New Mars Forums

I predict that this technology goes nowhere, if applied to aviation, until such time as 15kW/kg fuel cells become reality- roughly double the highest fuel cell gravimetric power density figure achieved to date.  However, a 40kW/kg electric motor would be a major improvement for ship and power plant operators.  I wish these MSN propaganda articles would quit indulging the "green fantasies" of their readers and instead focus on solvable problems that newer or more refined technology like this superconducting electric motor could realistically solve.

If I was an investor, I would demand to see a working prototype of a 15kW/kg fuel cell or electro-chemical battery cell before giving money to anyone purporting to develop superconducting electric motors for aviation.  I would tell them that if they change their target market to ship and power plant operators, cease and desist with all claims that they're doing anything remotely useful for aviation, and sell the product on the basis of its absolute efficiency and improved transportability by truck or rail, then doing all of those things would cause me to take them seriously as people who were working towards a practical design goal by creating a more efficient motor design solution to an existing real world problem that plagues all ships and power plants.  The issue is that they're not doing those things.  They're either knowingly or ignorantly asserting that their motor design will somehow become the enabler for electric aviation that it never will be, absent a 15kW/kg fuel cell, which combined with 40kW/kg electric motors, would then produce equal output for equal mass, when compared with a modern 10kW/kg gas turbine engine.

It's a little disheartening that so much time and effort is wasted on pointless pursuits when there's so much good that would be done by catering to markets where a new motor technology, all by itself, actually can "turn a page" on ship and power plant efficiency.  I guess they don't do that because they don't want to compete with the likes of Siemens and General Electric, but those are the exact two marine and power plant solutions providers which should be taken down a peg or two if they fail to innovate by bringing new technologies to bear on markets that are screaming out for major changes that have a significant impact on fuel economy.

Sharrow upended the propeller market with a markedly more efficient propeller design for boats and ships that minimizes cavitation.  20% to 30% power reduction to achieve a given rpm, especially at high speeds where fuel consumption becomes considerable, is nothing to sneeze at.

Speed-of-Air Pistons created pistons that deliver 20% to 30% more power at a given engine rpm by optimizing air-fuel mixing prior to ignition.  The result is that the engine can be throttled back by that amount for better fuel economy at the same output level, and emissions are dramatically reduced due to more complete combustion across the entire rpm range.

NASA's 3D SOFC, at 8kW/kg and up to 85% efficiency, is a serious contender for continuous power generation for ships and commercial electric power plants that burn natural gas, kerosene, or diesel fuels, provided that they have low Sulfur content to avoid damage to the cells.

Hinetics could be a serious contender for compact / lightweight marine propulsion systems and frequency stabilizers for electric grids that no longer rely on massive chunks of steam-powered spinning metal.  Spain and Portugal recently had their "come to Jesus" moment on grid stability using photovoltaics and wind turbines.  What they did should have theoretically worked, but it didn't work in the real world.  The monetary cost of that very brief event was measured in billions of dollars, to say nothing of the human cost in terms of lives lost.

Whatever its other faults may be, Bloom Energy actually sells real world commercial SOFCs to companies to generate their own onsite electric power using a natural gas line.  Their fuel cells, which are not the same as the ones NASA developed, are about 52% efficient at converting natural gas into electricity, which is still well above the average efficiency of simple cycle gas turbines intended to handle variable load from things like server farms and cities.  When you need 300MW+ gas turbines that power entire cities, then you might see an efficiency figure like that from a gas turbine while it's running at full output.  Unfortunately, thermal efficiency takes a nosedive at reduced load, because that's the nature of continuous combustion engines.  Their product is expensive at $4/W of output, but even with maintenance costs a company using their tech still saves about $100K per 200kW unit per year in total delivered energy cost vs purchasing power from a city running a gas turbine.  Breakeven point is about 8.6 years, but companies running server farms are going to need electricity to run them forever.  People shipping cargo by sea are going to run their cargo ships between ports until Star Trek teleporters become real.  Military ships are going to patrol the sea lanes until real world starships make ocean conveyance an anachronism.  Since neither of those things are likely to happen in humanity's near future, fundamentally more efficient tech is still better, but only when you get to market with a practical product that real customers can afford to pay for.

