Fuel Economy

One of the most significant obstacles to the introduction of an SST is the perceived environmental impact of what will inevitably be considered a ‘gas guzzler’…
One of the most persistent criticisms of Concorde was its thirst for fuel: it’s true that a supersonic aircraft will burn more pounds of fuel per hour than an equivalent sub-sonic, you are paying for the extra speed. The very same relationship between power and economy is true for any vehicle – probably the most tangible example of this would be when riding a bicycle.
If you are cycling from point A to point B, of course if you are in no particular hurry, you can gently turn over the pedals without exerting yourself and arrive without having broken much of a sweat – the watts of power you exerted, will have been less than if you were running late for a train for instance, and you were pedaling with haste. The additional effort you exert when hurried, or when racing, will translate you greater output power, but then also a greater loss of energy through the increased friction of the tyre on the road, the chain on the gears and the bearings in the wheels – most noticeably, the increased speed will incur greater air resistance, which will in turn limit your speed – if you try to overcome the air resistance, your effort will increase yet further, and so on. Arriving sooner entails greater expense of energy, of any vehicle – no better example can be physically experienced, than when under your own propulsion.
Quoting from Douglas Ross in the Bulletin of the Atomic Scientists in ‘The Concorde Compromise’, 1978 here...
‘Assuming a full load for each plane, Concorde obtains 15.8 passenger miles per gallon compared to 33.3 for the old Boeing 707; 44.4 for the DC-8-61; 46.3 for the Boeing 747 and 53.6 for the DC-10.’
More recent aircraft have reduced this fuel burn yet further, with some more recent aircraft in the same range bracket achieving almost 100 passenger miles per (US?) gallon…
Perhaps it is not fair to judge all potential future SSTs economical viability on the performance basis of the first generation designs, innovations in materials, FADEC engine systems and e-taxi ground propulsion may negate a great deal of the inefficiencies in future SSTs, but there is still no avoiding the fact that, in order to reduce your journey time by half, your ‘carbon footprint’ will be roughly 4-6 times greater.
If we casually ignore the fact that there is a market for Boeing Business jets (including 747-8s), Bugatti Veyrons and luxury yachts in spite of their opulent costs and extravagance, we must somehow justify or mitigate the anticipated resistance to the emissions argument against use of SSTs if we are to have a chance of winning their acceptance.

Carbon Emissions

More than any other measurement, the so called ‘carbon footprint’ seems to be more important in the realm of environmental advocacy than any other, with nitrogen dioxide rapidly gaining in alarmism as well.
Nitrogen dioxide, with its 48 hour half life, is easier to explain away, but carbon dioxide is enemy number 1.
Alternatives to traditional kerosene burning jets, for the power levels we need include the options of nuclear power, such as that researched on the NB-36H and the HTRE3 project of the 50’s, but was quite categorically proven to be a non-starter.
Elon Musk also quite famously mooted his idea for ‘an electric jet’, which even made a Cameo in ‘Iron Man’, but the technology behind this daydream is as yet unrealised.
Options of hydrogen fuel and liquid natural gas have been researched as well, but also have been shown to be unfeasible due to the need to store the fuel in pressurised refrigerated large tanks.
So, there are no real alternatives to hydrocarbon based fuel for the purposes of our modern SST, the answer to our problem of carbon footprint lies in the source of those hydrocarbons.
The solution then, is to use carbon-neutral* biofuels, which still result in the same emissions as regular crude-oil derived kerosene, but at least in theory create a smaller upset in the carbon budget that environmental advocates are most concerned about.
*Carbon neutral being the fuel itself, not the industrialisation of producing it, which may still be fueled by crude-originated diesel powered equipment, often in effect rendering it non-carbon nuetral.
Biofuels for US Airforce
The engines to power our SST then must necessarily be modified to accept biofuel-derived kerosenes, from base stock origins of algae, switchgrass, jatropha, and have appropriate pumps and filters designed into the fuel system. (Biofuels tend to me of higher viscosity than crude-derived oils)
Optionally, there is one other possible source in lieu of crude oil, but it reintroduces the carbon footprint problem, of sourcing kerosene from coal via the fischer-tropsch process…
Fischer Tropsch
This process was famously invented during WW2, when Germany had difficulty sourcing crude oil, but had plenty of coal – nonetheless this does not resolve our environmental footprint problem, so should be cast aside – however, ultimately the market decides which source becomes the most prevalent, and although an intent to use carbon-neutral biofuels should be kept in mind, the ultimate winner may well simply be the cheapest option.

What Does this Mean for our SST?

With respect to the Grassroots SST concept keeping the above in mind, the fuel burn will be high, and CO2 emissions will be high – we know that, it is unavoidable, and this will be a limiting factor in the usable range of the aircraft and it’s inevitable ‘carbon footprint’.
The high fuel burn can be partially negated simply by carrying more fuel – carry enough to get where we’re going, shift the payload-range diagram to the right a bit – for our SST, I envisaged that at a minimum, like the F-104 starfighter, Learjet 25 or even the Jet Provost, the aircraft will gain wingtip range-extender tanks – which conveniently also reduce some transonic wave drag due to Sears-Haack effect.
The high CO2 emissions are indefensible, we can’t hide them, and we can’t pretend they won’t be there, because they will be. You create CO2 from burning hydrocarbons to get energy. C’est la vie. BUT – as already mentioned, carbon nuetral hydrocarbons can be sourced from Biofuels, and some Airlines and Air Forces are already doing this, with Jatropha, Algae or Switchgrass derived kerosenes. This will be our approach too.