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I honestly don't know, Quaoar. An explosion-drive vehicle that size could certainly be built. What Isp it might have, I don't personally know how to calculate.
GW
The rule of thumb I learned for meteors is pea size (quarter inch)-and-up makes it to the ground, if a solid rock. Basically, rocks moving at meteor speeds will ablate away 1/8 inch on radius during atmospheric entry. This ablation is independent of whether there is actually any oxidation at all. It's a heat transfer/fluid shear force/phase change thing.
It's smaller stuff that "burns up" completely, which is most of the stuff out there. Many are not solid, and break up. Big ones breaking up violently are the bolides. The term "burn up" is extremely imprecise and misleading.
It's actually rather similar for entry of things moving only at orbital speed.
As for "combustibility" (as in coal in the argument a few posts above), that definition is subjective in the extreme.
Coal burns at very slow rates (lb/hr in several-second residence times) in an already-established fire, in large chunks. Coal can be burned at much faster rates (near a lb/sec at 1 sec residence times) in power plant furnaces if it is pulverized, and if the flame is piloted-off with oil or gas.
Very fine (colloidal-scale) carbon soot can be burned at even higher rates (a few lb/sec at residence times of few millisec) in ramjet engines, if there is a gaseous-fuel pilot flame to provide ignition, and if there is not too much soot.
So, time scale and size scale play critical roles in combustion, as does the overall (global) chemical kinetic rate description, along with mixing (however that might be be measured). None of those things is specified by the word "combustible". Kinetic rates for solids burning in air or oxygen are 10's to 100's or maybe 1000's of times slower than kinetic rates for gaseous fuels, typically.
GW
The Red Mars/Blue Mars/Green Mars trilogy does indeed point out the consequences of elevator failure. That tale describes one falling on Mars. Wherever/whenever such a thing is first attempted, it shouldn't an inhabited body, precisely because of the side effects of a failure-and-fallback event.
GW
The Orion explosion drive is a really oddball piece of physics. It works more efficiently the bigger your ship. For the small vehicle NASA looked at that would take a small crew to Mars, it didn't look that much different from the nuke thermal rocket. That's where your numbers like 1850 sec come from.
In much larger sizes is where numbers like 10,000 sec or 20,000 sec come from. The baseline design of 1959 was 280 feet long, 185 ft diameter, and about 10,000 tons at launch. That goes with 10,000 sec Isp, and T/W around 2 to 4. Bigger still is where the 20,000 sec Isp comes from: above 20,000 tons.
The nukes they use for this are fractional-kiloton in the atmosphere, a few kilotons in space. They are "shaped charges" in the sense that you can design for a spindle-shaped radiation "blast", meaning two intense spikes of it 180 degrees apart. Things like that make very lousy blast weapons down here. Atom bombs of the type we have for warfare down here make lousy Orion-drive devices, precisely because the radiation "blast" (and the real shock wave blast) is omnidirectional.
The original 10,000 ton design of 1959 was intended for a 3 year mission to Saturn and back, stopping off at the moon and Mars "on the way". I think they intended to land the thing, but that function would be better served with a landing craft, parking the Orion in orbit. Remember, this is 1954-1959 we are talking about.
They weren't supposed to test the principle back then, but they did it anyway. 1-meter flying model propelled by pulsed high-explosive blasts. It worked. The survival data for the pusher plates came from the surface tests of nuclear weapons in Nevada in the 1950's, plus some of the damage assessment data from Hiroshima and Nagasaki, and from the early tests against naval vessels in the Pacific.
GW
Hi Quaoar:
Yep, Orion-drive vessels would be made of steel, and closer to plate than sheet metal, too. Some of it would be heavy armor plate. At the time, they thought these things would be build in a shipyard, pretty much the way heavy naval war vessels are/were built.
GW
Watch that density factor. 0.6% is .006. All airfoil lift, long or short, is proportional to density x velocity squared.
GW
Sure, a tablet computer is (now) the modern, better answer. But back then? There were no pocket calculators, much less portable computers, in those days.
Today, your portable computer is the best answer for all sorts of tasks. Till the battery fails, for whatever reason. Then you need a backup. Depending upon what caused the failure, your backup ought to be a different technology or principle “just in case”.
I have a calculator or two around here that I use for running all sorts of numbers. But, if they fail, I kept my slide rule. I can run numbers no matter what. Doesn’t matter that your backup might be an obsolete technology. Who cares? What’s important is having that backup.