That is how I view superconducting electric motors- a measurable and practical improvement for marine propulsion and power plants, but still firmly in the realm of fantasy for aviation.  If we implement superconducting electric motors in ships, then the impetus exists to continue to refine them, so perhaps one day when fuel cell power-to-weight doubles, they can then be applied to aircraft as well.  Until that day arrives, we need to quit entertaining nonsense propositions that ignore the inconvenient parts of technological reality related to power generation, and perhaps devote a lot more money to fuel cells, which can be and frequently are measurably more efficient than the alternatives.

tahanson43206,

Generating lift in a bulk fluid is a matter of generating a pressure gradient through relative motion of a lifting device moving through that fluid.  The fluid's density, viscosity, compressibility, the shape of the lifting device, angle-of-attack relative to the oncoming flow, and its relative velocity all affect the magnitude of the generated pressure gradients.  For any reasonably well-designed lifting device, velocity has an outsized effect on the lifting force being generated.

Asking someone to "just throw out a number" for a given fluid density, without specifying all of those parameters, is simply not possible, because the magnitude of the lifting force depends on all of them.  If we further bound the answer to this question by specifying that the lifting device only works when the fluid flows over or through it at subsonic velocities, then we're more constrained in what our answer will be.  However, there are in fact examples of wings that, at least at reduced velocities relative to much faster cruise flight, will generate substantially more lift than prototypical airfoil shapes (slats, fowler flaps, flaps on the top of the wing rather than only hinged to the trailing edge to enable lift generation at much higher AoA before significant flow separation occurs, vortex generators, blown lifting surfaces, and shape morphing surfaces that use aeroelastic materials):

Wings that will ordinarily only achieve Cl between 1 and 2 in cruise flight can use those various aforementioned lift-enhancing devices to increase Cl to as much as 4 or even 5 without a corresponding increase in Cd.

Dr Clark,

How would you go about building an engine nozzle capable of changing its shape in 3 dimensions to match that 1D theoretically ideal expansion vs altitude graph, even now that we have lightweight materials able to withstand exhaust heat without regenerative cooling?

To the extent feasible, I would imagine that engine designers bias the nozzle expansion ratio toward the lower bound of the altitude range where the engine spends most of its firing time.  If you spend far less time below 30,000ft than you do above 30,000ft, then it makes a lot more sense to optimize the nozzle's expansion ratio for 1/3rd of sea level pressure, however much you can get away with before significant flow separation occurs at sea level.

This topic comes up again and again...

The ultimate answer is that it's possible to create a practical crewed aircraft on Mars, but far from easy.

The Raymer Manned Mars Airplane: A Conceptual Design and Feasibility Study

Concept Aircraft Presentation Slide Deck:
AIAA Aerospace Sciences Meeting, January 2021

A large collection of beautifully rendered concept artwork showcasing the Raymer Manned Mars Plane Concept:
The Raymer Manned Mars Plane (RMMP)

My personal opinion on this matter is that we require the strongest composites money can buy, an energy-dense monopropellant, such as Hydrazine, fed through a gas turbine to maximize energy extraction while minimizing engine / propellant / propellant tank mass, and a multi-wing design to prevent the wings from becoming impractically large to transport to Mars or mandating risky VTOL maneuvers.  My variation on his original concept uses 6 foldable / stowable wings instead of 1 giant wing, similar to dragonfly wings, in order to transport the aircraft to Mars.

Dr Raymer's concept plane has a wingspan of 105.136m, tip-to-tip, which is longer than a football field.  Simply put, that's an impractically large wing which is driving airframe mass, at 848.8kg.  If we had 6 stowable wings, then each wing is about 17.523m long, and therefore fits inside the payload bay of a Starship.  This should also result in iighter / stiffer wings that don't weigh as much.  His design used IM7 fiber, but I think we need T1200 fiber, which is roughly 50% stronger and about as stiff.  These are still very long wings, each one about 4.725m longer those of the Perlan II high altitude glider.  However, fabricating such wings is not outlandish since Perlan II's total wing length is 25.55m and I believe it was fabricated as a single piece design.

His design used 8X CO/O2 rockets for vertical takeoff and landing, plus 2X horizontally mounted rockets for acceleration to flying speed.  I think that's a kluge, which is what I think about all VTOL aircraft that are not helicopters.  Stall speed for his concept was 115 knots.  C-130 approach speed is 130 knots and landing speed is around 110 knots.  C-130s are proven capable of performing rough field landings, and their minimum rotation speed is around 97 knots.  Most of the time you're going to rotate closer to 110 knots when fully loaded.  Therefore, a 120kt to 130kt rotation speed is not outlandish.  Takeoff and landing at that speed is fine, provided you have very large / low-pressure tires that can roll over large rocks.

