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Mark Goll's Rocket Science for Earthlings (late 90s vintage) is still fun to read, although it flies in the face of recent SpaceX design philosophy.  Mark is a fan of MCD Big Dumb Booster rockets.
https://storage.googleapis.com/trctsi.net/rse0.htm

Now that Musk is close to perfecting fully reusable rockets, it appears questionable that expendables could actually be minimum cost design.  But Goll's calculations do appear to show the benefits of frequent staging, with 3 stage to orbit having the benefit of allowing for higher structural mass, which is beneficial to reusability.  From memory, about one third of total propellant mass burned during a shuttle launch was used accelerating to 1000mph.  A very cheap, high dead weight but high thrust lower stage, could provide the initial boost needed to accelerate a rocket to 1000mph.  Not quite SSTO, but more like 1.2STO.

Mercury is a volatile rich planet.  There are even speculations that life could exist beneath its surface.  Who would have guessed that even a few years ago?

This will be a tough place for humans to visit, partly because it is so deep in the sun's gravity well.  Any ship entering orbit of Mercury would need to rely on numerous gravity assists with Earth and Venus to dissipate sufficient orbital energy such that relative speed to the planet is low enough to enter orbit.  Landing on the planet won't be easy.  Mercury has substantial gravity and no atmosphere at all to brake a spacecraft.  So all dV must be propulsive.

That being said, the planet could offer opportunities for human settlement.  The lack of atmosphere is actually a bonus so close to the sun.  There is nothing to distribute heat, allowing areas close to the poles to remain temperate or cold.  The planet has virtually no axial tilt, so does not experience seasons.  That is an advantage as well, as it allows for predictable temperature ranges that never get too hot at high latitudes.  Closer to the equator, patches of the planet can be kept cool by suspending reflective mirrors above the surface.  These mirrors would be aluminised foil suspended on steel cables running between towers or pillars.  Solar energy provides all of the energy that human colonies will ever need.

Without any sensible atmosphere, mass drivers would work efficiently on the surface.  These would need to be housed in tubes and covered with a metre of regolith to keep the structure cool.  But export of materials from Mercury surface into solar orbit will be much easier than from Earth or Venus.  Mercury would be a good place for optical and infrared astronomy during its long night.  After sundown, the surface begins to cool, reaching temperatures of around 100K.

Helium is a rare and non-renewable resource on Earth.  I don't think there would be enough of it to make airships anything more than a niche curiosity.  The Hindenburg was huge.  Its gas cells had a volume of 140,000m3.  It also vented hydrogen as it flew and fuel consumption reduced its mass.  That wouldn't be economically possible with helium.  Helium also generates less lift than hydrogen.

I think the question with airships is whether hydrogen can be used more safely with modern materials?  We won't be making gas cells out of cotton lined with cow guts anymore.  We have advanced polymers, carbon fibre and super-strong maraging steels.  We also have gas turbine engines that are lighter and more powerful than the diesels on the Hindenburg.  How much difference would all of that make if we built a Hindenburg sized rigid airship today?  Probably quite a lot.

But safety was only one of the problems with rigid airships.  Who woukd want to cross the Atlantic at 60-70mph, when there are jets that can get you there 8-9x more quickly, probably at a lower cost?

This graph from Peak Oil Barrel is sobering.

This is world crude oil production from 1900 until 2023.  It looks to me like we are past global peak oil.  Were it not for North American shale & syncrude, the peak year would have been 2005.  Those things pushed it out another 13 years.  God bless America!  The graph refers only to crude.  There are other liquids like NGLs, condensates, biofuels, etc.  But the direction of travel is clear enough.  Could solar synfuels fill the gap?

This article discusses the possibility of cold district heating for densely populated urban areas.
https://theconversation.com/no-space-fo … ing-180005

In this concept, every dwelling would have a heat pump.  But the heat supplying the heat pumps would be drawn from a water pipe carrying cold water that supplies entire streets.  These water mains would extract heat from boreholes, which would store summer heat for use in winter.  The distribution pipes don't need to be anything special.  Just a big concrete underground pipe that serves as a heat source for individual heat pumps.

One thing I did notice when reading about heat pumps, is that larger systems appear to achieve scale economies.  The purchase cost of a heat pump providing twice as much heat will be <2x greater.  It therefore makes sense providing heat pumps that serve multiple homes.  That is the most capital efficient solution.  But there does need to be an authority that does this on behalf of a town or it won't happen.  It requires collective action.  Probably the best solution would be to run the cold water pipe down main streets.  Heat pumps would be located at street intersections and would provide heat to entire branching streets.

