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As far as I know efficiency is what has kept civilian supersonic flight from being a thing. Even military aircraft tend to only go supersonic when needed becaus

by api 6y ago

As far as I know efficiency is what has kept civilian supersonic flight from being a thing. Even military aircraft tend to only go supersonic when needed because it guzzles fuel. My biggest skepticism about Boom is that I don't see them even talking about this issue much. If they can't get big efficiency improvements I see this being a niche toy for the super rich rather than a mainstream transport option. I'm not an expert, but I wonder: can you get large efficiency improvements by going to really extreme altitude where the air is thinner? Maybe that's the long term plan. But as far as I know that may require a novel engine design or the adaptation of military engines, and the latter is a minefield if classification and ITAR regulations. Extreme altitude also means you're dead if you get a major hull breach, but the increased risk there may be statistically mitigated by decreased overall flight time... and it's still going to be safer than driving a car.

tim333 6y ago
Extreme altitude also means there's less air to hold the plane up which seems to require a lot of power for level flight. I'm not sure about ballistic flight paths - a lot of fuel to get up there and then none for a while. Apparently re Musk's old Telas that he launched towards Mars: >It has achieved a fuel economy of 9,886.9 miles per gallon (4,203.4 km/liter, 0.02379 liters/100 km), assuming 126,000 gallons of fuel. https://www.whereisroadster.com/ https://www.whereisroadster.com/ But it's not on a very popular flight route. Should be kinda near Mars Oct 7th.
- GuB-42 6y ago
  > Extreme altitude also means there's less air to hold the plane up which seems to require a lot of power for level flight. There is also less drag, and both lift and drag are calculated using the same formula: Cl * A * .5 * r * V^2 - Cl (Cx for drag, Cz for lift) is a coefficient depending on the wing shape, among other factors - A is the area (thickness for drag, wing surface for lift) - r is the air density - V is velocity If you divide the air density by 4 by going higher, you get the same lift and drag by going twice as fast. Which is great, and a good reason for fast planes to fly high. The reason it is so great is that in order to fly level, lift cancel gravity and thrust cancel drag. Because by going higher, we managed to keep the same lift and drag with higher speed, it means we got faster using the same amount of thrust. There are certainly complications somewhere. For example at supersonic speeds you get shock drag but suffice to say, high altitude is good for speed. Still there is a good reason why planes don't just get higher to get speed for free. And that's because planes are not rocket powered. Air breathing engines lose efficiency the faster you get, they are also designed to operate at specific speed ranges.
mpweiher 6y ago
> Even military aircraft tend to only go supersonic when needed because it guzzles fuel. Yep. In fact, this is one of many things that makes the Concorde so special: being able to cruise for a long time at Mach 2+. Supercruise has only come to fighter jets fairly recently, and there it is usually in the Mach 1.4-1.6 range. This means that if the Concorde has even a small head-start, it can outrun most fighter jets that might want to intercept it. The SR-71 could keep it company but is unarmed. Maybe the Mig-31, although its range also diminishes rapidly with increased velocity. > efficiency is what has kept civilian supersonic flight from being a thing This was an issue, but less than you might suspect, for example it didn't actually burn that much more than its contemporaries. The planned Concorde B [1] was to have more efficient engines that would have allowed the same performance without afterburners. Along with having larger fuel tanks, this would have led to a dramatic increase in range and therefore in the possible routes. Alas, the US effectively killed Concorde before that could happen by not allowing overflight, due to noise, particularly the sonic boom, and one suspects also the fact the the US SSTs weren't making much progress. And of course the oil price shock. Concorde tickets were actually quite a bit cheaper than the public expected, BA (and AF, presumably) later raised prices to match public perception, with a nice boost to profitability. > extreme altitude where the air is thinner Jets in general (~35K feet) and Concorde in particular (~60K feet) already do this. IIRC, jet engines are actually not very efficient, but they continue to operate well at high altitudes where the air is so much thinner that the plane becomes more efficient overall, whereas both piston engines and the propellers they drive become significantly less efficient at altitude. [1] https://www.heritageconcorde.com/concorde-b https://www.heritageconcorde.com/concorde-b
- rbanffy 6y ago
  > and one suspects also the fact the the US SSTs weren't making much progress. I also can't quite avoid that suspicion. The Concorde was noisy, but not that noisy and Concorde B should be quieter from the aerodynamic improvements.
TylerE 6y ago
Concorde already flew at 60,000ft. Go much higher than that and you're going to need to put the passengers in space suits, as any cabin depressurization would be...really bad.
- liability 6y ago
  Would it really? Subsonic airliners already fly high-enough to kill passengers if they depressurized. The oxygen masks that drop are only a stop-gap to give the pilot time to drop down to a safer altitude (and the pilots have an independent, superior, source of bottled oxygen.) Humans can survive a hard vacuum briefly, if they're repressurized promptly. Perhaps putting the pilots in pressure suits would be sufficient. I think it's a matter of how quickly pilots could descend with the plane, which is probably a complicated question for aviation engineers to solve. This all said, only pilots getting pressure suits probably wouldn't inspire passenger confidence.
  - api 6y ago
    Also unlike Concorde today it is quite possible to make the plane fly itself if the pilots and crew all pass out. If all the meat sacks pass out, drop out of mach and drop down in altitude, send SOS, find somewhere to land. At this point ground control could start commanding the plane remotely too. We are not quite ready for pilotless passenger planes, but pilotless as a fallback is viable.
  - TylerE 6y ago
    At the 60k ft the Concorde flew at, a loss of pressure results in loss of consciousness in 10 seconds (or less). They already had unusual emergency procedures. You can't just mindlessly descend because that would overstress/overspeed the airframe.
    - liability 6y ago
      The passengers don't strictly need to stay conscious, though the pilots certainly do need to. I do wonder how likely a total and rapid loss of pressurization is to coincide with the plane remaining airworthy at all though. I'm guessing most survivable scenarios involve a less than immediate depressurization, which would buy the pilots more time to save the passengers. And of course I doubt the pilots would be doing anything mindlessly, let alone responding to an emergency.. In any event, I think the Concorde proves that operating at that altitude is feasible. It's not like 60,000 feet at supersonic speeds is survivable. The pilots would have minutes to get down before passengers were brain damaged. The chemical oxygen generators giving oxygen to the passengers don't last long, not nearly as long as the oxygen bottles for the pilot/crew, and that's assuming the passengers can get the masks on.
Ajedi32 6y ago
I checked their website and it does link to an article which explains the improvements they made over the Concorde in regards to fuel efficiency: https://blog.boomsupersonic.com/the-big-3-components-of-supersonic-aircraft-c1c614aeedbb https://blog.boomsupersonic.com/the-big-3-components-of-supe...