GW
Looks to me like the only things not true about the space pen story were (1) how much money, and (2) who actually spent it. Once there is such a pen, it is better than pencils simply because of broken leads floating about. Broken leads are very common using pencils.
However, as for pencil-sharpenings being contained or not, I personally have owned multiple hand pencil sharpeners that were enclosed. No graphite dust or wood shavings ever escaped unwanted. No moving parts, either.
Little tiny plastic boxes about 1 inch x 2 inch by 0.5 inch, with a conical hole for the pencil tip, and a razor blade in that hole. You do have to empty them now and then, but that can be done inside a plastic bag.
These things would have worked fine in zero gee and vacuum. Maybe they did, I never heard. Use the right plastic, and it'll work hot/cold, too. Although most writing is done inside the cabin, not out on an EVA.
GW
"Solid Booster Rocket monopoly? Where have they all gone?" - combination of attrition and completely-unfettered consolidation, which latter is a code word for buying up competition.
Same thing happened in aircraft. In the US, there are but Boeing and Lockmart for military aircraft (foreign: a couple of European outfits, plus Russia), there is only Boeing for commercial aircraft (with but Europe's Airbus as competition).
In space launch there was for a long time nothing but Boeing-Lockmart together as the monopoly ULA. Now there is Spacex getting a decent foothold and providing some competition here in the US. Foreign competition comes from France and Russia, maybe Japan. Perhaps soon China and India. That's the one market going the right way.
GW
My own preference is for very simple solutions. They usually cost less, which makes them more probable. Footholds and strap-your-self-down points at strategic locations are pretty well-proven and easy-to-do.
Often I like to "think Russian". Example: back in the 1960's, NASA spent several million dollars making a ball point ink pen that could function in zero-gee and in vacuum. The Russians used pencils (and solved the sharpenings-containment problem instead, pretty much with a simple enclosed sharpener).
GW
Aerodynamical lift means are hard to come by with "air" density 0.6% that of Earth. My be better off with simple jet thrust from rockets.
GW
All it means is that solid SRB's come from a monopoly supplier.
GW
So what's wrong with a light gas gun for payloads hard enough to withstand 1000's of gees?
That's something that could be built "right now", and with all existing technologies and materials.
GW
The main thing about magnetic boots in sci fi was that everyone thought spaceships would have steel hulls. The V-2 was steel construction. The presumption was that humans in zero-gee would need to be anchored in some way to a surface to prevent disorientation. Both assumptions proved to be incorrect by the time we started shooting things into space "for real" in the 1950's.
What we have found is that for a person to exert forces effectively upon objects, we do need an anchor point. The first was the grate decks and oddball interlocking shoes on Skylab. Once shuttle flew, it became the footholds inside, and the shuttle's manipulator arm.
If we ever go back to spaceships of steel construction (such as a nuclear explosion drive vessel), magnetic boots might once again provide a solution, especially if the magnetism can be turned off and on, as in an electromagnet.
GW
I think there's role for both motorcycles and bicycles on Mars, maybe tricycles or quadricycles instead. The human-powered stuff will require graded roads. I rather doubt that'll work well in the rough, off-road. Not wearing spacesuits.
Paul Webb's "elastic space suit" of 1968-vintage didn't look all that impractical to me, especially in the film clip of the test subject pedaling a bicycle ergonometer inside a vacuum tank at the air pressure equivalent to 87,000 feet. It was just a bitch to don the thing. He had all kinds of mobility not seen since the full p-suits of Mercury, which would not have been adequate for an EVA because of no cooling for the wearer. The suits grew in bulk and restriction when they were made adequate for EVA, starting in Gemini, and culminating in the Apollo moon suit. It's only gotten worse since then.
To go outside in space with Webb's elastic thing, all you needed to add was an ordinary white insulated coverall (or insulated pants with a separate coat), a helmet visor, insulated booties, and insulated gloves. To go EVA on the moon (what Webb was working toward), swap out insulated hiking boots for the booties. If it would work on the moon, it would work on Mars. Too bad nobody ever tested further to find out.
All immature technologies need some development to make them "practical". MCP is getting a pittance at MIT for Dava Newman's work, but could be matured quickly if multiple avenues were funded significantly.
