda can on my desk right now. Gravity is definitely pulling down on it, right? But the aluminum walls are strong enough to hold up under their own weight. If I want to crush the soda can, I will need to make the force pulling down on it greater than the strength of the aluminum walls. One way to do that is to increase the gravity. If Earth was fifty times as heavy but still the same size, the gravitational force on the can would be fifty times greater and the walls would collapse. There's an easier way to increase the force, though. If I place an object on top of the soda can that is much more massive than the can, then all that weight will also push down on the can (due to gravity) and it will collapse. If I'm doing the same thing on a world with lower gravity, I will need to add more mass to produce the same amount of force. But that's easy enough to do; just keep stacking stuff on top. Venus has lower gravity than Earth, but its atmosphere has a lot more "stuff" in it, so it weighs much more.

Nah, the atmospheric pressure on Venus crushes you, not the gravity. The gravity on Venus is lower than Earth's. When you crush a soda can with your foot, you're not changing gravity for it; you're simply crushing it. Same with Venus; the very thick atmosphere is heavy enough to crush you. Not really. You can have a breathable oxygen atmosphere as long as you have the right partial pressure of oxygen; the overall atmospheric pressure doesn't need to be particularly close to Earth's. Of course, having a higher proportion of oxygen in the atmosphere makes fires a bit more...fiery. And you don't need 1 g to have an atmospheric pressure equal to Earth's, either; you just need a thicker atmosphere.

Yeah, inclination changes take too much dV to be useful, even with ion engines. That's what I was thinking -- it isn't cost-effective because you already have the station-keeping capabilities. Are there any sorts of scenarios where it would be cost-effective? That would be a much bigger project than a comsat tug.

The parent probe would have been well past Pluto long before the ejecta cloud would rise.