sevenperforce

Yep. If you have a bunch of antimatter laying around then your best route to orbit and beyond is probably a LOX-afterburning liquid-hydrogen antimatter-thermal rocket.

applicable for a necro like this

A fair point. I have yet to see a really solid analysis of what kind of performance you would get out of an engine with full antimatter containment. You could synthesize some anti-carbon, mix it, and make anti-steel. You could then take your anti-carbon and react it using energy with anti-hydrogen to make anti-methane, and then if you could make some anti-oxygen you could react the anti-methane with the anti-oxygen in an anti-steel engine to make an anti-rocket stage.

I don't think hitting isolation and contact tracing is ever going to be sufficient to beat it. It is contagious enough, and infection is broadspread enough, that we are going to need to hit herd immunity, period. Ultimately, either everyone gets it or we dev a vaccine, whichever comes first. I know you panned the earlier comparison to the flu R0, but a comparative analysis is really our best bet here. Once we have serological testing, we can start to take seriously the question of how many people need to have immunity in order to bring the average person's chances of catching it down to seasonal flu levels. I saw this in the nextstrain data a few days ago, before it was reported. The WA outbreak came from China and then triggered CA; the NYC outbreak came from France and then triggered NJ and other areas along the eastern seaboard. The "early" one decidedly did not help. By the time travel from China was restricted, new domestic case rate in the US was growing faster than the rate of new cases coming in from China. It was just too late. If they had completely shut down travel from Europe at the same time they half-restricted flights from China, it might have been early enough to prevent the catastrophe in NY.

Yes, definitely. As I have posted upthread, there is a lot of data NASA has collected on L/D ratios as a function of Mach number and fineness ratio for cone-tipped cylinders. Start there.

What I mean is that due to PV=nRT, running the helium through the engine's heat exchanger already increases the pressure and thus the expansion on outlet, and thus increases the available thermodynamic work per unit mass of helium, reducing the needed initial mass of helium. If you step down the pressure in a turbopump, you gain mechanical work which you can use to step up the pressure between the tank and the combustion chamber, but you lose pressurant capacity in the helium. Of course, that's fine -- it is what you were trying to get, anyway -- but my suspicion is that you lose more pressure in the tank than you gain in the turbopump, requiring you to carry more helium. The question is whether the mass of additional helium you need is greater than the mass you save by having less massive tanks. It may not seem like a Carnot cycle because it's not immediately reversible, but it is reversible in the context of the work it does. If the helium was expanding into a piston inside the propellant tank to fill the volume, it would do the same amount of work but would be reversible. To a first approximation, I would suspect that the square-cube law will dictate whether you lose or gain with this approach. OTRAG was kerosene plus hypergolic oxidizer. Armadillo's QUAD is room-temp ethanol plus LOX. But yes, most pressure-fed solutions are hypergolic.

Antimatter is just matter that is impossible to contain and explodes catastrophically via e = mc2 whenever you sneeze in its general direction. It is not magic.

The lift is generated regardless; whether the lift results in corrective torque is dependent on the center of pressure. That graph uses the center of mass for the first stage (S-I) only, because they were analyzing how the center of mass changes in the first stage. The Saturn V first stage would not have been stable on its own if the rest of the rocket was simply an empty fairing. Fortunately it was not, and so the center of mass was always well forward of the center of pressure when the Saturn V was in flight. You may be able to find it on Google Scholar or elsewhere. With the global pandemic, libraries are all fubar, but if you find a couple of papers you really would like to see then PM me and I will see if I can get them through my university on digital interlibrary loan.

That Measles R0 is a huge range, hence "garbage" IMO. Varicella is 10-12, which is a low range; measles is 12-18, which is a higher range but not as huge as 10-18. Here's an article explaining why the measles number is so broad. But the lower bound of 12 is still bad. R0 for COVID-19 is estimated between 1.4 and 3.9: https://pubmed.ncbi.nlm.nih.gov/31995857 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7001239 https://www.nature.com/articles/s41591-020-0822-7 I would wager money that the biggest reason why R0 is higher for COVID-19 is the lengthy, contagious, asymptomatic incubation period. It's like HIV: you don't know you have it until after you've given it to three other people.