That being said, the same immaturity vs maturity is true of nuclear rocketry. That's a different ball game cost-wise, but the same sentiment applies. There was some effort funded by one of the intelligence agencies in the 1990's to resurrect a variation on the old NERVA work, I think in response to some 1980's work by the Russians in the same area. I don't think either the US or Russian efforts ever led anywhere significant. But ours did confirm decent life, decent Isp, and better thrust/weight than the old NERVA rig. NERVA was close to flight test (but not spacegoing application) in 1973 when the effort died. It was about a year from its first flight test, as a replacement 3rd stage for the Saturn-5.
Now gas core was (and still is) quite exploratory, and that situation is still unchanged, because no one has worked on it. All that ever got done was academic-scale funding of two bench-top tests. One was a plasma flow experiment to test the fluid dynamic scheme necessary for essentially perfect containment of a U-233 (? not sure, might have been HEU) core in an open-cycle design. The other was a successful demonstration of a controllable HEU reaction in the gas phase. Both were successful feasibility demos, not mature technology demos.
At the time, the best estimates of contractors like Pratt was that regenerative cooling would be adequate in gas core up to about Isp 2000-2500 sec (with engine T/W around 30:1). Above that power level, some sort of active radiator cooling was needed. They thought Isp up to 6000+ sec might be possible, but that the radiator system cut T/W well under 0.1, maybe under 0.01. No test data ever backed these assessments up though. Gas core was a neglected and underfunded second stringer to the main event, which was NERVA. We know just enough to know it would probably work at one level or another, if we were to actually develop the thing.
The other widely-ignored nuclear option is nuclear explosion propulsion. This was General Atomics in San Diego, starting about 1954. It culminated in USAF-funded work from 1959-1965, when USAF had to turn all its space stuff over to NASA except for MOL and the spy satellite programs. NASA saw it as a competitor to NERVA, instead of the complement that it really was, and so NASA killed it by defunding it.
Enough experimental work was done to verify the principle: (1) they flew a 1-meter model with pulses of high explosives, and (2) there was data from the atomic bomb tests to well-support the survival of the pusher plate.
Enough design analysis was done to find out the explosion propulsion principle works far better in larger sizes (10,000 tons and up). It wasn't any better than NERVA in the small sizes NASA was considering for its then-planned Mars trip in 1983. That was used to help justify killing it.
Enough design analysis work was also done to figure out that the total fallout from surface-launching one of these 10,000 ton ships to LEO was equivalent to only one 9 megaton warhead atmospheric test today. No one would have been hurt by our building a fleet of a half dozen of these things over a decade. We're talking about a giant spaceship built of steel the same as a naval warship.
It wasn't until the Starfish Prime test over Johnston Island that anybody understood what the real side effect to be feared actually was: EMP effects. This can be managed, but the flight path restrictions, and the restrictions on construction shipyard locations, are very severe. And that applies to departure from orbit, too. I wouldn't recommend this technology for exploration missions, but for major colonization efforts maybe a century hence. And bear in mind that we can build better explosion devices than that 1950-vintage fission stuff.
One last comment about nuke explosion drive. The efficient form of the devices is a strongly-shaped charge. Those are lousy as blast weapons, and omni-directional blast weapons make lousy explosion drive devices. I know both are "nukes", and the psychology is against doing this, but the devices aren't really suitable as weapons.
Of all the technologies being discussed here, the MCP suit and the explosion drive could likely be brought to maturity quickest, probably as quickly as a ready-to-apply SEP drive. I think we should be doing all of them. They all have niches to fill.
GW
This is a very interesting discussion going on about materials of construction for a spacegoing habitat module. One should note that the requirements are light years apart for a hab that stays in space “forever” vs a hab that must enter atmosphere and land somewhere. Those requirements will likely never be resolved for some centuries yet.
What I don’t understand is insisting on exposing plastics to vacuum and UV and wild temperature swings where they will degrade at one rate or another. There are some good ones now available that will last a while, but nothing one could construe as “permanent” for any kind of space station-like structure that never lands. Put the plastics on the inside, and put the glass and metals on the outside.
Based on nothing but existing materials and common sense, I’d suggest a thin aluminum pressure shell surrounded by a thick layer of ordinary pink fiberglass insulation, just like we use in our attics, paper backing and everything. Vacuum won’t hurt glass fiber or paper. Make it quite thick; the nominal commercial batt thickness is 6 inches, double-layer it if you need more.
