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President Trump's tariffs are intended to make it cheaper for American businesses to return manufacturing to America.  All industrialized nations require robust domestic manufacturing bases.  The end goal is to make America self-sufficient so that it does not need to import anything from other countries.  If other nations are upset that America's government is now creating conditions favorable to American businesses manufacturing their products here in America, for consumption by Americans, that's their problem.

Rather than treating this as an opportunity to make themselves self-sufficient as well, other nations like Canada are treating it as a pissing contest with President Trump.  President Biden's administration was busily implementing the same policies for microchips, munitions to feed the war in Ukraine, and other tech items we feel we need.  RobertDyck is upset that Canada will ultimately need to become self-sufficient, even though Canada has the energy, raw materials, education, manufacturing base, and technology to make Canada broadly self-sufficient.  It will cost more money to do, but at the end of the day Canadians will have an independent and self-sufficient nation that is uniquely Canadian and not dependent on remaining in the good graces of any other nation.

Maybe Canadians don't think that will ultimately benefit Canada, but I would like the counter-arguments for why it won't benefit Canada.  I would also like those arguments from the standpoint of long-term generational prosperity- something that goes beyond the "here and now".  A lot of problems have been artificially created by very short-term thinking.

America is now treating our military and economic alliances as, "Let's collaborate if all parties think they're getting fair value for whatever they contribute, or go our separate ways and remain as friends / colleagues if we don't think any particular deal is mutually beneficial."

As an American, I don't want my own fellow Americans to continue to suffer through this systematic "siphoning off of accumulated wealth".  I do not want Canadians to suffer, either.  I have no bone to pick with Canada or Canadians.  That said, my first and most important loyalty and duty is to my fellow Americans.  I cannot control how people in other nations view America's imposition of tariffs on foreign-made products.  Foreign nations have imposed tariffs on American products for many decades now.  If it's good for the goose, then it's good for the gander.

If tariffs are "good / necessary" when Canada applies them to American-made products, then they cannot be "bad / pointless" when America applies them to Canadian-made products, by that very same logic.  Regardless or moral valuation on trade policies, what we're presently doing is slowly but surely draining the wealth out of America.  That process will inevitably end, so whatever benefits it brought to other nations will end as well.

The Rocket Labs Electron rocket is LOX/RP1 TSTO with propellant tanks made entirely from CFRP.  It's successfully delivered payloads into orbit 64 times now.  The turbopumps are electric and use Lithium-ion batteries to power them.  Engine Isp is right up there with the best staged combustion engines, meaning 311s to 343s.  I think we can say that it works now.

As the above comparison shows, our RP1 fueled TSTO is delivering 3X more payload mass per cubic meter of propellant volume, as compared to LH2.

Using the Silverbird Astronautics Launch Vehicle Performance Calculator:

Upgraded Staged Combustion Merlin-powered SSTO
Dry Mass: 17,088kg (40% lighter than Al-2195 tanks using CFRP; Merlin engine mass same as existing engines)
Propellant Mass: 518,400kg (same propellant mass as both stages of F9B5)
Thrust:5,913kN (same as Merlin-1D sea level thrust X9)
Isp: 324 seconds (Merlin-1D converted to staged combustion 360s * 0.9 for time-avg thrust output to orbit)
Launch Site: Cape Canaveral / KSC (USA)
Default Propellant Residuals?: Yes
Restartable Upper Stage?: No

Orbit: 185km by 185km, 45deg inclination

Estimated Payload: 10,654kg; 95% Confidence Interval: 5,704kg to 16,639kg

Why might we still want an expendable SSTO version of Falcon 9?

If we can figure out how to get the engines back, once on-orbit, CFRP tanks could be made 10% lighter than the mass estimate provided here for a single use.  Total stage mass could be as little as 14,940kg, with 10,710kg being mostly CFRP.  At $100/kg, if we figure on throwing away $1M per launch to discard the propellant tanks while recovering the engines, then I think we can live with that.

For this lighter single-use vehicle...
Estimated Payload: 12,802kg; 95% Confidence Interval: 7,852kg to 18,784kg

For RCC vs stainless and Nickel-Copper engines:
1,057.5kg (engines) + 10,710kg (propellant tanks) = 11,767.5kg dry stage mass
Estimated Payload: 15,970kg; 95% Confidence Interval: 11,023kg to 21,957kg

50% stronger T1200 fiber with CNT-infused resin vs IM7 fiber:
1,057.5kg (engines) + 6,426kg (propellant tanks) = 7,483.5kg dry stage mass
Estimated Payload: 20,254kg; 95% Confidence Interval: 15,310kg to 26,241kg

If we can make the tanks light enough, then at some point we give up on full reusability because it's cheaper to just discard the propellant tanks, collect the engines with a space tug, and return them to Earth using a HIAD.  Alternatively, we accumulate some engines in space for use to propel ships to other destinations.  Storing RP1 in space is relatively easy.

Someone also recently devised a method to make RCC 100X cheaper than it previously was, so fabricating high-temp engine components from RCC is now a plausible means to greatly reduce engine mass and cost.

SSTO will never be as-performant as TSTO, but the payload performance difference between the partially reusable TSTOs we have today and the expendable SSTOs we could have tomorrow may not be that great.  If the cost is lower than the cost of stage recovery, that's still a "win" in my book.

tahanson43206,

Using RP1 (kerosene) as the fuel of choice permits rocket vehicle designers to minimize the propellant volume and maximize the total units of thrust generated per unit of engine mass and total vehicle dry mass for an orbital launch vehicle.  This feature of RP1 is particularly important for SSTOs, but also important for TSTOs.

Vehicle dry mass scales by engine thrust-to-weight ratio and propellant tank volume, because for any significant payload to orbit, the propellant tank mass represents the majority of the vehicle's total dry mass (engines + propellant tanks).  The cube-square law does help limit propellant tank mass increase as the propellant tank volume increases, but the end result will still be heavier propellant tanks and therefore higher vehicle dry mass when substantially less dense propellants, such as LH2, are substituted for RP1, when the goal should be equal or better payload performance with RP1.  The vehicle dry mass comparison between Delta IV Medium and Falcon 9 Block 5 outright proves this.  This is merely another way of saying that you cannot increase propellant tank volume by 10% or more, on account of LH2's exceptionally low volumetric density, yet still somehow arrive at a lower vehicle dry mass, as compared to RP1.

D4M Booster vs F9B5 Booster Propellant Volume and Tank Mass Comparison
D4M: 450m^3; mass, less engine: 22,110kg
F9B5: 370m^3; mass, less engines: 21,370kg or ~19,370kg without reusability hardware installed
F9B5 tank mass is 87.61% that of D4M
F9B5 tank volume is 82.22% that of D4M
F9B5 payload to orbit is 2.68X greater than that of D4M, but only 2X the gross liftoff mass of D4M

D4M Upper Stage vs F9B5 Upper Stage Propellant Volume and Tank Mass Comparison
D4M: 76m^3; dry mass, less engine 3,189kg; 23,264kg LOX + 3,956kg (561,752MJ) LH2
1X 301kg RL10B-2 engine, 110.1kN, 465.5s Isp; 24.1kg/s mass flow rate
1,129.460580912863071s * 110,100N = 124,353,610N-s
226,770kg total LOX/LH2 (both stages) / 8,510kg = 26.64747kg propellant per 1kg of payload
28,850kg + 3490kg = 32,340kg total dry mass

(447,150,942N-s + 124,353,610N-s) = 571,504,552N-s Total Impulse (both stages)
571,504,552N-s / 8,510kg = 67,157N-s/kg of payload