The biggest consideration (and possible stymie) to your idea is that heating the helium in an exchanger at the engine and then dumping it directly into the tank will immediately do Carnot-cycle work against the propellant, without the need for any turbopump at all.

I agree 100% that the true number of cases is substantially higher than the number of cases ascertained via testing. That's not at issue. The question is probability space. Would it have been reasonably possible (absent containment/mitigation measures) for the US to have a CFR similar to Italy and an ascertainment rate high enough to get 2M deaths with under 50% population infection? I don't think we can say with any degree of certainty that the answer is no. R0 is a huge variable here. Measles and varicella have an R0 of 10-18 in a completely susceptible population. The only reason they don't sweep across the country in massive epidemics is that we have 90% or more herd immunity via vaccination. The value you're really looking for is Re, or effective reproduction number. The operative equation is Re = R0X, where X is the percentage of susceptibility in the population. The average flu vaccine is 67% effective and on average 63% of the population is vaccinated, which gives you an X = 0.58. Seasonal flu has an R0 of up to 2.1, so that's an effective reproduction number of 1.22. Compare to COVID-19's R0 of up to 3.9 and the gravity of this comes into light. Let's think about it this way. At what point will the odds of catching COVID-19 be equivalent to the odds of an unvaccinated person catching the flu? The math is simple. If we use the upper bounds for the flu and COVID-19, then you need 68.7% population infection; if we use the lower bounds (Re_flu = 0.522, R0_C19 = 1.4), then you need 62.7% population infection. In other words, your odds of catching COVID-19 become no greater than your unvaccinated odds of catching the seasonal flu when 63-68% of the U.S. has been infected. That's 215 million infections. If InfectionFR > 0.93%, that's two million deaths.

For reference, the "standard" pressure-fed cycle assumes that a heat exchanger will be used at the engine to increase efficiency: The Apollo Descent Module engine tanks used a helium tank that circulated through the propellant tanks, as you propose, to bring them to ambient temperature before exhausting to increase efficiency. I'm digging around to try and figure out how much helium is used in a pressure-fed stage that doesn't already run it through a chamber heat exchanger.

This is why I was saying we should start with a known and work forward. We certainly don't want to end up bringing more helium to run our system, so if we start with the amount of helium that comes standard in a pressure-fed stage, and it isn't enough to give us a meaningful pressure drop, then we know we are dead in the water (or, in the alternative, we need to look into something like an engine heat exchanger). Understood -- although the autogenous pressurization of the LOX tanks on Falcon 1/9/H is a very minor contribution. I will say that of all the hot fluid streams you can possibly have in an orbital launch vehicle, superheated helium is probably the least problematic thing.

What you need to know is how much helium pressurant a pressure-fed stage needs in comparison to a pump-fed stage of equivalent size. Is it twice as much? Ten times as much? This tells you how much helium you have to work with, and then you can apply a Carnot cycle to determine how much total mechanical energy you can extract from the pressurant mass at your disposal. You can then divide by the total propellant mass to figure out what kind of pressure drop you can produce from that amount of energy, which tells you the pressure reduction (and associated weight reduction) in your tanks. Note that there is a positive but diminishing feedback loop: the mechanical energy available from your Carnot cycle is an inverse function of the outlet pressure, and the more energy you have, the lower that outlet pressure becomes. It's entirely possible that pressure-fed kerolox stages already utilize heat from the kerosene to their advantage, at least on one side. If helium pressurant is exhausted into the RP-1 tank directly at LOX temperatures, it will warm and expand, adding supplementary pressurization, meaning you need less of it to get the job done. As a corollary, if you're exhausting helium into the LOX tank after running it through an RP-1 heat exchanger, you're going to warm the LOX, boiling small amounts of it and causing it to self-pressurize. The helium pressurant system on the Falcon 1, with the Kestrel engine, already used a titanium heat exchanger around the engine to transfer heat to the helium pressurant -- far more than you could get from the RP-1 tank.