This needs an over-wrap to protect it from UV. Make that out of simple textile-reinforced mylar (I’d consider using something woven of simple cotton yarns), aluminized on one side. Face the aluminized surface outward, and glue Velcro strips where geometrically appropriate when you fabricate it. Wrap this around your fiberglass layer, and overlap it over itself for securing with the Velcro. Nothing but aluminized mylar faces space and its vacuum and UV, and the mylar part is underneath the aluminum. If this degrades ever so often, it is quite easy to replace, and extremely light and compact to ship. Meteor hole? Stuff some more fiberglass batting into the hole.
On the inside is where you arrange your plastic furnishings that can help with radiation shielding effects. Although you’d get even better effects by arranging your water, wastewater and frozen foods as part of your shielding, which things you have to have anyway, although not enough for the whole hab; so just around the designated flare shelter space.
Inside, these plastics see no UV, no wild temperature swings, and no vacuum. Now you can use any appropriate plastic for any particular detailed purpose, exactly the same way we do down here on Earth. Why make things hard putting plastics outside in space when you don’t need to? (I gotta ask.)
Whatever you do, do NOT mount anything permanent to the inside of this aluminum pressure shell! Everything must be quickly removable, because you have to reach that shell quickly to patch punctures. There isn’t time for an EVA to do that, and besides, from the inside, the air pressure helps hold and seal your patch. Put your equipment and stores down the core, and put the people and their operating spaces around inside of the pressure shell.
As for windows, pick a transparency. But add an exterior metal (or composite build-up) shutter that you can operate remotely from the inside. When you’re not using the window, close the shutter. Your transparencies will last a lot longer in a very hostile environment that way. UV and meteoroid impacts are the threats. You can even multi-layer the shutter as metal foils and Kevlar. Just don’t expose the Kevlar to UV, make sure the metal layers cover up all the Kevlar. The shutter doesn’t need to hold pressure, vacuum won’t hurt these materials. If you never put much force on them, then brittleness in the cold is no problem either.
(What you would design for an article that lands is quite different.)
GW
We've discussed what's wrong with NASA etc elsewhere. The behavior certainly suggests there's no real intent to send men to Mars, and there's an entire spectrum of reasons as to why that would be. As for ULA (Boeing/LockMart), they make more money off gravy train technology developments than they do actually doing anything substantive. That's been the history the last 4 decades. The way we as a society do business in this arena has to change, which is rather fundamental (and unlikely), I think. I think it more likely that a visionary private entity will go first.
As for artificial gravity, two things: (1) it does not have to be a full wheel. Any long vehicle with a habitat at one end can spin end-over-end to put about 1 gee in that habitat. 4 rpm at 56 m is one gee. That's actually quite do-able, right now. (2) Until we know any better, I'd go with one full gee, both ways. We do NOT know that 0.38 gee at Mars is enough to be therapeutic. The returning crew, even from LEO, will see 4+ gees on reentry, maybe 10-15 if on a free return. They will die if unhealthy during that ride.
Radiation protection is more scare than real. Galactic cosmic radiation (GCR) is too energetic for any practical methods of shielding, you're just going to get a dose. How much you get depends upon when you go. GCR varies sinusoidally between 24 and 60 REM, in phase with the solar cycle. An active stormy sun is associated with the 24 REM, and vice versa. Current astronaut yearly limit is 50 REM, and there's a career max that varies with gender and age. A Mars crew does not need to fly outside the Van Allen Belts a second time. If they do get a bit too much in any one year (60 vs 50 REM), it's only a little bit too much. And the structures around them actually do have a tiny shielding effect.
Solar flare during active sun periods is the lethal item. This is far less energetic, and we can shield it. About 20 cm of water works fine for the worst flares, and most are nowhere that big. But some are, and you must have a way to shelter from them, or it will kill you in a matter of hours. Water, wastewater, frozen food, all qualify. You have them with you as part of your long-term life support, so use them as shielding materials, too. Not every habitable space needs shielding, just a spot where everyone can hide for a few hours. I'd make it the flight deck, so that critical maneuvers could be flown, regardless of the solar weather.
One of the remaining bugaboos is lightweight astronaut food. It only lasts 12-18 months in edible condition. Frozen and canned stuff lasts the years we need, it's just heavier. So bye-bye minimalist minimum-thrown-weight ideas!
The other bugaboo is space to stay sane, and this is usually underestimated, and it isn't arranged well, in most of the designs I have seen. People need spaces to congregate, and they need spaces to be alone (and that's not just a bunk!). They need an organized workstation, and they need something generalized and reconfigurable for recreation. The total volume per person ought to look about like the old Skylab. 3 was great in there, it'd be marginal at 6.