His design uses 500Wh/kg Lithium-ion batteries, which we actually have now (yay!), but even future projected battery technologies are pitiful when compared to Hydrazine's gravimetric energy density.

500Wh/kg Lithium-ion batteries: 1.8MJ/kg
1,000Wh/kg Lithium-O2 batteries: 3.6MJ/kg
CO combustion with O2: 10.1MJ/kg
Hydrazine decomposition over a catalyst: 19.5MJ/kg

HAN or AF-M315E monopropellant is considerably denser than Hydrazine, with much lower toxicity and no concern over it freezing in the tank.

Dr Raymer's design requires 200hp, provided by 4X 50hp electric motors, 2 inboard and 2 outboard, presumably with 2-bladed props for maximum efficiency.  Motors are fantastic when it comes to power-to-weight, better than anything except a rocket engine, but their battery power source is pathetic.

Space Shuttle Hydrazine-fueled APUs weighed about 40kg each, less gearbox, and produced 135hp. RCC vs superalloy components would reduce mass by a factor of about 4, so only 10kg per APU.  At 80,000rpm, the gearbox would need to provide a 110:1 reduction to swing a 4m diameter prop, since local Mach 1 is only 788fps.  Max prop rpm is therefore ~725rpm.  The Shuttle APUs could generate up to 150hp for short periods of time.  Regardless, fuel flow was 3lbs/min, so 810lbs for 4.5hrs, meaning less than the battery mass for the same 4.5hrs of flight.  Deleting the electric motors, power conditioning equipment, photovoltaics, rocket engines, and CO/O2 propellants and tanks will save quite a bit of mass.  HAN provides a healthy energy density increase over Hydrazine, plus 45% less volume than Hydrazine for the same total energy storage.  This is starting to look doable with a more or less "conventional" design.  95 gallons of Hydrazine buys us 4.5 hours of flight time.  Since we deleted so much mass elsewhere, all the rest of our components can become smaller and lighter- a virtuous circle.

tahanson43206,

I will generally agree with your premise that it shouldn't take weeks to discover the cause of an explosion within a rocket sitting on the test stand with no engines running, provided that Starship 36 had sufficient instrumentation attached to it for testing purposes.  Something capable of instantly releasing tremendous energy, which at least did not superficially appear to involve combustion, occurred while the rocket was loading propellants.  The force generated was sufficient to split the entire upper hull open like a busted pinata.  Since this same failure mode has happened before to other launch vehicles, a COPV rupture is the most probable explanation.

Space Shuttles, Falcon 9s, and now Starships have all suffered casualties as a result of COPV ruptures.  Once the COPV ruptures and throws high velocity tank fragments through paper thin propellant tanks, it's only a matter of moments before those highly volatile cryogenic fuels like LCH4 or LH2 find an ignition source and rapidly combust.

If it seems like we experience these failures more frequently than other space programs do, that might be because we fly more often than anyone else does, except for the Russians, who don't typically use COPVs in their launch vehicles.  I've no idea how many other launch vehicles use COPVs, but this same sort of failure happened to Falcon 9s when frozen LOX became trapped between the CFRP overwrap and the tank liner, causing the COPV to explode and destroy those vehicles.

GW,

If getting accurate reports on a giant moon rocket that blew up on live television here in America is more difficult than it ought to be, imagine how difficult it might be to get accurate reports on far more complex geopolitical topics.

Most of the video you're watching, whether on TV or online, is actually slow-motion footage of what happened, which is why there appears to be a brief delay between the initial and primary / major explosion.  The clip on Scott Manley's YouTube channel that came directly from the NASA Space Flight TV channel shows the explosion in real time at least once, and that happened in almost the blink of an eye.

Start watching this video from Scott Manley at the 1:05 (one minute, five seconds into the video) time marker:
SpaceX's Latest Starship Explodes During Ground Test

The two NSF Channel narrators are casually talking with each other about the particulars of the test, the screen goes white in less than a second, and then one of them screams, "Whoa!  Whoa!  What!?  No!".

You're correct when you state that this was NOT an engine failure.  No engines were running at the time of the explosion, because propellant was being loaded into Starship 36 (a V2 variant of Starship) on a test stand.  No Super Heavy booster or Raptor engine was involved in this explosion.

