Suborbital Airliners?

[shynung]

I recently read about Reaction Engines A2, a hypersonic (Mach 5+) airliner using the Scimitar engine, which is essentially a precooled turbofan engine. Then, I came up with a slightly different idea.

Rather than going hypersonic, why not go suborbital?

Such a plane, I think, would look similar to A2 or Skylon, but would need hybrid engines, such as SABRE or air-augmented rockets. It would take off from a typical runway, then quickly accelerate upwards, setting itself on a semi-ballistic trajectory aimed near the destination airport. After that, it would turn off its engines completely, gliding at the thin part of the atmosphere, then fire its engines once more at final approach. Electricity demands in the gliding phase could be met from either fuel cells (if using LH2/LOX), or deployable solar panels (if apoapsis is high enough). If using air-augmented rockets, it could either take off on fuel-oxidizer mix, or use some sort of bypassable turbocompressor mechanism. Then, it would transition to ramjet mode at about Mach 3, then to rocket mode when the air becomes too thin for jet engine operations.

Possible advantages include the capability to fly hypersonic over land, something which is forbidden under current laws. However, the need to carry oxidizer, along with the plane needing a heatshield for reentry, and a possibly complicated electrical system could offset it. Although, if the goal is going over very long distances for the shortest time possible, this design could do much better than Concorde or A2, at the cost of complexity.

What do you guys think?

[Nibb31]

Nowadays, it takes you at least an hour to get to the airport, and 2 hours to check in and board. Then it's one hour to get from the airport to your final destination. So travelling from London to New York in 2 hours instead of 8 only makes the total travel time 6 hours compared to 12.

How much more expensive would a 2 hour flight be compared to an 8 hour flight? I'm guessing a lot, judging by the complexity of the aircraft as well as the infrastructure (you would need to upgrade your airports with LH2 and LOX handling and storage facilities for a single aircraft type). How many people would pay the premium?

[MaverickSawyer]

Forget the LOX. If you use scramjets, you can get the same effect without the need for rockets.

I suspect that it would be a largely business-class affair, used for people to commute across the Pacific swiftly.

[shynung]

The London-New York route is just above open ocean, on which supersonic flight is permitted. In that case, a Concorde could take advantage of its speed to zip through quickly without the need for LOX or LH2.

The routes that are viable for suborbital airliners would be London to Tokyo, or London to Sydney, where most of the route goes over land. Here, supersonic transport is not permitted due to noise from sonic booms. The A2 plan to get around it by travelling northwards, over the North Pole, through the Pacific, then arriving at Sydney from the North. A suborbital airliner could, in theory, simply fly over them at the upper atmosphere, where the air is too thin to transmit the sound to the ground, therefore taking a much shorter direct route.

Also, air-augmented rockets are flexible on the fuel. Jet-A/LOX could be used, eliminating the LH2 infrastructure necessity. Jet-A/N₂O₄ is also possible, if cryogenic tanks are to be avoided.

[shynung]

Forget the LOX. If you use scramjets, you can get the same effect without the need for rockets.

I suspect that it would be a largely business-class affair, used for people to commute across the Pacific swiftly.

Scramjets works only in an atmosphere. Once the plane gets to a critical altitude, where the air is too thin, it would stop working. In theory, if the scramjet's initial acceleration is powerful enough, it could also do the job.

[MaverickSawyer]

That's the thing: they CAN do that... but you could be kicked into your seat at loads above 3Gs at peak performance of the engines.

Tradeoffs.

[Tiberion]

On a slightly related note; NASA is working on aero designs that lessen or remove sonic booms anyway, so supersonic flight could be less restricted.

[shynung]

On a slightly related note; NASA is working on aero designs that lessen or remove sonic booms anyway, so supersonic flight could be less restricted.

You mean the Quiet Spike?

[Lukaszenko]

If you have the engines that are capable of sending you fast and high enough, suborbital transportation would be the logical application. I actually made a drunken bet with a guy that we will be doing this in 20 years. Might be a tad optimistic, but one can only wish

[K^2]

Nowadays, it takes you at least an hour to get to the airport, and 2 hours to check in and board. Then it's one hour to get from the airport to your final destination. So travelling from London to New York in 2 hours instead of 8 only makes the total travel time 6 hours compared to 12.