F9B5: 98m^3; dry mass, less engine 3,450kg; 75,200kg LOX + 33,200kg (1,427,600MJ) RP1
1X 550kg Merlin-1D Vac engine, 801kN, 348s Isp; 236.6kg/s mass flow rate
454.353338968723584s * 801,000N = 363,937,025N-s
518,400kg total LOX/RP1 (both stages) / 22,800kg = 22.73684kg propellant per 1kg of payload
25,600kg + 4,000kg = 29,600kg total dry mass (with booster reusability hardware installed)
23,600kg + 4,000kg = 27,600kg total dry mass (expendable)

(1,178,870,316N-s + 363,937,025N-s) = 1,542,807,341N-s Total Impulse (both stages)
1,542,807,341N-s / 22,800kg = 67,667N-s/kg of payload

Total Propellant Volume Required per kg of Useful Payload
D4M: 526m^3 of total propellant volume; 16.17871kg of payload per 1m^3 of propellant volume
F9B5: 468m^3 of total propellant volume; 48.71795kg of payload per 1m^3 of propellant volume

Total Thrust Required per kg of Useful Payload
D4M: 67,157N-s/kg of payload
F9B5: 67,667N-s/kg of payload

D4M "beats" F9B5, a vehicle pushing 2X as much total mass into the sky, in this one category that doesn't help make the case for LH2 fueled SSTOs, nor TSTOs for that matter.  It's fractionally better than RP1 here, meaning you could run the main engine(s) for about 1.5 seconds longer, or less, on a LH2 fueled vehicle before total thrust (an indirect measure of total energy expenditure) per kg of delivered payload equalizes between both vehicles.

Hopefully we now understand how greatly vehicle dry mass affects our vehicle's total energy expended per kg of useful payload.  We're getting 3X more payload mass per cubic meter of propellant volume for the F9B5 when we include both stages of the D4M and F9B5 rockets.  That's a reasonably good efficiency metric if you ask me.  Rather than showing how great LH2 is, we're continuously illustrating how freakishly efficient a modern RP1 fueled vehicle truly is.  Even for a TSTO, every little bit of vehicle dry mass increase has a serious effect on vehicle performance.

RP1 has an acceptable Isp (total units of thrust generated per unit of propellant mass expended) for SSTO and TSTO.  It's not even close to being "the best", yet LH2's much higher Isp doesn't seem to help improve total vehicle dry mass.  Delivery of payload to orbit is primarily a question of Total Impulse delivered per kilogram of vehicle dry mass, and this is where RP1 is a stand-out performer.

RP1 allows the vehicle's propellant tanks to occupy the least amount of physical volume, therefore reducing their mass, amongst all of the common liquid rocket propellants.  Amongst the propellants I listed, RP1 (kerosene) is the most widely used energy dense fuel combusted in gas turbine engines.  The turbopumps that feed liquid rocket engines are specialized gas turbines capable of force-feeding a tremendous mass of gaseous propellants into the main combustion chamber.

Calliban,

As an engineer, do you understand all the calculations I presented to evaluate Total Impulse per unit of vehicle dry mass?

tahanson43206,

Let's compute the volumetric energy density efficiencies for the following fuels:
LCH4 (not the same thing as LNG), at 422.8kg/m^3 and 55.5MJ/kg
HTPB (solid propellant), at 920kg/m^3 and 20MJ/kg
N2H4 (Hydrazine), at 1,021kg/m^3 and 19.5MJ/kg
HFO (Heavy Fuel Oil / "Bunker Fuel"), at 1,010kg/m^3 at 41MJ/kg

LH2: 71kg/m^3 * 142MJ/kg = 10,082MJ/m^3
HTPB: 920kg/m^3 * 20MJ/kg = 18,400MJ/m^3
N2H4: 1,021kg/m^3 * 19.5MJ/kg = 19,909.5MJ/m^3
LCH4: 422.8kg/m^3 * 55.5MJ/kg = 23,465.4MJ/m^3
RP1: 810kg/m^3 * 43MJ/kg = 34,830MJ/m^3
HFO: 1,010kg/m^3 * 41MJ/kg = 41,410MJ/m^3
CBP (Carbon Black Powder): 1,700kg/m^3 * 32.8MJ/kg = 55,760MJ/m^3

Volumetric Energy Density of Alternative Fuels Compared with RP1
CBP: 160.092%
HFO: 118.892%
RP1: 100%
LCH4: 67.371%
N2H4: 57.162%
HTPB: 52.828%
LH2: 28.946%

When over 90% of the vehicle's volume is filled with propellant, as is the case for all orbital class rocket vehicles, vehicle dry mass becomes sensitive to propellant density.  Since CBP and HFO would pose a severe coking problem for a rocket engine, and HFO is also loaded with Sulfur in most cases, RP1 is the most energy dense and readily available propellant for minimizing vehicle dry mass.

There are no clever "tricks" to skirt around major volumetric energy density differences when fractional improvements in propellant mass efficiency can't overcome volumetric inefficiencies which are multiples of propellants with much higher volumetric energy densities.  You need as high an Isp as you can manage, but more importantly, you need sufficient density to minimize vehicle dry mass if you intend to accelerate 100% of the vehicle dry mass, plus the useful payload, to orbital velocities.  The only known way to do that is to minimize the vehicle's volume, which is enabled by RP1 and hampered by far less dense propellants like LH2 and LCH4, or propellants with much lower thermal output per unit of mass, such as Hydrazine and solids like APCP and HTPB.

Let's consider the energy efficiency of RP1 vs LH2, purely from an energy consumption per kilogram of useful payload perspective.

D4M = Delta IV Medium (all stages fueled with LH2)
F9B5 = Falcon 9 Block V (all stages fueled with RP1)

Note: D4M is a single Common Booster Core powered by a single RS-68A, with no strap-on solid rockets, and an upper stage powered by a single RL-10.  D4M's vehicle dry mass is significantly greater than a F9B5, despite delivering less than half the payload to LEO of a F9B5.  Fuel mass for both stages of D4M is 4.9X less than both stages of F9B5, and total propellant mass for the D4M's Common Booster Core is almost exactly 50% that of a F9B5 booster core.  One would think that the superior Isp of Hydrogen burning engines and half the propellant mass would translate to a lighter dry stage mass for the D4M Common Booster Core, yet the opposite is true.  Both stages of D4M are significantly heavier than both stages of F9B5.  Both the propellant tanks and engines of D4M are heavier than the propellant tanks and engines of F9B5.

F9B5 Energy Economics
Fuel Masses: 123,500kg (booster); 34,275kg (upper stage); 157,776kg (total)
157,776kg of RP1 * 43MJ/kg = 6,784,368MJ of total fuel energy
6,784,368MJ / 22,800kg to LEO (fully expended) = 297.56MJ/kg of payload to LEO
297.56MJ / 43MJ/kg = 6.92kg (2.24 gallons of RP1 per 1kg of payload to LEO, at 3.084kg/gal)

D4M Energy Economics
Fuel Masses: 28,400kg (booster); 3,763kg (upper stage); 32,163kg (total)
32,163kg of LH2 * 142MJ/kg = 4,567,146MJ
4,567,146MJ / 8,510kg to LEO (expendable-only) = 536.68MJ/kg of payload to LEO
536.68MJ / 142MJ/kg = 3.78kg (14.12 gallons of LH2 per 1kg of payload to LEO, at 3.733gal/kg)

LH2 vs RP1 Payload-to-Orbit Fuel Efficiency Comparison
536.68MJ/kg (D4M/LH2) / 297.56MJ/kg (F9B5/RP1) = 1.80360

D4M vs F9B5 Payload to Orbit
8,510kg (D4M payload to orbit)  / 22,800kg (F9B5 payload to orbit) = 0.37325

D4M vs F9B5 Booster Total Impulse
447,150,942N-s (D4M) / 1,178,870,316N-s (F9B5) = 0.37930

D4M vs F9B5 Booster Dry Mass
28,850kg (D4M) / 25,600kg (F9B5) = 1.126953125

Conclusions
D4M consumes 80% more fuel energy than F9B5 while delivering only 38% as much payload to orbit, with a mass increase of 13% over F9B5.  On top of that, D4M also costs more than F9B5, which is why it was retired.  If you increase the booster dry mass of the LH2 solution by 338%, using a triple booster core Delta IV Heavy configuration, then you can get 5,990kg more payload to orbit than F9B5.