The source of the initial explosion was either the header tanks or the COPVs sitting right next to the header tanks inside the Starship, which Scott Manley has detailed footage of, later on in the same video I linked to above.  Whatever initially "let go" inside of Starship 36 was powerful enough to open up the top half of the vehicle like an empty beer can, followed almost immediately by an explosion of the LOX and LCH4 in the primary propellant tanks.  Those troublesome COPVs are under something like 400 to 500 bars of pressure, so I'm thinking that was sufficient overpressure, most likely accompanied by COPV shrapnel, to rupture the propellant tanks inside Starship and ignite the LOX/LCH4 vapors.

GW,

The Democrats lost on all accounts- House, Senate, Presidency, popular vote, and Electoral College.  Republicans not winning the popular vote has been a favorite Democrat talking point over the last decade or so.  Your attempt to move the goal post is very predictable, but still falls flat.

By that metric, there was never a mandate for all the nonsense imposed on us by the Democrats, either.

Off the top of my head:
1. Creating a pathogen in a Chinese lab, spreading knowingly false lies about COVID's origins and vaccines, lockdowns and resultant economic destruction
2. Allowing biological men pretending to be women into women's bathrooms, locker rooms, and sports
3. Spending hundreds of billions of dollars on "green energy", that ultimately produced a lot of nothing
4. Blatant racism against white and Asian people in hiring practices, recently overturned by SCOTUS in a 9-0 decision
5. Attempts at de-facto firearms bans which were repeatedly struck down by SCOTUS
6. Using the FBI, IRS, CIA, and NSA against political opponents using political smears as "evidence"
7. Attempts to convict a President of a crime where no codified law was ever broken, nor injured party
8. Abandoning our allies in Afghanistan to the Taliban, in order to start the next pointless war in Ukraine
9. Defending anti-semitic violence perpetrated on Jewish students in our universities
10. Purposefully lying to the American people about the mental faculties of President Biden

Democrats never had a mandate from anyone to do any of that idiotic nonsense.  If they ever thought they did, I hope President Trump being elected to a second term in office by the majority of the electorate disabused them of that notion.

That's your personal opinion, stated as if it was a fact, but you've lost objectivity so you cannot tell the difference.

Do you make any distinction between arrests of organized foreign criminal gang members and drug cartel members, who claim to be members of said organization, and "ordinary" illegals?

Do you remember "Maryland man"?

That narrative was Democrat media knowingly falsely portraying a foreign criminal gang member as a local family man who just happened to be an illegal, after he'd already told our federal agents exactly who and what he was, because he didn't think it would have any effect on where he ended up.  He was wrong, and so were all the Democrats who were lied to by their own media.  They were convinced that little cretin was some kind of saint who was unjustly targeted because he was an immigrant.  He was a human trafficker, someone who was involved with selling children and young women into sexual slavery, who beat his wife so badly that she called the Police asking for a restraining order, before figuring out that they felt no obligation to protect her from her abuser.  That's the kind of filth our Democrats and their propagandists carry water for.  I never was the least bit confused over who / what he was.  Democrat media is just repulsive filth.  That's all.

US Immigration and Customs Enforcement Administrative Arrest Statistics

When more than 10 times as many arrests were being made between 2020 and 2024, under President Biden's administration, because his ICE appointees weren't actually removing said criminals from our country or incarcerating them, is your assertion that all those arrests were completely above board because President Biden was in office, but now that we're actually getting rid of real criminals under President Trump, and we require fewer and fewer arrests as a result, you think that's indicative of Gestapo-like malfeasance?

Anybody equipped with a pair of functional eyeballs can look at those real facts and figures provided by ICE, but I don't think you ever have.

How many interactions have you ever had with multinational gang members and drug cartel members?

I'll bet that number is zero.

Speaking of nobody being entitled to their own facts, you, sir, are not entitled to call a riot a "loud public protest", nor any kind of a protest, when there are literally thousands of pictures and videos of people dancing on burning cars in the streets that don't even belong to the government they're supposedly protesting against, throwing Molotov cocktails at the Police and assaulting them, looting stores, and preventing hundreds of thousands of people from getting to work.  Words have meanings, so let's start honoring the meaning and intent of the words.

That is a real fact, not my opinion.