GW
In all societies during and since the stone age (and probably before), people have had to have a "job" of some sort (generalized definition) in order to live. It is hardly possible for a single individual to supply himself with all his needs for all of his life. We learned to live cooperatively in groups, each doing different "jobs" so that all could live better, long before we were ever human. Many species have done this, because it works.
The flip side is that you don't eat (etc) if you don't do your job. I don't care what economic system you want to talk about, that's still fundamentally true. So, everybody has to have a "job" to do. Period. That's just life.
OK, now automate all the jobs with robots of one sort or another. So what are we humans supposed to do for a living? Didja ever think of that? I think robots are fine, as long as there is something else useful for us humans to do for a living. When there isn't, I am dead set against automation. Period.
Western civilization has been down this road several times before. Each time it did not turn out well.
The last time is when they automated manufacturing to the greatest extent possible, without any thought at any level anywhere in society, as to what else the fired workers would do for a living. What they couldn't automate, they outsourced overseas to the slave labor societies.
That is precisely why for the so-called "middle class", it now takes 2 full-time workers to support a family, instead of the 1 full-time worker that it took when I was a boy.
"Automation for profit without regard to the human consequences" is the most precise way of saying what I so strenuously object to. And that's precisely what is being discussed above, in this thread.
Beware, you are planning your own demises, should this speculation actually come to pass.
GW
You do realize that this outcome happened precisely because that court refused to dismiss Spacex's lawsuit. Rather than go to court and give the lawyers a chunk out of their budgets, USAF settled, which means they have to act more expeditiously on the certifications, and more fairly on the contracting. Or get sued again.
They (USAF) didn't want to settle or do anything. That's why this has dragged on for so long. ULA is where USAF retirees go for civilian careers, not Spacex. Giving work to Spacex cuts into potential second retirement benefits when you go to ULA after retiring from USAF. So far, Spacex has no place for USAF retirees. Boeing and Lockheed have had places for military retirees, for 7 decades now, when you consider those two gobbled up everybody else that there used to be.
Monopoly actually cuts both ways, sometimes. But only sometimes, and lawyers are usually involved when it does.
GW
kbd512:
What you say is perfectly true. The problem is that ignoring outside advice is part and parcel of the "not invented here" arrogant attitude that US government labs and agencies are so infamous for (for well over 7 decades now), out in the contracting industry. The public is relatively unaware of this, but needs to be made aware. However, any contractor who tells the truth in public gets no more business. And that's the plain truth of it. It an evil, but it is quite real.
GW
"been there and seen it on the inside many, many, many times"
Your whole habitat does not need to spin. The bed rest microgravity studies do in fact show that we do not benefit from gravity while sleeping. There's no need for artificial gravity in sleeping quarters. There is for your daily work shift. I'd guess that off-hours leisure time should have some spent under artificial gravity, but some could be spent in microgravity. The real question, unanswered even today, is how much artificial gravity is enough to be therapeutic? That really has a huge impact upon designs and their fundamental feasibilities.
GW
Re: thrust oscillations in SLS SRB's. It is my second-hand understanding that the offending mode in the 5-segment motors is a quarter-wave fundamental longitudinal instability that didn't show up in the shuttle 4-segment SRB's. Such resonances tend to be fairly sharply defined by susceptible geometries.
Maybe some sort of baffle might help. It does not necessarily have to be variable geometry; some fixed baffles have quieted oscillations in tactical missile size motors before. This is a solution decades old. It won't be a reusable item, though. The environment in an aluminized solid is quite harsh. Fast moving very hot solids of soot and aluminum oxide droplets are quite erosive, and the gas temperature is pretty near 6000 F. The radiation heat transfer from the solids to the structures is also super extreme.
One should bear in mind that the organ-pipe mode is not the only mode in which solid motors "sing". At least in the smaller sizes, radial and circumferential modes at far higher frequencies are the most common "offenders". You distinguish them partly on the basis of their frequencies, after having analyzed the geometry and conditions for what all the possible modes could be. That kind of analysis is pretty common in the small motor houses, not very common among the big motor boys, sorry to say.
Re: safety / redundancy practices vs program management -- The truly arrogant ones regarding the Columbia disaster were the managers, the engineers were simply ignorant (they could not believe that foam could break carbon-carbon, a 500 mph impact being outside ordinary human experience). The same management arrogance was true for Challenger, with the engineers not having ignorance for an excuse.