Key Takeaways
Whatever propellant mass efficiency advantage LH2 provides over RP1, it clearly doesn't translate into lower vehicle dry mass, lower total energy consumption, nor even improved payload performance.  If anything, the opposite appears to be true.  As the size of the vehicle scales-up to deliver more payload to orbit, that vehicle dry mass problem only gets worse, never better, because materials don't become any stronger as you increase the size of a structure.  You will see a relative improvement in vehicle dry mass as you increase the propellant tank diameters, thanks to the cube-square law, yet any LH2 powered vehicle is still going to end up being significantly heavier than a RP1 powered vehicle for the same delivered payload mass.

Even when we compare the total impulse of a notional RS-25 powered D4M operated purely in a vacuum vs a Merlin powered F9B5 operated purely at sea level, the total impulse generated per unit of vehicle dry mass advantage is lopsidedly in favor of F9B5.

If you invoke a D4M with Carbon Fiber propellant tanks with only 60% of the mass of the actual historical D4M's Aluminum propellant tanks, powered by a pair of lighter and higher-Isp RS-25D engines, you will still fail to generate a total impulse per unit of dry vehicle mass figure that's higher than a Merlin powered F9B5, constructed primarily of Al-2195 alloy.  That's how horrendously awful LH2 is as a fuel choice for a SSTO or TSTO.

Total impulse generated per unit of dry vehicle mass, not propellant mass, is the figure of merit that either makes or breaks SSTOs, and the primary reason why there are none.  That figure of merit is either sky-high, as it would be for a RP1 fueled rocket with Carbon Fiber propellant tanks and staged combustion engines, or SSTO doesn't work.

My personal take on Russian armor is that their T-72 is a roughly correct design, but even their frontal armor is ineffective against modern sabot and shaped charge rounds, their fire control system needs modernization, and their thermal imagers used to surveil the battlefield and acquire targets are woefully inadequate.  The 125mm smoothbore cannon is adequate, but their sabot rounds could be improved.  I would prioritize the improved thermal imager and fire control upgrades.

Here's how I would overhaul a NATO tank program:

1. Give up on sabot rounds for direct penetration of modern composite armor.  Kinetic penetrator rounds do work, but the extreme heat involved rapidly "burns out" the barrels through erosion caused by hot high pressure gases, especially the latest and greatest sabot rounds, which renders accuracy unacceptable after far fewer rounds fired.  Barrel life has seen a steep decline with successive "improved penetration" versions of the M829 "silver bullet" sabot round and similar analogs.  M256 L44 cannon barrels are now seeing service lives in the low thousands of rounds at most.  Each M829 now costs as much as a subcompact car.  The Tungsten penetrators fielded by other NATO nations have only made the barrel erosion problem worse, because those require extra velocity to equal the performance of our DU penetrators, which is why you see longer L55 barrels on the latest version of the Leopard 2, prompting Rheinmetall's experimentation with 140mm caliber guns.  Either way, the gun tubes are becoming unwieldy in urban warfare scenarios and if we're increasing caliber to 140mm, then why not use 155mm and call it a day?

2. Standardize on 155mm caliber for all towed artillery pieces, self-propelled guns, tank main guns, naval guns, and airborne gunships.  The US Army discovered long ago, much to its chagrin, that their tanks were nowhere near as well protected against 155mm artillery shells as they thought they were, whether fired at high angle or low angle, whether air burst above the tank or allowed to make contact and detonate.  Air burst 10m to 20m above the target proved to be the most destructive, but direct hits also rendered the target tanks inoperative as tanks.  155mm did not penetrate the frontal armor, but splinters from the shells punched right through the main gun tube of the target tanks, totally wrecked their optics and radio antennas, completely sheared off parts of the running gear (road wheel bogies and tracks), and punched through the engine deck in airbursts, setting fire to the vehicle after destroying the engine beneath the thinner engine deck armor.  Given that the M48 and M60 tanks used in the test were no longer usable as tanks, and would require depot level repairs to return to service, I'd say that still counts as a "kill".  No modern Russian or NATO tank would fare any better when hit with 155mm.  The M776 cannon that equips the M777 towed howitzer is a L39 weapon that weighs 3,700lbs for the barrel and breech, about 6,370lbs in total when configured as an artillery cannon.  The M256 L44 cannon that equips the Abrams weighs 8,330lbs.  The L55 cannon of the Leopard 2 weighs 9,170lbs  I would reduce the M776 cannon's length to L29, same as ye olde M109s, and Charge 4 to reduce the length of the breech and recoil.

3. Cease and desist with loading up every imaginable type of weapon and round of ammo aboard every vehicle.  The sheer quantity of equipment we try to cram into each vehicle is, in point of fact, driving the horrendous cost and weight of our tanks and other armored vehicles.  A tank should have its driver and vehicle commander located inside the hull and nobody inside the turret, which implies an autoloader for the main gun, a high quality thermal imager, panoramic fiber optic cameras that provide a 360 degree view of the battlefield without having the crew constantly expose themselves, a top notch fire control computer, and a battery of grenade launchers for smoke and HE or buckshot for close-in vehicle defense.  It doesn't need multiple machine guns in different calibers, a small caliber automatic cannon, Javelin missile launchers, and partridge in a pear tree.  The more people and weapons you stuff into a tank, the higher its silhouette becomes, which makes it easier to spot from the ground by casual observation, and the heavier yet more vulnerable you make the vehicle and people in it, simply because you cannot adequately armor the top and sides of a tank with a gigantic 3-man turret.

For a tank to support infantry assaults, you need a big gun that fires powerful shells, mostly to deal with light vehicles and dug-in infantry using fragmentation effects.  You can and will fire that $15K M829 "silver bullet" at a Toyota pickup truck if that's all you have, and it will do what it does, but to what end?  Whether or not it can outright "punch through" opposing tanks is largely irrelevant unless you're fighting hordes of tanks over open ground.  Even if you are, that's what Javelins and airborne tank destroyers are for.  What's the point of deploying infantry and combat drones in support of armored ground vehicles if you're not going to use all of their strengths to deliver asymmetric counter-attacks to defeat opposing forces?  The western world has an unhealthy ongoing fascination with the idea of "dueling" and "aces".  War is, "We're using everything we have against everything our enemy has."  It's not, "We're going to defeat your tank with our superior tank in one-on-one combat."  You get "quality" not only from technological superiority, but also from exhaustive realistic training, well-maintained equipment with plentiful spares, local numerical superiority, and the element of surprise.