by api 6y ago

tim333 6y ago

Extreme altitude also means there's less air to hold the plane up which seems to require a lot of power for level flight. I'm not sure about ballistic flight paths - a lot of fuel to get up there and then none for a while. Apparently re Musk's old Telas that he launched towards Mars: >It has achieved a fuel economy of 9,886.9 miles per gallon (4,203.4 km/liter, 0.02379 liters/100 km), assuming 126,000 gallons of fuel. https://www.whereisroadster.com/ https://www.whereisroadster.com/ But it's not on a very popular flight route. Should be kinda near Mars Oct 7th.

GuB-42 6y ago
> Extreme altitude also means there's less air to hold the plane up which seems to require a lot of power for level flight. There is also less drag, and both lift and drag are calculated using the same formula: Cl * A * .5 * r * V^2 - Cl (Cx for drag, Cz for lift) is a coefficient depending on the wing shape, among other factors - A is the area (thickness for drag, wing surface for lift) - r is the air density - V is velocity If you divide the air density by 4 by going higher, you get the same lift and drag by going twice as fast. Which is great, and a good reason for fast planes to fly high. The reason it is so great is that in order to fly level, lift cancel gravity and thrust cancel drag. Because by going higher, we managed to keep the same lift and drag with higher speed, it means we got faster using the same amount of thrust. There are certainly complications somewhere. For example at supersonic speeds you get shock drag but suffice to say, high altitude is good for speed. Still there is a good reason why planes don't just get higher to get speed for free. And that's because planes are not rocket powered. Air breathing engines lose efficiency the faster you get, they are also designed to operate at specific speed ranges.