Just another unsurprisingly typical image of the aftermath of your fellow leftist's riots, falsely labeled as "mostly peaceful protests" by yourself and your fellow leftists:

Nothing says "proud to be an American" like burning someone else's car in the street, looting, and waving Mexican flags:

More "mostly peaceful" arson:

Mostly destroyed Police vehicles that the tax payers get to dig into their pockets to replace:

Just look at how peacefully that building is burning to the ground:

Those Waymo cars must really be threatening to your fellow leftists:

Oh, look, they even respect law enforcement:

Those are real "facts", captured in photos and on video tape, for the entire world to see.  I can and will keep posting more, endlessly if required, until you get the point.  Those are all images of R-I-O-T-S, taken over the past week, from places like Los Angeles, New York, and Chicago.  There's nothing peaceful about them.  I can tell you for a fact that I value the lives and property of leftists, which is why I never treat their communities the way they treat them- like trash.

What you are receiving here are images of all your "mostly violent riots", where there does in fact seem to be a rather unhealthy obsession, amongst those on the left, with actual "flaming" of the property of others.  Those images are not right-wing or left-wing in nature.  However, they are entirely the work product of real left-wing extremists.  You'll have to pardon me if it's a bit hard to hear the actual "protest" over the Police, Fire, and EMS sirens responding to all the looted stores, burnt cars and buildings, and broken or dead bodies those riots leave in their wake.

If you don't like America's immigration policies, then petition Congress to change the laws.  The Democrats have held the Presidency, Senate, and Congress during periods of time when all their fake whining and crying over our immigration laws took places, yet your Democrat politicians, who you voted for, quite clearly refused to simply change the law to say that anyone is allowed to come into the United States for any reason.

BTW, join the crowd, bubba.  My wife's sister has been patiently waiting her turn to immigrate while all these criminals bum rushed our border.  I stopped counting how much money we've paid to the US government and the Vietnamese government for paperwork processing fees and background checks, and how many years we've patiently waited our turn.  Oddly enough, she's already learned to speak English, she has a college degree- a science degree rather than a lesbian interpretive dance degree, she's been employed for her entire adult life, and she'll be waving an American flag, not a Vietnamese flag, if/when she finally enters our country- LEGALLY!

I'm married to a LEGAL first generation immigrant, who treats both her American citizenship and our flag as priceless personal treasure, worth any amount of sacrifice.  The only time I've ever seen her with a Vietnamese flag was to place one at the war memorial for her father and uncle, along with an American flag.

I don't take any issue with him relaying his opinions about Ukraine and Russia.  He's entitled to express them like everyone else is.  The only issue I take is over all the half-court tennis games you guys like to play.

tahanson43206,

I was thinking truly superlative spacecraft radiator panels are the most obvious first application for the "blackest of black pigments".  Near-perfect black bodies are superb at both absorbing incoming radiation and re-radiation, though not necessarily at the same wavelength as the incoming photons.  I guess that's a roundabout way of saying that while black pigments readily absorb visible spectrum photons, they also tend to be really good at both absorbing and re-radiating infrared photons.  If said panel was held in shadow, then it would very efficiently radiate heat into space.  If the panel was exposed to sunlight, then that same black pigmentation property would work against you, unless you actually wanted it to absorb heat from the Sun.  Ordinarily, rejecting excess heat is the primary environmental control problem for a spacecraft, even in deep space far from a star or other warm planetary body.

I don't know how "ultimate black" pigments will help or hurt stealthiness because almost nothing in nature is black.  I would think that it would make visual observation exceptionally difficult at night, but that same optical property draws the eye during daylight hours.  Modern stealth aircraft significantly reduce their IR signatures by rejecting heat through fuel-air heat exchangers that use their fuel as a heat sink to absorb and concentrate heat from aerodynamic heating, onboard electronics, and the portion of the airframe surrounding the engine(s).  A very black pigment could make said heat exchangers more efficient at thermal power transfer into the fuel or incoming airstream.  Automotive radiators are painted black for this reason.  If we had "super black" radiators, then they'd be more efficient.