My concept of "useful combat drones" involves small-ish (~750lbs) remotely-piloted fixed wing piston engine aircraft that can fly at speeds up to 200mph or so for up to 1 hour, and drop a single 155mm glide bomb shell on enemy armor or field fortifications.  If the air defense threat is minimal, perhaps Apaches and Warthogs can fulfill our "airborne tank destroyer" doctrinal role to protect our combined armor / infantry advances.  Apache and Cobra gunships ultimately replaced the M18 and M36 tank destroyers of WWII, as well as armored cavalry scouts for most missions, but now those roles appear ready to assign to small combat drones.  I would retire the Apaches and Warthogs at this point because the threat posed by radar-aimed autocannon and shoulder-fired missiles to piloted close air support assets is far too great against a peer level adversary.  Afghanis may not have any Stingers.  Russia and China, however, have thousands of them.  Flying at treetop level against them is a better than average way to lose good men and machines on every mission.  When, not "if", some of our combat drones get shot down, we build more and our drone operators live to learn from their failures since they weren't aboard the machines that were shot down.

We'll use trucks and cargo ships to transport / fuel / arm / repair our combat drones.  We'll launch them from gun-based catapults that use fuel and compressed air in conjunction with the high-low pressure principle our 40mm grenades use for "soft launch".  We'll recover them with nets so we don't need runways.  This worked well for drones of that approximate size carried aboard American battleships used during Gulf War I to spot the fall of rounds from the 16 inch guns.

The XM-803 (forerunner to the M1 Abrams, equipped with a 152mm rifled gun):

This is what we need to get back to, but without the oversized / overweight 3-man turret, meaning only 2 men in a highly protected hull compartment.  We must use electro-optical sensors to surveil the battlefield instead of exposing tank crews to enemy small arms and artillery fire while they're actively operating their tank.  The entire point of putting them in a tank was to protect them so they could focus on neutralizing targets that are holding up infantry advances.  Keeping all their hatches open, because they're still using their eyeballs to observe, has become a highly lethal anachronism.  This new tank concept deals with enemy field fortifications and infantry by direct-firing 100lb HE shells at them.  When you start direct-firing 155s at people, then everyone gets down, or everyone dies.

The infantry themselves will start manning self-mobile 30mm cannons that the Apaches previously carried, using small arms for close range self defense only.  As they advance, their first objective is to tear apart light vehicles, machine gun nests, mortar pits, and infantry formations using HEI cannon shells.  The Chinese are sure to fight island defense campaigns in the South Pacific roughly the same way the Japanese did during WWII, because there's not many viable alternatives to what the Japanese did, even with modern weapons.

This new way of fighting is firepower-based, rather than a specialized / limited supply weapon for this / that / the other.  Most ground targets can be dealt with using 155s.  Anything that requires a JDAM or bunker buster or cruise missile will be delivered by F-35s, B-21s, B-52s, ships, and submarines.  Missiles will be used sparingly, mostly limited to attacking enemy air defense systems, ships, and aircraft.  There are not enough of these advanced weapons, we cannot easily and rapidly produce more, and huge amounts of resources are captured in their production, stockpiling, and maintenance.  Iron bombs and artillery shells can be manufactured by the millions for comparatively little cost, especially if the shell bodies or bomb casings are stockpiled without filling them with explosives.

We need all services to adopt 155mm.  L29 155s for our tanks to minimize barrel length / weight / recoil force, L39 155s for our towed M777s and M109s, perhaps a few L52 M777ERs for niche applications, and 58 caliber 155s for the US Navy to replace their less effective 5 inch Mk54 guns.  When everyone is using 155, figuring out logistics becomes rather easy.  You need 155 shells and their modular propellant charges.  For longer shots, we'll splurge on Excalibur munitions so we don't burn out the barrels while "firing for effect".  All the services have spent mad money on special application guns.  The US Navy spent "stupid money" trying to make a 155 fly 100 miles.  Each 110lb guided shell cost $1M, as much as a Tomahawk cruise missile that flies 1,500 miles with a 1,000lb warhead.  A 58 caliber cannon can still reach out 40 miles or so with a $50K guided shell.  That would have been a very useful capability for the US Navy to have aboard ships, because it roughly doubles engagement range over the standard 5inch L62 Mk54 gun.  That would put the ship that fired it over the horizon, against most islands.

The objective of this program is to build lots of guns and shells, not "special guns" that require their own "special shells".  Since 155 works for the Army and Marine Corps, it works for the Navy, too.  Heck, we're going to figure out how to mount 155s in Spectre gunships so we can shell people from the air as well.  Against adversaries like Russia and China, more shelling equals "more better".  Russia is not wrong, as it pertains to their liberal use of artillery.  They don't call artillery the "King of Battle" for nothing.  The fact of the matter is that a 100lb HE shell is adequate to kill most military targets, if accurately delivered.  Bunkers, ships, and certain kinds of area targets like runways are the only kinds of targets that require more powerful guided bombs and missiles or torpedoes.

tahanson43206,

I've still seen no response to the fact that RP1 very clearly provides a lot more Newton-seconds of total impulse per unit of dry stage mass, as compared to LH2.  A Falcon 9 Block V booster core with sea level Merlin-1D gas generator engines and the reusability hardware attached still delivers 48% more Newton-seconds of total impulse, per kilogram of dry stage mass, using sea level thrust / Isp values, as compared with a pair of RS-25D staged combustion engines attached to a notional re-powered Delta IV Common Booster Core, when the basis of comparison is with the RS-25D's Vacuum thrust / Isp figures.

For any rocket, Newton-seconds of total impulse per unit of dry stage mass determines how much payload you can deliver to orbit.  More total impulse per unit of dry stage mass is "more better", especially for a SSTO, which is carrying the entire vehicle mass all the way to orbit with the useful payload.

How would I attempt to produce one of these "eVTOL" machines if I was running the development team, with an eye towards achieving production of a practical electric aircraft that uses a lot less fuel?

1. Use a Jet-A / kerosene burning high temperature Solid Oxide Fuel Cell (SOFC) that directly consumes kerosene, without prior reformation into Hydrogen gas, instead of electro-chemical batteries of any description.  There are no batteries in existence that are worth the cost, mass, volume, operating temperature limitations, and numerous operational challenges involved, such as the total lack of charging infrastructure and sufficient electric power generating capacity for recharging them.  That would be the absolute best place to start working on an "all-electric" society.  Provide the electricity first, rather than excuses for why it's not available, and then we'll pursue electrification of vehicles.

I've seen credible (independently tested and verified) 1,500Wh/kg figures posted for single-use Aluminum-air batteries that the Israelis experimented with here in America, installed in electric cars that drove over 100,000 miles in total.  Experimentation lasted a number of years until it became apparent that there was no economical solution for recharging them.  The process for recharging them is the same process used to smelt fresh Aluminum metal from Alumina ore (oxidized Aluminum), meaning 10X as much input energy is used to re-separate the Oxygen from the Aluminum as the batteries can actually store / output.  The batteries were easily swapped for fresh ones, by design, but the battery would last for two weeks to a month in a BEV under normal daily driving, and then the Alumina dust had to be emptied from the battery casing and recycled at the factory.  There was some minor disassembly involved, but no messy liquid or gel electrolyte as such.  The battery had to be "re-manufactured" (not quite the right word since it was more like rebuilding a fuel cell than true re-manufacturing) using fresh Aluminum metal "dust".  The parts of it that I saw made me think it could be done outside of a factory environment, but only with appropriate equipment.  To my untrained eye, it did not appear to be dangerous, merely time consuming and a little messy.  I think a mechanic with appropriate training could do the job, but I'm not an expert so go talk to the experts if you need a better evaluation.