I believe it was the first East Coast droneship landing and they were still working the kinks out of the software.

No data?? The global CFR is 5.79%. Italy's curve is finally starting to flatten thanks to their extreme (if too late) social distancing measures. The national CFR in Italy, currently, is 12.6%. Assuming conservatively that the 4% of cases currently listed "Serious or Critical" have the same 41% CFR as closed cases in Italy (this assumes hyper-conservatively that no recovered patients were ever listed as "Mild Condition"), that ticks up to a national Italian CFR of 13.8%. If the US had not taken any mitigation measures, we likely would have been worse-off than Italy. A national CFR of 13.8%, using your numbers ("the actual number of cases was probably 10X tested"), corresponds to a national IFR of 1.4%, which gets you 2M dead with just 44% of the population infected. Remember that you need ~50% population infection to get passive herd immunity and bring transmission rate down to flu-like levels. The only way to argue against these numbers is to say that it is unrealistic to think that the US would have been worse off than Italy without mitigation measures. Even though Italy has already started to flatten the curve, its death rate for closed cases is 41%. The US has a death rate for closed cases of 37.2% and its curve is still concave up. @tater, you can continue to insist that the US could never have been as bad off as Italy even if we had done nothing at all, but that's a hard sell when we have had mitigation measures for weeks and our CCFR is already within 4 percentage points of Italy's.

What's the basis for saying 2M deaths w/o mitigation was a nonsense projection? Serological testing will help but what about respiratory deaths in patients not tested for the virus?

That is some true fanfic.

The center of mass does move forward as propellant burns, which is one reason why you need active stabilization during boost. I am not certain of this, but I suspect divert thrusters are used either during coast, when the center of mass (CoM) is fixed and unchanging, or during a boost period where airspeed is so extreme that any torque is rapidly damped by the airflow. Even if the divert thruster doesn't point exactly through the CoM, it will apply less torque if it is close to the CoM, and at supersonic speeds any change to the AoA will quickly be scrubbed out. Here's an article that looks at plume interaction with airflow for kinetic kill vehicles -- according to their paper, a proper understanding and utilization of the plume interaction and associated aerodynamic effects can increase the divert capability by 100% without any added propellant.

You probably know this but humans only exhale trace CO2. We mostly exhale nitrogen and oxygen.

To that point -- @Chequers, are we allowed to recover the cost of the launch vehicle?

Looks like I was prescient. https://medium.com/@davidstarlingm/no-you-dont-need-masks-2f8335702f66

Right. It's no longer a pressure-fed engine at all; it's now a proper turbopump-fed engine, with correspondingly better performance. The trick, as I understand @Cunjo Carl's idea, is to use the temperature differential between the LOX tank and the RP1 tank to operate a heat engine, essentially a helium-based expander-cycle turbopump, and allow the helium to exhaust into the tanks as it ordinarily would. I'm not sure whether the heat differential between the LOX tank and the RP1 tank would provide enough power. The Falcon 9/H already expands its COPV helium through a heat exchanger at the engine to increase its volume and use less as a pressurant; it would be straightfoward enough to design a similar setup but use it to run a turbopump before it is dumped into the tank. The question is whether the decreased tank weight, decreased helium requirements, and increased efficiency would outweigh the mass of a turbopump. Triprop engines are a thing. You'd need a LOT to make it work, though. Yes, you definitely need the back pressure but this way you don't need the weight of pressure-fed tanks.

I don't have the energy, time, or mental capacity to run the numbers right now, but I like the idea! Back of the envelope says it is good.

Rotating skyhook is surprisingly workable.