GW,

There was no booster underneath the Starship that exploded.  It was a Starship sitting on a test stand by itself, not the Super Heavy booster, loading propellant into its tanks.  The explosion appeared to start near the nose of the Starship where the header tanks are located, and it happened live on-air, on the NASA Space Flight channel.  If whatever you watched showed a booster stage underneath it, then that was AI-generated nonsense.

tahanson43206,

The M10 Booker was always a bad idea, one previously killed by the Army decades ago when it appeared in the form of the M8 Buford, which was arguably a better light tank that met more program targets, since the M8 was at least C-130 transportable, mounted the same 105mm main gun used by the earlier versions of our Abrams at that time, had excellent cross-country mobility on account of its light weight / low ground pressure, and could cross most bridges in Europe without damaging them.  The M10 is neither a weight-optimized light tank like the M551 Sheridan, which is what our airborne forces requested, nor does it mount a modern 120mm caliber gun more suitable for taking out the latest generation of main battle tanks.  The Sheridan's 152mm main gun was optimized for supporting infantry assaults by busting field fortifications and keeping heads down with highly effective HE shells, with a secondary capability to fire gun-launched ATGMs at enemy tanks.  In contrast, the M10 was never optimized for any specific target set that our airborne infantry wants to take out.  It's more expensive than the Abrams main battle tank, lacks both the 120mm-class main gun armament and heavy composite armor of an Abrams, and its HE shells are much smaller than they could be if it was optimized to fire 155mm caliber shells over short distances from a compact and light direct-fire howitzer type gun.

After seeing what our new 50mm Bushmaster chain gun can do to armored vehicles and drones, and watching Ukrainian Bradley IFVs smoke Russian T-72s using their far less powerful 25mm chain guns in Ukraine, merely by flanking the Russians to take shots at their weaker side and rear turret armor, my argument would be for a light semi-autonomous uncrewed mobile gun system, similar in concept to Russia's SPRUT 125mm anti-tank field gun.  SPRUT is nominally a towed variant of the T-72's main gun, but with its own onboard engine to move itself about the battlefield without a separate prime mover vehicle, except for long distance travel.  Our take on their idea would be even more mobile, so able to go most places infantry can go on account of its small size and low weight.  Our miniaturized version of Russia's self-mobile direct-fire anti-tank field gun concept would minimize its mass and physical size by not attaching a giant attention-getting vehicle to the gun.  We'd use the same tricycle wheel arrangement of the SPRUT with a small onboard engine providing running pace motive power.  If the gun can move itself at 10mph over rough terrain, basically as fast as a man can sprint, that's all we really need it to do.  The gun moves with the infantry, not ahead of them.  Short of a mountain top or house, such a small gun can go pretty much anywhere else that our soldiers can go, so they always have a medium caliber cannon nearby that can fire programmable airburst HE shells at field fortifications or helicopters and APFSDS at armored targets.

Russian SPRUT 125mm Self-Mobile Anti-Tank Gun

It's not a perfect solution, but a C-130 could easily carry multiple air-droppable guns onboard, something not remotely feasible with a light tank, which is far too heavy to parachute deploy.  The UH-60 can easily sling-load such a light gun as well.  Each self-mobile gun system would weigh 550-600kg.  That's about 50-100kg lighter than the old but still useful 1927 vintage 653kg 75mm M116 pack howitzers that were man-handled through mountains during WWII, and remain in service to this day.  The entire weapon would be 4.5-5m in length, with a width of about 1.5m across the outside of the main wheels.  A small tire would be locking collar-mated to the muzzle for transport.  Onboard ammo supply would be limited, no more than 20 rounds or so, but that's sufficient.  The gun would be operated in single shot mode, rather than fully automatic, to minimize stabilization weight.  Every soldier in an airborne unit would carry 3 to 5 additional rounds of ammo in their pack to feed their unit's cannon.  In the same way that WWII Wehrmacht units were organized around their machine gunner or light wheeled autocannon or pack howitzer, American units would be centered around their autocannon.

In our coming fight with the Chinese over Taiwan, we would "mix-and-match" units equipped with 30mm (a belt-fed version of the Apache's autocannon), 25mm (Bradley), 50mm (Bradley replacement), heavy machine guns or grenade launchers (normally used to equip jeep-like vehicles), and Javelin (anti-tank) or Stinger (anti-drone / helicopter) launchers.  Heck, we can probably make a version that mounts a few Peregrine missiles to pop any enemy fighters that venture too close.

M242 25mm Chain Gun

XM913 50mm Chain Gun

Cannon Size Comparison

Note:
The 35mm and 50mm chain guns are almost identical in size.  IIRC, the 35mm and 50mm caliber units only need to be " barrel swapped" to change calibers.  The 35mm cartridge's base diameter and overall length is the same size as the 50mm cartridge, merely "necked down" to 35mm.  The Germans use 35mm caliber for short range air defense, but we don't.  We prefer IR-guided missiles.