To this day, no readily rechargeable Lithium-ion battery comes within a country mile of Aluminum-air's gravimetric energy density.  Amprius has developed a 500Wh/kg and 1,300Wh/L Lithium-ion cell, which is a real commercial product you can buy with money, not a lab prototype or future promise.  This tech ought to be a boon for portable electronics, but they're fixated on BEVs and other inappropriate uses for batteries.

I see lots of enthusiasm for batteries because they're neat toys for electrical engineers to tinker with, but there's little acknowledgement of the fact that nobody knows how to solve their fundamental energy density and recharging problems.  We need an order of magnitude energy density improvement.  General Motors continues to be correct in their assertion that 2,500Wh/kg batteries are necessary for practical BEVs that perform similarly to gasoline fueled vehicles in all respects, with similar curb weight and cost.  For aviation, we need battery energy density to almost match hydrocarbon fuel energy density.  That prospect is much further into the future, assuming it's possible.

Let's compare that with hydrocarbon fuels (BTU/gal figures provided by "The Engineer's Toolbox" website):
Gasoline: 125,000BTU/gal or 36,634Wh/gal; 6lbs/gal avg; 13,461Wh/kg
Kerosene: 135,000BTU/gal or 39,565Wh/gal; 6.8lbs/gal avg; 12,827Wh/kg
Diesel: 139,000BTU/gal or 40,737Wh/gal; 7lbs/gal avg; 12,830Wh/kg

Modern turbocharged direct-injected liquid-cooled gasoline automotive engines are 35% to 45% thermally efficient, which means they convert 4,711Wh/kg to 6,057Wh/kg of energy from their gasoline into mechanical power output.

Micro gas turbines, less than 0.5MW, are typically pretty horrendous when it comes to fuel efficiency, usually 20% or lower thermal efficiency.  Small gas turbines, 0.5MW to 25MW will be about 30% thermally efficient, such as the T406 that powers the V-22, so 3,848Wh/kg can be converted into mechanical horsepower output using kerosene as the fuel.  Medium-sized gas turbines, 25MW to 75MW, are about 40% thermally efficient.  LM2500 gas turbines or equivalents typically power warships, cruise ships, and large offshore oil drilling platforms.  Large natural gas fired gas turbines, in the 75MW to 500MW range, can approach 50% thermal efficiency.  All of that is simple cycle efficiency without significant recuperation, although it is increasingly common to use recuperators in marine applications and gas turbine power plants, which can improve combined cycle efficiency up to about 65%.

Diesel engines that power semi-trucks, construction equipment, and ships are now up to 50% thermally efficient without doing anything too spectacular or expensive.  Unlike gas turbines, which are most thermally efficient only at max power output, diesels are typically highly efficient across most or nearly all of their operating power band.  Warships smaller than aircraft carriers frequently use diesels to economically steam at speeds up to 15-20 knots, then use gas turbines to pour on the power to attain that last 10-15 knots of their flank speed.  Steaming at 30 knots typically requires about 8X more power than steaming at 20 knots.  Delivering that much additional shaft power using diesel engines alone makes the propulsion plant impractically large and heavy.  To the best of my knowledge, nobody uses geared steam turbines any more, except in conjunction with naval nuclear reactors, because marine steam boilers are also very heavy and take up too much space inside the hull.  For ships designed to move at significant speed, medium-sized marine gas turbines have become the power plant of choice for virtually all conventionally powered warships and cruise ships.  Cargo and tanker ships typically use a single very large diesel engine powered by bunker fuel / residual fuel oil.

SOFCs can be 50% to 85% thermally efficient, dependent on type, so 6,414Wh/kg to 10,903Wh/kg of mechanical work output can be extracted from kerosene or diesel fuel.  That's 4 to 7 times as much energy per unit weight as the most energy dense single-use Aluminum-air battery cell.  For an electric aircraft, that translates into 4 to 7 times less "fuel" weight, or 4 to 7 times greater useful payload-to-distance for the same "fuel" weight.  Practically speaking, there is no physical way to increase the size and weight of the aircraft by 4 to 7 times and retain the same energy usage efficiency.  Increasing an aircraft's physical size increases the aerodynamic drag because you need more wing area to make it fly, as well as more power.  Apart from takeoff power, which typically is significantly greater than cruise power, power required is a function of the drag force generated at the design cruise speed.

The only reason Joby Aviation's S4 can fly 150 miles using its 125kWh battery pack (I've recently learned that the actual production vehicle will be equipped with a 150kWh pack, though range remains unchanged) is that most of its flight time is spent in forward flight.  However good they are at hovering, all helicopters, regardless of what powers them, are incredibly inefficient "forward flight machines".  As a general rule of thumb, you can fly 2 to 4 times further using a purpose-built fixed wing aircraft, or merely one better optimized for forward flight than one using a purpose-built rotary wing, with half as much horsepower, at the same takeoff weight.

Joby Aviation's S4 2.0 pre-production prototype, with its 4,000lb MTOW, has an installed power of 1,899hp (6X 236kW max output 28kg electric motors, each motor driving a 5-bladed composite constant speed propeller).  Direct competitors to S4, Robinson's R44/R66 (270hp) and Bell's Model 206 JetRanger (400hp), do not require anything approaching the S4's power output figure because they're purpose-built helicopters with main rotors providing dramatically greater swept area.  The S4's stated service ceiling is 15,000ft, presumably in forward flight.  The R66 service ceiling is 14,000ft.  The JetRanger can reach 20,000ft.  Induced drag must be horrendous with 30 propeller blades, which explains why the S4's top speed is only 200mph, while the lack of swept area explains why the S4 requires 1,900hp to hover.  With optimal blade length for hovering, the R66 can still fly 400 miles.  From a mission fulfillment standpoint, the S4 looks like a brute force approach to what should be a more elegant solution, meaning either an electric V-22 facsimile or a real electric helicopter.  Anyone who thinks a 1,900hp VTOL machine will require dramatically less maintenance than a 270hp VTOL machine is in for a rude surprise.

Thousands of R44/R66 and JetRanger models, as well as thousands of examples of similarly capable foreign helicopter designs, have been operated within the confines cities for many decades now- the exact operational environment where these new S4 eVTOL vehicles are supposed to "make a splash" by providing a vastly more economical air taxi service.  That's the only way for these all-electric air taxi services to become profitable.  Forgive me, but this entire concept looks like Moller Sky Car 2.0.  Dramatically larger sums of money have been invested into the S4 in comparison to the Sky Car, but there's no clear market for thousands of electric V-22s that cost as much as a gas turbine powered helicopter to purchase.  Given the sheer numbers of helicopters already operating within cities, fuel and maintenance costs alone cannot possibly be the only reasons why most people are not flying to work every day.

When people say "beyond oil", I don't know what kind of nonsense they're spouting off, unless they mean nuclear power.  If we had a miniature fusion reactor we could stuff into an aircraft, that would be ideal.  Unfortunately, we don't have any of those.

The RR300 gas turbine that powers the R66 burns 23 gallons per hour, so let's do some basic math to see if a SOFC and electric motor can deliver equivalent power output for less total mass.