For residences that don't have space for a ground source heat pump, maybe a tank of water could be used as the heat source?  Lets say we start with water at 7°C and use a heat pump to suck heat out of it freezing it.  For each litre of water frozen, the heat pump will extract 0.1kWh of heat.  A cylindrical tank 3m in diameter and 3m tall, would provide some 2100kWh of heat through its phase change to ice.  That is enough heat to meet the heating needs of a well insulated house for about 1 month.  When temperatures outside rise above freezing, we would open vents and use a wind catcher to blow air through the ice store, gradually remelting it.  During summer, we would use the wind catcher to collect warm air, preheating the tank ready for the beginning of heating season in the autumn.

Something like this would work in locations where temperatures only occasionally dip beneath freezing and only stay beneath freezing for a few weeks at a time.  Most of Europe meets that description.  Deep ground temperatures average about 10°C.  Here is seasonal average temperatures for Edinburgh, which is colder than most of UK.
https://www.metoffice.gov.uk/research/c … /gcvwqum6h

The tank is effectively a thermal capacitor, drawing energy from the wind.  If we take the average windspeed to be 6m/s, the specific heat of air to be 1KJ/kg.K, its density to be 1.22kg/m3 and assume we drop its temperature by 1°C.  Assuming a 1m2 catchment area, how much energy flux can we get into the tank when outside temperatures are above freezing?

Q = 1 x 6 x 1.22 x 1000 = 7.32kW

This comfortably exceeds the heating loads of most residences.  Drawing heat from a source at 273K and providing heat at 303K (30°C) woukd give a Carnot COP of about 9 in tge dead of winter.  In autumn and spring, temperatures will be higher and COP will be better.

Installation costs are a lot cheaper than other estimates I have heard in the past.  But they won't include all of the changes that have to be made to a property in order for heat pumps to provide enough heat.  Properties must be well insulated, because achieving a high COP is not compatable with high heating temperatures.  The lower the temp difference between radiators and the room, the lower the heat transfer rates.  In fact, the best COP would be achieved by heating water to room temperature.  But the amount of radiating area needed would be huge.  Underfloor heating might work.  Embedding heating pipes in walls would turn the walls into radiant heaters.  That would be a good option for new builds.

In areas where urban development is too dense for ground source, a pipe containing flowing sea water could provide the heat source.  It doesn't have to be warm, though it always boosts COP if it is.  We could dump nuclear waste heat into a city sized salt water main.  Suppose we have a pipe 50cm (1.5') in diameter, with saltwater flowing through it at 3m/s.  Its temperature is 10°C and we remove heat from it until temprrature drops to 5°C.  How much heat could it provide?  Ans = 12.37MW.  That is enough for a large town.  The COP will beat air source heat pumps by a large margin, because there is no need for a high dP fan in a ground source heat pump.  During summer, we would run the salt water mains in reverse, taking summer heat from the tarmac of roads and carparks and using it to recharge subsurface heat reservoirs.  This ensures that come winter, the salt main temperature can be kept at 10°C, regardless of the outside air temperature.

Vacuum insulated thermal storage.
https://www.researchgate.net/publicatio … al_Storage

I have not read this yet.  I will post a synopsis when I have.  It is certainly an interesting idea.  Steel vacuum tanks are relatively expensive because of the problem of buckling instability.  Concrete tanks are far more affordable and can be built to almost arbitrary size.  A thermal mass contained in a vacuum tank would only lose energy by conduction and radiation.  Polished aluminium surfaces can reflect most radiated heat.  So a vacuum insulated thermal mass could be used to store energy for a long time.  This would seem to me to be a good option for interseasonal energy storage.  This is one of the few ideas that might work.

A concrete vacuum tank some 100m in diameter wouod have a minimum thickness of 0.25m, which is tad under 1'.  We would probably built these tanks with multiple cross-braced layers to make them stable against buckling.  The easiest way would be to cast hollow hexagonal blocks that are then fitted together with cement.  The inner surface would then be clad with aluminium coated rockwool slabs.  Within the vessel would be a spherical steel shell, containing a mixture of rock, sand and compacted clay.  This would be heated to temperatures of ~600°C.  The whole assembly would sit of a steel sheet, mounted onto a layer of vermiculite, which in turn transfers load to a concrete foundation.  Beneath the steel sheet would sit cooling tubes containing sodium nitrate, which would transfer heat to an S-CO2 power generation loop.