Depends on which engineers, though: Thiokol engineers told them not to fly, the NASA engineers know a whole lot less about solids.
But in both fatal accidents, it was managers who ignored technical advice and did what they wanted to do. In the case of Challenger, the scapegoat was to be the engineers, and the bad management was to be covered up. That got exposed during the hearings.
GW
Here’s why I think the kind of Isp comparisons we typically see are often deceptive, because the conditions and assumptions are unstated. Doing the basic ballistics gets you the real comparisons. The measure most independent of nozzle conditions is chamber characteristic velocity c*, which does still depend weakly upon chamber pressure, so it needs to be stated when quoting a number.
Here are some 1000 psia chamber pressure values for c*, and the oxidizer/fuel ratio r that goes with it. These r’s are not stoichiometric, they are always a little fuel-rich. Both r and c* are weak functions of chamber pressure Pc. A good empirical correlation is c* = k Pc^ m, where typically m is a small-number exponent on the order of 0.01. It is entirely empirical, and determined by tests. It is always larger than the value you would estimate from thermochemical calculations made at different chamber pressures.
Oxidizer fuel c*, fps r
O2 H2 7950 4.0
O2 RP-1 5900 2.55
O2 NH3 5880 1.41
N2O4 UDMH 5680 2.6
HNO3 RP-1 5180 5.0
All else being equal (which is a lot to worry about), the 1000 psia c* (and thus Isp) of O2/H2 is about 134.7% that of O2/RP-1. That’s for the same Pc, the same ambient backpressure Pamb, and the same best-expansion nozzles at the two slightly-different specific heat ratios.
What you do with this kind of data is use your ratio Pc/Pamb and the specific heat ratio of the gases to determine your thrust coefficient. That has to include a nozzle kinetic energy efficiency (which is half-angle dependent) that applies to the mVe term in thrust, but not the (Pe – Pamb)Ae term. These calculations are fairly standard in the textbooks. The trickiest part is incorporating correctly the nozzle kinetic energy efficiency. It’s often left out of textbook equations.
Once you know your thrust coefficient at your design chamber pressure, and at your design ambient pressure (at which thrust is to be rated), then you can size the engine: F = Pc At CF. Then use the c* to set propellant flow rate: w = Pc At gc / c*. Then finally the Isp is F/w at that point.
For other operating points at different ambient backpressures, you have to refigure CF, then use the thrust equation F = Pc At CF to find that thrust, and the flow rate w = Pc At gc / c* is unchanged. Isp then changes. What you don’t want is an overexpanded exit nozzle where Pe < Pamb, because the (Pe – Pamb)Ae term in thrust goes negative. Overexpand enough, and the nozzle flow shock-separates, so that thrust is drastically reduced. There are empirical correlations for this, but precision isn’t possible.
Adding throttle-down to the design complicates this further, and reduces performance, simply because Pc is lower when you throttle down. That limits your expansion and thus your CF, to much lower values, which in turn lowers Isp. You may have to tolerate some overexpansion, but you need to stay far away from pressure ratios that would risk separation.
All these things affect thrust and Isp drastically. Chamber pressure affects c* weakly. So comparing c*’s is the more reliable means to evaluate propellant combinations. Thermochemical codes often report theoretical c* among all the other results. Rocket c* efficiencies are usually pretty high, especially in the larger sizes.
GW
I have nothing against using obsolete equipments or technologies if they serve well. (I'd just as soon still use steam locomotives for heavy loads on hills, for example.) I think it's sad we in the US must buy big kerolox engines from the Russians. "Business decisions" have really failed us in that area.
Experience shows that the best workhorse two-stage boosters use kerolox in the first stage core, and LOX-LH2 in the smaller second stage, where the lower density of LH2 doesn't hurt you, up to a stage diameter equal to the first stage. I temper that design advice with solid strap-ons, that stage off before the first stage liquid core burns out. There I went and described Atlas-5. Surprise, surprise.
The old F-1 is a bit big for that application, at 1.5-1.7 million pounds of thrust each. Maybe not too big for a real heavy lifter. It could use a little modernization in its equipment and controls, that's for sure. Although there is not much you can do about chamber pressure (the real driver for Isp). Modern kerolox chamber pressures are just higher than F-1.
GW
Well, a pre-pressurized "one-shot" approach with a convenient liquid makes a lot of sense, weight-wise. They just underestimated how much the grid fin actuators might demand, I guess. Maybe a seal leaked a bit and increased the effective demand, too. Who knows?
GW