People who can afford sub-orbital aren't the kind of people who wait in lines to check in. They are the kind of people who are going to fly in to NYC or London on a private jet or a heli. Oh, and ICBMs from Russia don't take 2 hours to get to NYC. Sub-orbital to London will be under an hour with the takeoff and landing.

Cost is another matter. People who could afford this are also not the sort of people who are going to pack like sardines into tiny capsule like some sort of peasants that have to fly economic. Nor wait for a week for a convenient flight time. If you can't organize at least one flight a day with first class sitting for a reasonable price, it's not going to work.

With cryo LH2/LOX, composite tanks, composite structure, and something like an aerospike engine, preferably one that can run as a scram jet to improve efficiency, you might be able to just make it viable. But this is a huge risk on a fairly small and unpredictable market that no private company is ready to take.

There is some military interest in a similar vehicles. USMC has payed good money for various studies on the matter. A vehicle that can deliver a small platoon of Marines would be just perfect for such a venture. If they have actually started research on it and they go to building prototypes in secret right around now (this isn't something you can test without it becoming public) they can have one in, optimistically looking, a decade. If they are working with one of the aerospace contractor giants, like Boeing, and part of the agreement is them being able to make use of the tech, (not unprecedented, consider AgustaWestland AW609) we might be looking at private operations of sub-orbital shuttles within two decades. And that's a very optimistic prognosis for it.

Another route would be to try and use something like Falcon 9R first stage to get a boost for something with a much simpler build, and that can get you something like 20T sub-orbital, which can fit a small airliner-worth of people to make the whole thing relatively affordable, but we're back to the sardines scenario here, which I don't think people who can afford it would go for.

[Red Iron Crown]

We have the technology to make suborbital airliners, just not the economic incentive to do so.

It turns out the market that is willing to pay 3x or more to reduce intercontinental flight time by 75% is vanishingly small. Airliners are designed for efficiency, not speed, because ticket price is very much driven by fuel cost.

That and atmospheric airliners are more comfortable. Reasonable G-forces and no space sickness are the big factors here.

[peadar1987]

People who can afford sub-orbital aren't the kind of people who wait in lines to check in. They are the kind of people who are going to fly in to NYC or London on a private jet or a heli. Oh, and ICBMs from Russia don't take 2 hours to get to NYC. Sub-orbital to London will be under an hour with the takeoff and landing.

True, but ICBMs experience upwards of 30g on reentry AFAIA

[K^2]

True, but ICBMs experience upwards of 30g on reentry AFAIA

On a short jump like that, you don't need more than 5-6km/s. So it's going to be less than 10 minutes to accelerate and just as much to decelerate at 1 g. That's not where bulk of the flight time is going to go. Though, approach and landing can easily take another 20 minutes or so.

[Idobox]

There is some military interest in a similar vehicles. USMC has payed good money for various studies on the matter. A vehicle that can deliver a small platoon of Marines would be just perfect for such a venture. If they have actually started research on it and they go to building prototypes in secret right around now (this isn't something you can test without it becoming public) they can have one in, optimistically looking, a decade. If they are working with one of the aerospace contractor giants, like Boeing, and part of the agreement is them being able to make use of the tech, (not unprecedented, consider AgustaWestland AW609) we might be looking at private operations of sub-orbital shuttles within two decades. And that's a very optimistic prognosis for it.

Another route would be to try and use something like Falcon 9R first stage to get a boost for something with a much simpler build, and that can get you something like 20T sub-orbital, which can fit a small airliner-worth of people to make the whole thing relatively affordable, but we're back to the sardines scenario here, which I don't think people who can afford it would go for.

I'm pretty that's what the X-37 is all about. An unmanned shuttle makes no sense for satellites (it's extra weight not needed to bring people back), but it's the perfect vehicle to send stuff anywhere on Earth very quickly.

[Nibb31]

That's certainly not what the X-37 is all about. If it was made to "send stuff to anywhere very quickly", it wouldn't need to mess about getting to orbit and manoeuvering around.