They might do better storing anhydrous sulphur dioxide as a chilled liquefied gas.  Its boiling point is -10°C. Concentrated sulphuric acid will dissolve almost any container on a timeframe that is short compared to the life of the plant.  Decomposition of sulphur dioxide requires temperatures up to 950°C.  That is difficult to achieve even with concentrated solar and corrosion will be a severe problem.  I can't see this solution having much milage.  The iodine sulphur cycle was once touted as a promissing method of converting concentrated solar heat into hydrogen.  It has never advanced very much, because the temperatures needed are challenging and sulphuric acid is too corrosive.  Same problem here.

One thing that molten salt has in its favour is that it is non-toxic and does not produce dangerous gases.  Nitrate salts are also non-corrosive to stainless steels.

I have watched the first episode and part of the second.

Synopsis of No 1: Architects design buildings to win artistic awards.  Very little attention is paid to how well the design fits end user requirements.  So buildings are often a failure for their users, who attempt to change them straight away.  Surprisingly, architects rarely revisit past work after construction to see how successfully they work for users because it is 'too depressing' to see their hard work modified and screwed up.  Another important point from the episode is that building usage can change with time.  Architect designed buildings are often difficult to modify.

None of this is surprising, as we have all seen it in action.  It is however bewildering that these sort of practices have gone on for so long.  They suggest that architects have consistently failed to deliver good value in their products.  In most other industries that would put them out of a job.  This series was aired in the late 90s.  Maybe things have changed since then?

I have only seen the first 10 minutes of the second episode.  It is discussing peripheral buildings - things like workshops, garages, outhouses, containers, etc.  These are not usually architect designed.  The narrator makes the point that this is where the real life of a society is because people have the space and freedom to do things here without it costing a fortune.  This resonates with me, because it is these spaces that are really lacking in British towns and cities.  Everyone is crammed into flats and terrace houses like sardines.  The average person just doesn't have the space for a workshop or a studio that they can use to develop things.  This has a crushing effect on creativity and productivity in the UK.  We have less living space per capita than just about any other developed country save Japan and Holland.  Startup companies need places to start up.  There is only so much you can do in the spare box bedroom.

One thing I neglected to include in the opening post of this thread, was the amount of heat consumed by food as it cooks.  Ignoring the effects of evaporation, the specific heat of food varies significantly, but 3.7KJ/kg.K appears to be a good average.  Let's say we cook 1 tonne of food per day - enough for 1000 people.  The amount of heat the food will absorb just by heating from 10 to 90°C would be:

Q = 1000 x 3700 x 80 = 296MJ (82kWh).

That is almost twice as much heat as the store loses to conduction in the base case.  This suggests to me that there is limited point in making the thermal store much larger than 4m in diameter or using much better insulation.  The cooker is already almost as efficient as it can possibly be.

District heating has long been popular in Nordic countries. In Denmark, some two-thirds of households are connected to a district heating network. Early systems distributed heat through steam with heat supplied by combined heat and power plants. The use of solar, geothermal and pumped heat, have prompted interest in distributing heat at lower temperatures. The logical end point of this is cold district heating.
https://en.m.wikipedia.org/wiki/Cold_district_heating

In this scenario, heat is distributed at 10-25°C and heat pumps use the cold water as a heat source, producing warm water at temperatures 30-60°C for heating. This is much more efficient than using cold air as a heat source, because water is denser, does not require high pumping power blowing it over a heat exchanger and is likely to be warmer than outside air if it is drawn from the sea and passed through the ground layer. A temperature of 10-25°C requires a smaller dT to provide warm water at 30°C, than air at say 5°C. So heat pumps supplied in this way will be more efficient.

Cold district heating is in some ways easier than hot water distribution, because the distribution pipework need not be heavily insulated. The soil above a buried pipe may provide sufficient insulation. But it has the disadvantage of requiring the use of heat pumps at the consumer end. In densely populated cities, we could compromise and install heat pumps that provide warm water to entire streets, using a cold water main as the heat source. In this scenario, cold water mains probably containing sea water, will pass beneath main streets. Heat pumps would serve branching roads, which would carry heat pipes at 30-40°C. The cold water mains would carry sea water, sourced at a temperature of about 10°C. We could boost this temperature up to say 20°C, by passing the pipe through soil that has been used to store summer heat. The heat collecting surfaces would be roads, carparks, building roofspace and decicated flat plate concrete solar collectors, which would reach temperatures up to 30°C in the summer sun.

It is noteworthy that district heating could provide heating for cooking applications as well. In this case, we want a minimum temperature of 74°C to slow cook food. A district cook house could take heat from a heat main at 30°C and use a heat pump to raise temperature to 74°C. Carnot COP would 6.89. I think a real system should be able to give us at least half of that COP.