RR300 gas turbine engine weighs about 91kg, can generate 270hp for takeoff and 224hp at max continuous power, and burns about 23gph in cruise flight, when installed in the R66.
23gph = 909,995Wh of thermal energy input
224hp (max continuous power for the RR300) = 167,036.8W
167,036.8W / 909,995Wh = 18.36% engine thermal efficiency in cruise flight
That explains why nobody uses a gas turbine less than about 550hp for conventional aircraft.  The fuel burn rate is too high.
Let's use Joby Aviation's electric motors.
236kW = 316hp, so 1X of Joby's electric motors is sufficient to power the R66 and it weighs 28kg.
We'll use 5kW/kg as our SOFC's power-to-weight value and 70% as our efficiency figure, since both values have already been demonstrated in a singular SOFC design developed by NASA.
270hp = 201,339W
201,339W / 5,000W/kg = 40kg
68kg is the mass of our fuel cell and electric motor combination.  This combination is looking like a rather promising for our electric R66, because it's lighter than the mass of the gas turbine engine it's replacing.
The R66's max fuel capacity is 73.6 gallons, which weighs 500lbs / 227kg.
167,036.8W / 0.7 (SOFC's electrical conversion efficiency) = 238,624W
238,624W / 39,565Wh/gallon = 6 gallons per hour
The RR300 powered R66 can fly for just over 3 hours before it's out of fuel.
We're going to carry 20 gallons / 136lbs of fuel to provide 3 full hours of flight at max cruise speed, plus a reserve of 2 gallons.
R66 empty weight is 1,290lbs and MTOW is 2,700lbs.  We'll assume our "eR66" has an identical empty weight as the gas turbine model because our power plant mass is so close that we're unlikely to save a lot of mass on the fuel cell and electric motor.  Technically, we're 23kg lighter than the gas turbine, but we'll presume that 23kg covers any electric power cabling, power inverters, or other unanticipated mass increases.
With full fuel, our eR66 only weighs 1,426lbs, which leaves us with 1,274lbs of useful load.  That means each of the 5 occupants could weigh 254.8lbs.  However, the real R66 is only rated to carry 1,200lbs, so we have a full fuel / full occupancy aircraft, which is simply not possible with the gas turbine engine because 73.6 gallons of fuel weight cuts into our useful load weight.  Regardless, we can now fly a full occupancy aircraft for 3 full hours while burning only 18 gallons of fuel vs 69 gallons of fuel.  Our fuel burn rate is reduced by 3.83X, as are our CO2 emissions for people who care about that.  On top of that, we made our electric "eR66" a "practical electric aircraft" which is "more useful" than the original gas turbine R66.

In comparison to Joby Aviation's S4, we're consuming 273,912Wh of energy for equivalent flight distance with equivalent payload.  That means we're consuming about 82.6% more energy.  The explanation for that is pretty simple, though.  First, batteries are more efficient than fuel cells.  Second, as previously stated, rotary wing aircraft require about 2X more power than aircraft which are optimized for forward flight.

2. Use air-cooled brushed direct current direct drive electric motors with permanent magnets.  Brushed DC motors are capable of high torque at low rpm, which is ideal for an aircraft application.  The utter lack of solid state power electronics and software required to control the fuel cell and electric motors dramatically simplifies the entire power plant design problem.  Could you theoretically achieve greater absolute efficiency with electronically controlled systems?  Probably, but then you have to pay for all the additional electronic components and software, which pile on weight / cooling issues / purchase cost / maintenance cost at a breathtaking rate.  When I say "I'm designing an electric aircraft", that statement is precisely what I mean.  I do not mean, "I'm designing an oversized electronic remote controlled toy that humans can ride around in."

Those brushes will have to be replaced on a regular basis, which is annoying, but so what?  Carbon brushes are dirt cheap mechanical parts that a human being can visually inspect for imminent failure using ye olde Mk I Mod 0 eyeball.  Nobody can "eyeball" a microchip or other solid state electronic device to evaluate when or even if it might fail.  I need a very expensive piece of electronic support equipment to tell me that.

My mission here is to produce an affordable, maintainable, and demonstrably "better", in every sense of the word, "electric aircraft".  That objective is incompatible with retaining standing armies of engineers engaging in pointless "make work" projects.  I consider all battery operated aircraft much larger than children's RC toys to be pointless endeavors.  My aircraft won't have electronic motorized door handles, "just because we can".  While I pay my electrical engineers, they're going to do something demonstrably useful for aviation.  Anyone who needlesly increases the weight of our airframes by attempting to use batteries for inappropriate uses will promptly be shown the door.

3. Decide if what we're building should be a true helicopter or an electrically-powered V-22 facsimile.  All the oddball eVTOL designs I've seen do not improve upon range, speed, or payload-to-distance over true helicopters or V-22 facsimiles or fixed wing aircraft, which means they're not "good aircraft".  The one market I see for eVTOL of the variety developed thus far, that seems almost entirely ignored, is as a "rapid response" air ambulance or firefighter delivery vehicle that can near-instantly liftoff and head towards the scene of an emergency.  Most emergency response vehicles do not get "used and abused", 24/7/365, the way commercial aircraft are.  They're meticulously maintained but infrequently used.  When used, they need to be ready to go, post-haste.  This is where a battery operated eVTOL really would be a meaningfully better aircraft, purely for its ability to rapidly respond.  You don't need to wait 5 to 10 minutes for the engines to spin up and pre-flight checklist completion.  You flip the switch and it's ready to lift off seconds later.

RobertDyck,

Hollyweird creates fantasy-based caricatures of what might otherwise be semi-realistic yet still somewhat futuristic military equipment because it's flashy and captures the attention of an audience who are ignorant of anything remotely related to war.  It's all hyper-exaggerated silliness from highly creative people who thrive on drama.

My favorite bit of their ridiculous nonsense was "Runaway" (a 1984 move starring Tom Selleck and Gene Simmons) where a "homing bullet" was fired from an absurdly large handgun, after which the projectile would "hunt down" its intended victim at a speed that the camera work indicated was barely above sprinting speed, it would somehow penetrate their skin despite having virtually no energy, and then detonate a "micro explosive" inside of it.  Said bullet would turn around corners multiple times, go through windows, and other similarly hilarious absurdities.  The funny part was that in one of the scenes where this "special bullet" was used, had the person fired any normal handgun, even a .22LR, the person that this "homing bullet" was fired at likely would've been dead right there.  I guess that would've made for a very short and boring movie since that obvious bit of reality would've killed our protagonist immediately.

Anyway...

The flying drone from the Terminator movie has no wings to speak of to provide aerodynamic lift, but sports a giant butterfly tail.  This "killer drone" was presumably designed by a self-aware AGI to hunt humans who are moving around at walking speed, so it's designed and built aerial drones equipped with a pair of horrendously loud jet engines.  In the apocalyptic post-nuclear war future where nearly all infrastructure has been purposefully destroyed by said AGI, most notably the fuel stocks and oil men who previously supplied the oil to make jet fuel, the same AGI "thought-up" the most energy-intensive flying machine imaginable to capture or kill people who are moving around on foot... and also thought that was the best way to find and eliminate them.  If that's the best solution this self-aware AGI can come up with, then I'm going to ask this AGI to "show me it's real war face", because I'm not scared.

If I knew that my prey can move no faster than 20mph, then I think I would employ minimally powered soaring drones that drop the equivalent of a mortar shell's worth of explosive on them, because I can probably find caches of millions of mortar shells to drop.  Said shells don't weigh that much, so my flying drone can carry several of them.  If I could somehow guide them to their targets, then the humans I'm hunting down would likely never see or hear this flying machine orbiting a couple miles overhead.  They would likely never see or hear the mortar bombs being dropped from it, either, until it was far too late.  I would certainly not willingly expose my "eyes in the sky" to a relatively easy counter-attack using their short-ranged "laser rifles" by flying it 25 feet above the ground.  I would not need any incredibly energy dense power source to power an onboard laser rifle or cannon, and there's almost nothing most humans could do to retaliate.  A Stinger might be able to shoot it down relatively easily, but they either have to find a cache of Stingers or restart manufacturing of Stinger missiles from scratch in a world where almost everything has already been destroyed by a barrage of nuclear warheads.