34 Round On-Gun Linkless Dual Feeder Example 35mm Oerlikon KDE Cannon

By trading absurdly conspicuous / expensive / fuel-thirsty vehicles, we'll save money on destroyed vehicles and fuel and destroyed ships that must supply the fuel and munitions.  Some weapons will inevitably be lost to enemy action, but we're about to fight another island-to-island campaign in closed terrain.  We don't need any more ponderous land vehicle units for the enemy to attrit.  We need more firepower that moves with and protects our infantry in close quarters.  Protection takes many forms.  One of the most underrated forms of protection is a reduced target signature and physical size, which is what this proposal would provide.  Any land that is not directly "taken" from the enemy by a man with a rifle in his hands is not something we actually control, and many of these places are simply too small to drag a critical mass of heavy mechanized units ashore to dominate the battlefield.  I've already been to some of these islands.  They're the size of postage stamps.  You can walk across them in an hour or two, and I have.  Even roaming around in a tank is akin to waving around a giant "Shoot Here!" sign.  The armored "thunder runs" to places like Baghdad were only possible due to the vast open spaces of Middle Eastern deserts.  We won't be fighting that kind of war.  WWII island hopping campaigns were all about bringing men and supplies ashore, using radios and offshore naval gunfire for heavy fire support, and traveling light and fast for shock effect.

Buran, Space Shuttle, HOTOL Size Comparison:

Gross Liftoff Mass; Orbiter Vehicle Mass; Max Payload Capability
Buran: 2,524,000kg; 75,000kg; 30,000kg (presumably 251km circular orbit)
Space Shuttle: 2,029,633kg; 78,000kg; 24,400kg (204km circular orbit) or 12,500kg (407km circular orbit)
HOTOL: 250,000kg; 50,000kg; 8,000kg (300km circular orbit)

HOTOL LH2 vs RP1 Mass and Volume Comparison for Equal Stored Energy
1. HOTOL had sufficient internal volume to store 995m^3 (70,495.75kg; 10,080,892.25MJ) of LH2 fuel, at most, ignoring the fact that some void space must be present to vent boil-off.
2. 10,080,892.25MJ / 43MJ/kg = 234,439.35kg (285.902m^3) of RP1
3. HOTOL also had up to 140m^3 (159,740kg) of LOX onboard, at most, which implies that 26,623.33kg (3,807,136.67MJ) of LH2 was consumed using pure rocket propulsion (presuming the typical 6:1 O/F ratio).  88,538.06kg of RP1 provides equivalent energy, which implies that 230,198.96kg (201.752m^3) of LOX (presuming the typical 2.6:1 O/F ratio for RP1 fueled engines) must be carried aboard an RP1 powered HOTOL variant for pure rocket propulsion.
4. That means total onboard propellant volume for a RP1 powered HOTOL equivalent is 487.654m^3, so 230,198.96kg LOX + 234,439.35kg RP1 = 464,638.31kg of total propellant mass.
5. The LOX/RP1 propellant mass is slightly more than double that of HOTOL's stated LOX/LH2 propellant load, but the total propellant volume in the vehicle is less than half of the LH2 volume alone.  The TWR of RP1 fueled engines is more than double that of LH2, so engine mass would be significantly reduced, which directly translates into greater payload mass since HOTOL is a SSTO.

The most obvious "better option" associated with choosing RP1 power is the ability to carry significantly more useful payload mass to orbit in a vehicle with the same internal volume as an equivalently-sized LH2 powered vehicle.

For clarity, in the first illustration above you will see a different variant of the HOTOL vehicle with 995m^3 of tank volume allocated to LH2 (70,496kg at most), plus 114m^3 (130,074kg) of LOX tank capacity, and possibly up to 140m^3 of LOX, although the actual capacity of the aft LOX tank near the engine cluster is not provided.  LH2 fuel mass for HOTOL is listed as 48t, but there were multiple variants of the vehicle and I don't have definitive data to know which propellant capacities applied to which specific variant.  What ought to be readily apparent is how much internal vehicle volume LH2 fuel requires.

Engine Type Specific Impulse with Hydrogen vs Hydrocarbon Fuels:

University of Texas at Arlington - Aerodynamics Research Center - Pulsed Detonation Engines

PDE = Pulsed Detonation Engine (RDE is a type of PDE)

Presumably, what the chart actually means is that a RDE being fed atmospheric Oxygen can achieve that kind of Isp, because pure rocket propulsion Isp is shown at the bottom of the chart, and ranges between about 360s (RP1) and 470s (H2).  Detonation engines don't achieve substantially greater Isp (thrust generating efficiency) than conventional deflagration-based chemical rocket engines when all of the oxidizer and fuel must be carried onboard the vehicle.  I think the term applicable here is "Effective Isp".  When the atmosphere can be used as reaction mass and to provide the oxidizer consumed to generate thrust, the Effective Isp for turbojets, turbofans, and pulsed engines greatly exceeds that of pure rocket engines, to include PDEs / RDEs.