[K^2]

Yeah, it's all sorts of wrong for a suborbital prototype. Not that a lot of valuable know-how needed for suborbitals can't be acquired from such a project. Though, if I had to point a finger at an X-something that most likely serves some of the military's suborbital research interest, I'd go with X-51, the WaveRider. DARPA's main interest in the project is probably as a cruise missile, but if you want a single-stage suborbital, you want hypersonic capabilities, compression lift, and scram jets. Well, air-augmented rocket, to be precise, but challenges in building one are mostly the same.

I should really sit down and run some numbers on that. Just to see what it'd take to get a good single-stage suborbital, see how much better that is than an SSTO, and try to guestimate the launch and operation costs from that.

[Tiberion]

You mean the Quiet Spike?

That is part of it yes, they had the SSBD platform before that on an F5 (linked form your article) and after Quiet Spike Gulfstream and NASA began working on an all new design, the X-54.

Concept: http://www.nasa.gov/sites/default/files/nasa_supersonic_concept.png

And its no accident that its Gulfstream working on this, since they're one of the leaders in the private jet industry. Some of the drawings show it as a passenger plane with at least 5 rows of seats: Link

So a return to supersonic travel is probably the "future" of upper-class travel.

"Airliners" that hold hundreds of people, like all mass transit, relies on being cheap, so it'll likely remain as is until there is a major technological leap and that tech is super cheap. Perhaps when hypersonic is mature and Skylon has revolutionized cargo transport (which is probably never) we might see a private shuttle armed with an internal booster to make that last suborbital jump. But I don't see any sort of disposable booster rocket being useful outside of novelty tourism uses in the "passenger" sector.

[Skyler4856]

You're all ignoring the novelty of sub-orbital flight, the richest of the rich would LOVE to brag about their exploits in space...

[Tiberion]

That is space tourism, which is different from traveling from point A to B.

Tourism is a real thing already, and has a hefty price tag attached. That too will get cheaper, but will always be more "adventurous" than any sort high-speed traveling (perhaps artificially so, to keep the premium price tag on it)

[Sillychris]

The only thing I have to offer to the conversation is this:

Naysayers say it's very complicated, requires infrastructure, etc. Engineering obstacles.

So let's do it! When has complexity ever stood in the way of human minds?

Fear of complexity is for farmers and dogs.

[Skyler4856]

That is space tourism, which is different from traveling from point A to B.

Tourism is a real thing already, and has a hefty price tag attached. That too will get cheaper, but will always be more "adventurous" than any sort high-speed traveling (perhaps artificially so, to keep the premium price tag on it)

Im well aware of the difference, however early aviation was undoubtedly looked upon with excitement, and a feeling of the exoctic, regardless of their effiecency.

[shynung]

Naysayers say it's very complicated, requires infrastructure, etc. Engineering obstacles.

So are trains and airplanes. They are complicated, needs significant infrastructures, and presents engineering challenges. Yet, we build them anyway.

[Sillychris]

So are trains and airplanes. They are complicated, needs significant infrastructures, and presents engineering challenges. Yet, we build them anyway.

And so are pretty good rocket science simulators! Can you imagine if Nasa had access to KSP and MEchJeb back in the Apollo days?

[Nibb31]

The only thing I have to offer to the conversation is this:

Naysayers say it's very complicated, requires infrastructure, etc. Engineering obstacles.

So let's do it! When has complexity ever stood in the way of human minds?

Fear of complexity is for farmers and dogs.

That's silly. Contrary to a widely popularized speech, we don't do things because they are hard, we do things because they are worth doing. Especially when it requires a substantial effort. In fact, the bigger the effort, the more justification you need.

We don't build suborbital airliners because there is no demand for $200 000 express plane tickets. Concorde was a complex machine, but it flopped due to lack of demand. Nobody is going to start building rocket fuel factories and storage facilities at Heathrow, JFK, and Dubai International Airport for a handful of passengers every year.

[Seret]

The only thing I have to offer to the conversation is this:

Naysayers say it's very complicated, requires infrastructure, etc. Engineering obstacles.

So let's do it! When has complexity ever stood in the way of human minds?

Fear of complexity is for farmers and dogs.

The main obstacles here aren't technical, they're economic.

Could we build a suborbital airliner? Of course.

Could we build one that made money? That's pretty dubious IMO. So efficiency and complexity are pretty key.

Edited March 4, 2014 by Seret