The hunter-killer vehicle is something we already have, minus the laser cannon or whatever from the movie:

China's testing their own copy of the Boston Dynamics "Big Dog" Robot:

USAF "Big Dog" Robot:

USMC apparently wants these little guys to fire rockets:

A much smaller vehicle that might be more generally useful for a ground pounder to have close at-hand:

When these robot vehicles breaks down because there's no spare parts, gets hit with a computer virus because it's networked software was compromised from the word "go" since the software developer's oblivious teenage daughter connected her China-hacked Tik-Tok app to her daddy's war robot software development laptop, or it can't figure out where to move so it just stops cold and does nothing like a Tesla that cannot identify a parked car, or lacks the ability to recognize that a Marine giggling like a schoolgirl while moving towards the robot under a cardboard box is actually a human and likely an enemy combatant, this is what I would like to have as a soldier:

I don't need it to fire full-auto, so it doesn't need a massive battery and drive motor.  I'd be fine with a recoil-operated semi-auto, an oversized .50 cal if you will.  I'm not going to mount that ridiculously large ammo box to it.  25 rounds in a belt is plenty.  If this gun is attached to some kind of semi-autonomous robot, then I want it to follow me around using visually recognized hand signals or verbal commands, stay low and out of sight if possible, and stay quiet.  If I happen to pop my head up and realize, "oh joy, there's 10 dudes with AKs and RPGs only 200 yards away, then I want it to pop up right next to me after receiving a simple verbal "gun up" command.  I don't want or need a ridiculously over-complicated computerized sight that makes the weapon unaffordable and therefore unavailable.  A large zero magnification red dot is fine for my purposes.  We already have those in the inventory.  In an ambush, I can immediately return fire with 30mm HE shells that only need to land within about 5m of my target to have a serious deterrent / wounding effect.  If you think that's overkill, have someone fire a PKM or RPG over your head, then tell me if you wished you had a real cannon available to convince the gunner to find someone else to shoot at.

This is the kind of optic I want (no computers to fiddle with, no magnification, just a simple AA battery that lasts for years and a wide field of view):

My proposed solution's not exactly the stuff of futuristic action movies.  Small caliber cannons have been used in the field by infantry since at least WWI, and the Germans made them walking speed mobile in WWII by adding a light wheeled carriage that could be man-handled by a couple of men.  It's not the sort of AGI-enabled "killer combat machine" our military brass dreams about.  Even so, if my enemy is equipped with normal infantry small arms, as most are, then when I show up with the Apache's cannon, they're likely to have second and possibly third thoughts about pressing an attack.  More importantly, I won't be calling for air support because I'm returning fire with the same kind of weapon attached to an attack helicopter.

The robot doing cartwheels is impressive, but I'd like to know what kind of dexterity its hands have, and if it can use normal infantry weapons.  I'd settle for it carrying my pack so I can move about with less difficulty, spare ammo / rations / batteries, and administering first aid.  I'd like to see how well first aid robots work before trusting them with weapons.  The girl AI robot is creepy as all hell.  Maybe we can use those to convince the enemy to leave by either nagging at them or asking for free dinners until they leave in disgust.

One kind of combat robot I still haven't seen fielded yet is a snake robot with a piece of embedded det cord that wraps itself around your leg or ankle and then blows itself up, a near-silent self-mobile land mine if you will.  Flying drones are quite noisy, as are wheeled and tracked robots.  Imagine a sort of heat seeking "toe popper" that silently moves across terrain, looking to "cuddle up" next to a sleeping soldier.  Those would be diabolical- truly scary in a way that larger and more noticeable machines simply aren't.

tahanson43206,

LOX/LH2 may be able to perform an expendable SSTO mission, but the real issue is the LH2 propellant volume and the low thrust-to-weight ratio (TWR) of LH2 fueled engines.

LH2 Engine TWR: 75:1
LCH4 and RP1 Engine TWR: 150:1 to 200:1

At the thrust levels demanded for a 100t class payload, about 1.5X whatever GLOW happens to be, using LH2 adds considerable engine mass.

LOX is 1,141kg/m^3.  LH2 is 71kg/m^3.  LOX has 16X the density of LH2 per unit volume, yet tank mass per unit volume of propellant stored will be the same between LOX and LH2, up to 3g of vehicle acceleration.  LH2 internal pressurization loads drives propellant tank mass.  This useful tidbit of information comes from NASA.

LOX/LH2 Isp (Vac): 450s
LOX/RP1 Isp (Vac): 360s
LH2 provides a 25% Isp increase over RP1.

1m^3 of LH2, at 71kg/m^3 and 142MJ/kg, stores 10,082MJ/m^3
1m^3 of RP1, at 810kg/m^3 and 43MJ/kg, stores 34,830MJ/m^3
RP1 provides a 345% volumetric energy density increase over LH2.

Can LH2's 25% fuel economy advantage over RP1, somehow overcome RP1's 345% energy density increase over LH2?

Delta IV (all LOX/LH2 stages): 249.5t GLOW; 8.51t payload to LEO (3.411% of GLOW)
Falcon 9 (all LOX/RP1 stages): 549t GLOW; 22.8t payload to LEO (4.153% of GLOW)
Delta IV Heavy (all LOX/LH2 stages): 733t GLOW; 28.79t payload to LEO (3.927% of GLOW)
Falcon 9 Heavy (all LOX/RP1 stages): 1,420t GLOW; 63.8t payload to LEO (4.493% of GLOW)

There's your answer.  RP1 fueled vehicles are heavier because they take more payload to orbit than the LH2 fueled vehicles.  Even as a percentage of GLOW, payload-to-GLOW is higher for RP1 than LH2.  Both rockets are fabricated from Aluminum alloys and both use gas generator cycle engines.

At some point, stage volume, thus stage dry mass fraction, combined with low engine TWR, really starts to become a problem for LH2 because it eats into your payload mass fraction.

Do we imagine that these problems will become more or less severe as we both scale-up the rocket to deliver a 100t payload and push the entire vehicle mass from the ground, all the way to orbit?

Why is the payload mass fraction of Delta IV Heavy lower than a Merlin powered vehicle that never achieves the sea level Isp of the RS-68A and RL-10 engines that power the Delta IV and Delta IV Heavy?

Using electric motors for marine propulsion has "battle heritage" with the US Navy:
NavWeaps - History and Technology - Turboelectric Drive in American Capital Ships, by Joseph Czarnecki

From the article:

Wartime experience fighting the Imperial Japanese Navy showed that their survivability was well above that of purely steam powered warships, and their implementation by the US Navy dates back to just before WWI.  The only reason they were not more widely implemented during the interwar years was the Treaty Era ship size / mass constraints imposed by all the signatories to the treaties.

If we had modern 40kW/kg electric motors and 8kW/kg kerosene or diesel burning Solid Oxide Fuel Cells, then we would definitely want said motors and fuel cells powering all of our warships, because they would maximize range and mitigate battle damage if ship and fleet tonnage restrictions were still in place.  In modern times, economy dictates fleet size and tonnage, but the restriction still exists, regardless of treaties.