mpweiher 6y ago

> Even military aircraft tend to only go supersonic when needed because it guzzles fuel. Yep. In fact, this is one of many things that makes the Concorde so special: being able to cruise for a long time at Mach 2+. Supercruise has only come to fighter jets fairly recently, and there it is usually in the Mach 1.4-1.6 range. This means that if the Concorde has even a small head-start, it can outrun most fighter jets that might want to intercept it. The SR-71 could keep it company but is unarmed. Maybe the Mig-31, although its range also diminishes rapidly with increased velocity. > efficiency is what has kept civilian supersonic flight from being a thing This was an issue, but less than you might suspect, for example it didn't actually burn that much more than its contemporaries. The planned Concorde B [1] was to have more efficient engines that would have allowed the same performance without afterburners. Along with having larger fuel tanks, this would have led to a dramatic increase in range and therefore in the possible routes. Alas, the US effectively killed Concorde before that could happen by not allowing overflight, due to noise, particularly the sonic boom, and one suspects also the fact the the US SSTs weren't making much progress. And of course the oil price shock. Concorde tickets were actually quite a bit cheaper than the public expected, BA (and AF, presumably) later raised prices to match public perception, with a nice boost to profitability. > extreme altitude where the air is thinner Jets in general (~35K feet) and Concorde in particular (~60K feet) already do this. IIRC, jet engines are actually not very efficient, but they continue to operate well at high altitudes where the air is so much thinner that the plane becomes more efficient overall, whereas both piston engines and the propellers they drive become significantly less efficient at altitude. [1] https://www.heritageconcorde.com/concorde-b https://www.heritageconcorde.com/concorde-b

rbanffy 6y ago
> and one suspects also the fact the the US SSTs weren't making much progress. I also can't quite avoid that suspicion. The Concorde was noisy, but not that noisy and Concorde B should be quieter from the aerodynamic improvements.

TylerE 6y ago

Concorde already flew at 60,000ft. Go much higher than that and you're going to need to put the passengers in space suits, as any cabin depressurization would be...really bad.

liability 6y ago
Would it really? Subsonic airliners already fly high-enough to kill passengers if they depressurized. The oxygen masks that drop are only a stop-gap to give the pilot time to drop down to a safer altitude (and the pilots have an independent, superior, source of bottled oxygen.) Humans can survive a hard vacuum briefly, if they're repressurized promptly. Perhaps putting the pilots in pressure suits would be sufficient. I think it's a matter of how quickly pilots could descend with the plane, which is probably a complicated question for aviation engineers to solve. This all said, only pilots getting pressure suits probably wouldn't inspire passenger confidence.
- api 6y ago
  Also unlike Concorde today it is quite possible to make the plane fly itself if the pilots and crew all pass out. If all the meat sacks pass out, drop out of mach and drop down in altitude, send SOS, find somewhere to land. At this point ground control could start commanding the plane remotely too. We are not quite ready for pilotless passenger planes, but pilotless as a fallback is viable.
- TylerE 6y ago
  At the 60k ft the Concorde flew at, a loss of pressure results in loss of consciousness in 10 seconds (or less). They already had unusual emergency procedures. You can't just mindlessly descend because that would overstress/overspeed the airframe.
  - liability 6y ago
    The passengers don't strictly need to stay conscious, though the pilots certainly do need to. I do wonder how likely a total and rapid loss of pressurization is to coincide with the plane remaining airworthy at all though. I'm guessing most survivable scenarios involve a less than immediate depressurization, which would buy the pilots more time to save the passengers. And of course I doubt the pilots would be doing anything mindlessly, let alone responding to an emergency.. In any event, I think the Concorde proves that operating at that altitude is feasible. It's not like 60,000 feet at supersonic speeds is survivable. The pilots would have minutes to get down before passengers were brain damaged. The chemical oxygen generators giving oxygen to the passengers don't last long, not nearly as long as the oxygen bottles for the pilot/crew, and that's assuming the passengers can get the masks on.

Ajedi32 6y ago

I checked their website and it does link to an article which explains the improvements they made over the Concorde in regards to fuel efficiency: https://blog.boomsupersonic.com/the-big-3-components-of-supersonic-aircraft-c1c614aeedbb https://blog.boomsupersonic.com/the-big-3-components-of-supe...