Anyway...

In my proposed SSTO vehicle design using vertical launch and pure rocket propulsion, propellant mass was 2,185,289kg to deliver about 100t to orbit.

SSTO Engine Technology Post #29

1,875.496m^3 of LOX/RP1 propellant was required.

SSTO Propellant Masses and Volumes for a STS Orbiter Equivalent Payload to Orbit:
LOX: 1,597,846kg (1,192.422m^3)
RP1: 587,443kg (683.074m^3)
Total: 2,185,289kg (1,875.496m^3)

Saturn V S-IC Stage Propellant Masses and Volumes:
LOX: 1,444,506kg (1,266m^3)
RP1: 647,800kg (790m^3)
Total: 2,092,306kg (2,056m^3)

This was intended to deliver the same total dry mass to orbit as the historical Space Shuttle, inclusive of its nominal max payload.  Effective Isp over the entire flight regime (sea level to orbit) was 90% of the RD-180's Vacuum Isp.  Engine mass was derived from Raptor 3's 200:1 engine TWR and from the requirement for a 1.5:1 vehicle TWR at ignition, in accordance with GW's admonishment to ensure that liftoff thrust is sufficient to get the vehicle "smartly moving downrange".  This was presumed to be feasible on the basis of Raptor 3's 200:1 TWR while burning LCH4 fuel, which is only about half as dense as RP1, and consequently requires larger and heavier turbopumps than a RP1 burning Raptor 3 thrust-equivalent.  Essentially, if a 200:1 engine TWR is achievable with LCH4, then it's also achievable with RP1.  If anything, the engine should get lighter and provide a higher TWR because the fuel pump requires less mass / volume / input shaft power to deliver the same flow volume to the combustion chamber.  There's nothing too outlandish about these assumptions.

When we talk about a pure rocket powered SSTO designed to deliver 500 passengers to orbit, we're discussing a vehicle with roughly double the Space Shuttle orbiter's total internal volume, on-par with the S-IC stage of the Saturn V, but using much lighter and stronger structural materials, modern thermal protection materials, and thrust equivalent to the 5X F-1 engines attached to the S-IC stage, albeit using much lighter / more powerful engines.  If we're able to substitute the onboard oxidizer mass and volume by using air-breathing engines up to normal booster burnout velocity, then our vehicle's total internal volume becomes more similar to that of the Space Shuttle orbiter or HOTOL design, and we can say with a fair degree of confidence that such a vehicle will be able to attain orbit and return to a runway on Earth, just as the Space Shuttle did.

Now back to the "big idea" behind using RDEs:

Thrust-equivalent RDE should require half as much input shaft power, thus much smaller / lighter pumps, and consequently, a bump in engine TWR from 200:1 to 400:1 ought to be possible using the same materials (high grade stainless steels and Nickel-Copper alloys.  RCC is only 2g/cm^3 vs 8g/cm^3 for stainless and Nickel-Copper alloys, so a 800:1 to 1,600:1 engine TWR becomes feasible.  Since NASA has already tested scale RDEs and RCC engine components (the combustion chamber and nozzle), I think the material and engine tech are "ready to apply", even though more development is still required.  RCC turbopump components were fabricated and tested for Project Timberwind, many moons ago.  It's not outlandish to think that all the major components can be fabricated from RCC and that regenerative cooling is largely optional.  We may still choose to extract heat from the combustion chamber and nozzle to reduce pumping power, not because the engine would melt if we did not.  At 800:1 TWR or higher, even when Saturn V thrust levels are demanded, engine mass becomes a minor fraction of total vehicle mass, so propellant tank mass and TPS mass will become the focus of development work.

All together now, we're using supersonic air compressors to extract atmospheric Oxygen and atmospheric Nitrogen to use as reaction mass, possibly driven by sCO2 gas turbines that transfer heat from the compressed air into the fuel, to force feed gobs of air and vaporized fuel into rotating detonation wave engines, so that our sea level to orbit propulsion system can generate the thrust required for a mass that a SSTO vehicle can tolerate.