Let's use the Kuznetsov NK-12 turboprops from the 1950s Tupolev Tu-95 "Bear" bomber as our "airliner-like" example:
Each NK-12 weighs 2,900kg, the Tu-95 has 4 of them for 11,600kg of power plant mass, each one generates 11,000kW of output, so 44,000kW of takeoff power.  Overall power-to-weight ratio is 3.79:1- not great by modern standards but acceptable for an airliner.

State-of-the-art PEMFCs created for Toyota Hydrogen-fueled hybrids can achieve up to 5kW/kg, so a 44MW fuel cell would weigh 8,800kg.  If each 11,000kW PMSM weighed in at 440kg (at 25kW/kg for takeoff power), then we can achieve 10,560kg for the installed power plant weight.  To this weight, we must either add compressed Hydrogen tanks or cryogenic liquid Hydrogen fuel tanks.  That will rapidly increase our installed mass to achieve the same payload-to-distance because Hydrogen has such absurdly low density, despite the fact that its very low mass makes Hydrogen an ideal fuel from a power-to-weight perspective.  Regardless, we can "just beat out" 1950s Soviet gas turbines that were installed in numerous military and civil aircraft, by about 1,000kg, for a vehicle with a MTOW of 188,000kg.  If we're reverting back to 1950s aircraft performance, then this may be an acceptable trade.  I would argue that nobody would willingly "time travel" back to the 1950s, as it relates to aircraft power plant performance, unless the "competitive advantage" was greatly reduced per-flight-hour operating cost.  For a variety of reasons, Hydrogen fuel cells and superconducting electric motors make that a highly unlikely outcome.

Modern high-output gas turbines such as the Rolls-Royce T406 turboshaft engine installed in numerous V-22 Ospreys will serve as an example of how far we've come since the early gas turbines of the 1950s.  T406's takeoff power is 4,586kW and it weighs 440kg (at 10.4kW/kg), so 880kg and 9,172kW (close enough to the 10MW airliner example you asked about) of takeoff power for both engines.  An equivalent pair of electric motors only weighs 367kg at 25kW/kg- a spectacular mass reduction over the T406.  However, our 5kW/kg PEMFC is a hefty 1,834.4kg, so 2,201kg is our final power plant mass.  At 40kW/kg, our electric motors now weigh 229kg, which reduces our total power plant mass to 2,064kg.  This is still a major fundamental technology problem.  We cannot double the power plant mass of a V-22 without severely cutting into its payload-to-distance capability.  Internal fuel capacity between the variants ranges between 1,450 gallons and 2,025 gallons, and 1,184kg equates to 384 gallons of jet fuel.  T406 thermal efficiency at cruise is only around 32%, however, so an 8kW/kg SOFC, with thermal-to-electrical efficiency of up to 85%, might make that trade worthwhile, with the understanding that as vehicle dry mass increases, so does cost.  Each V-22 costs about as much as the F-35 stealth fighter, which is why production ended.

Hydrogen is much lighter than Jet-A, but we're also carrying far less of it unless we dramatically increase the internal volume of the V-22, which is not practical to do.  Even moving the engines inboard and using pairs of generators and flight motors to eliminate the mass of the cross-connect shafting and gearboxes doesn't really solve our total installed power plant mass problem, because the weight of a gas-turbine-to-generator-to-motor setup still doubles total power plant mass with 40kW/kg electric motors.  This is why featherweight electric motors don't really solve any existing modern aircraft design problems.  The electric motor mass is not the primary design problem and hasn't been for at least a decade now, since the widespread adoption of axial flux PMSMs.  The issue is solely related to the mass of whatever generates the electricity, which is either a gas turbine with an attached generator, a fuel cell, or electro-chemical batteries.  None of those setups have total lower installed mass than gas turbines alone, but this is what matters for practical electrically-powered flight applications.  The only correct answer is to dramatically reduce the mass of the fuel cell or batteries.

To illustrate how acute the electric power generation vs power delivery problem is:
880kg - 229kg (pair of 40kW/kg motors) = 651kg
9,172,000 / 651 = 14,089W/kg

14kW/kg would be the required power density of whatever provides the electricity, when combined with 40kW/kg electric motors, for equal mass when compared with hydrocarbon fueled gas turbine power delivery solutions.  All the claims about "the future" and all the tantrums thrown by our "green energy" advocates over our use of gas turbines as the preferred power delivery solutions for commercial and military aircraft will never make electric aircraft a practical solution to any aviation problem beyond infrequently used very short range personal aircraft.  Personal aircraft are owner-operator aircraft taken out every weekend or two for what amounts to minimal currency training.  No kind of "working aircraft", which generates 90%+ of all aviation-related CO2 emissions, will ever be powered by something that roughly doubles the total installed power plant mass over present day gas turbine engines.

Anyone who thinks they can arbitrarily "hang" an extra 500kg worth of fuel cell off each engine nacelle, simply because we can burn less fuel when consumed by a more efficient fuel cell, doesn't know a thing about aircraft design.  The wing doesn't magically become any stronger than it already is.  CG changes won't improve stability in flight.  The pivoting nacelles could be replaced with pivoting rotor heads and electric motors only, as in the new Bell V-280.  However, that design uses similarly light yet powerful (10kW/kg) gas turbine engines.  The V-22's fuel is stored in its wings and in sponson tanks located on either side of the fuselage.  The "wing" of the V-22 had to be specially designed to make it much stronger than either a conventional aircraft wing or "stub wings" that large attack and transport helicopters use to both carry heavy ordnance and "unload" their main rotor during high speed forward flight, specifically to deal with the weight of the T406 engines, which rotate between horizontal and vertical positions, cross-connect shafting and gearboxes, in addition to the massive torques generated by its helicopter-length rotor blades.  Doubling the mass hanging off the tip of each wing is a non-starter.  Maybe we can make the wing stronger for the same weight maybe we can make the massive engine pivot bearings even stronger without increasing airframe weight too much, but I doubt it.  Every moving part of a V-22 operates perilously close to clearly defined materials limits- engines, shafting, gearboxes, rotors, and major airframe structures like the wing box.  If you exceed the specified operating hours between maintenance cycles on a V-22, something you can usually get away with on a conventional transport helicopter like a Blackhawk or Chinook, a V-22 stands a better than average chance of literally "falling out of the sky" when one or more of its highly stressed power plant and/or airframe parts fails catastrophically.  If the maintenance manual says, "replace this shaft after 2,000 flight hours", that's exactly what it means, no exceptions, no waivers, no additional service life margin remaining.  The Marines learned that the hard way, more than once.

If not directly behind the electric motors in the nacelles that previously housed the gas turbines, someone tell me where exactly we can fit roughly 1.223m^3 (at ~8kW/L) worth of SOFCs inside this thing:

A locomotive could readily be adapted to carry a fuel cell that burns low-Sulfur diesel fuel.  The fuel cell would be much lighter and more compact than a locomotive's diesel engine for equivalent electrical power output and requires no electric generator, so this application would benefit from a fuel cell installation.  Virtually all trains already burn diesel fuel and use electric traction motors.  Using superconducting motors would add quite a bit of pointless cost and complexity for a minor improvement in energy efficiency, but a diesel-burning SOFC is already significantly more efficient than the most diesel engine and electric generator combination, so why bother?

There are plenty of ways to reduce conductor mass and volume in an electric motor:
Radical electric motor runs without metal coils

Electric Motor Engineering - From nanomaterials to the new generation of electric motors

Carbon Nanotubes as an Alternative to Copper Wires in Electrical Machines: A Review

Enhanced Copper-Carbon Nanotube Hybrid Conductors with Titanium Adhesion Layer