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Aerospace Opinions on a Hypersonic Tunnel based Transportation System

  1. Oct 15, 2011 #1
    Hello Guys,

    Please provide your opinions on the following Hypersonic Tunnel based transportation system? I am a Computer programmer and not some aerospace engineer, so kindness is appreciated :-)

    The primary problem with hypersonic(or supersonic) travel is that we have to remove several tonnes of air in front of the vehicle every second at hypersonic speeds by ramming our aircraft into it. This requires a lot of power and structural weight etc. My design tries to bypass this primary bottleneck. In my design a hypersonic rocket powered aircraft moves inside a tube of large diameter. The tube spans the entire path of the journey. This tube is filled with hydrogen(or some other gas) at low pressure to reduce drag. This tube is also lined with metallic foam from the inside to reduce the shockwave inside the tube(like a suppressor of gun). The vehicle inside this tube feels that it is traveling at very high altitude(hence very low drag) throughout the journey. This setup should solve almost all problems associated with high speed flight by a big margin.

    My design is roughly equivalent to the design discussed at http://dsc.discovery.com/convergence/engineering/transatlantictunnel/interactive/interactive.html [Broken] and . But there are two major differences.
    Firstly, I am using an aircraft and not a train and secondly, I don't have a vacuum inside the tube but some gas that provides good lift to drag ratio(possibly hydrogen, but there are other contenders) at hypersonic speeds. This setup avoids the huge construction and running costs of a maglev system and is safer as precise tolerances are not required. Basically, the system is tube with hydrogen in it which simulates upper atmosphere.

    I have done the patent search and haven't found anyone proposing it. Please let me know if someone else has explored this concept before.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 15, 2011 #2


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    Welcome to PF.
    I can't comment upon most of your proposal, partly because I have a grade 11 education and partly because I've had 8 beers.
    I will, however, state one thing unequivocally: Stay the hell away from hydrogen! Weren't you paying attention when the Hindenburg had its little hiccup fit? :eek:
  4. Oct 15, 2011 #3


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    Well first, that isn't the primary problem with hypersonic flight. Hypersonic flight is much more complicated than that.

    With hydrogen, you will likely kill everyone involved. Filling a tube with hydrogen and putting a rocket in it will just ignite all the hydrogen in the tunnel.

    Additionally, putting all that air in a tunnel will INCREASE drag since you will have wall/blockage effects. You would make the problem worse.

    Also, density is not the only important factor in the drag on an object. Viscosity is also important, so you may end up just making things worse.

    Rocket travel is not a very efficient way to move and it generally isn't cheap to do in a reusable way, so this project would be very expensive. Air breathing propulsion would be a better choice, but that requires oxygen, which your tunnel won't have.

    How do you plan to steer this object through the tunnel? If you are putting it on a track then this just amounts to a rocket-propelled train. If you think it will control like a plane, then you will kill people that way too, as control is one of the chief barriers to hypersonic flight. It would be very tough to not slam into walls.

    Also, you won't dissipate shockwaves using a foam wall. That would reduce reflections a bit, which may give a slight benefit, but the shock will be just as strong as ever until it touches that wall.
  5. Oct 15, 2011 #4


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    Thanks for that post, Bony.
    I was just so overwhelmed by the question that I couldn't think straight. It looked reasonable at the very beginning, but I was going to mention the plans for evacuated tunnels. Then he addressed that, so I figured that he was thinking ahead of me.
    My brain started to rebel when he mentioned that it wouldn't be on rails, and with his idea of using hydrogen. He obviously isn't a pilot if he even for a microsecond considered flying in a tunnel. There's that pesky little critter called "ground effect" that would not only bite him in the ***, but devour it completely and then start working its way up his spine.
    Just as a point of curiosity, though... How much cheaper would it really be to evacuate a tunnel and fill it with hydrogen than it would be to evacuate it and leave it that way?
    And as another side note, coming from someone who has a passing relationship with firearms suppressors... The purpose of such is not to suppress shockwaves internally. The whole point is diffusion of the expanding gases, which are then released in a controlled manner. It has nothing to do with trains.
  6. Oct 15, 2011 #5
    Hi Guys,

    I can't do Quantitative drag analysis. So, I have no idea how severe wall/blockage effects are. But I am of the opinion that these effects can be mitigated to an extent by increasing the diameter of the tube. It ain't wind tunnel and rolled steel is cheap :-). As far as hydrogen is concerned we can replace it with any other gas or a combination of gases. Basically, what I am looking for is a gas(or gases) with good lift to drag ratio inside a tunnel for an aircraft at hypersonic speeds.

    Steering and piloting is going to be through control surfaces(may be retro rockets) driven by fly-by-wire systems and mission computers. Control loops in SAM systems are far tighter than this system would ever need as we are tracing a known path with no turbulence or side shear etc. inside a tube. I would call controls a non issue. I may be wrong so please comment.

    Ground effects would be a friend during landing and takeoff and maybe during the flight. I admit this is an unknown flight regime and would be tricky if not impossible. Again, we can increase the diameter of the tube.

    Shock waves inside the tunnel is a fundamental unknown to me. If someone is willing to do a quantitative analysis on this I am willing to program his/her website(or any other piece of software) in return ;-).

    I appreciate all your comments. However It would be better if you can also provide solutions to correct the flaws in this system(physics permitting :-)).

    Danger, don't worry about the beers, I am high too...
  7. Oct 15, 2011 #6


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    :rofl: Glad to hear it, because I'm now on my 13th and last can. So as soon as it's done, I will have a nap before hitting the bar.

    The simplest thing that I can say is that if you have to make your tunnel large enough to negate any ground effects (which will come from all directions, not just the ground), you might as well just stay above ground in the first place. Not only are you putting the cart before the horse, but you have the cart's brakes locked and the horse's feet nailed to the ground.
    I love the idea that you are thinking up things that don't exist. That is one of the best signs of intelligence. The trick is to learn how to filter out what is practical and what is not. It took me over 40 years to do that, and I still occasionally go for the long shot.
    You are definitely in the right place to learn what can and cannot be done, though. I'm the dumbest guy here; listen to the others and you shall soon be rocking big-time.
  8. Oct 15, 2011 #7
    One of the reasons for this thread is that it appears aerospace community has given up on the idea of supersonic/hypersonic travel. Although people are working on some ideas but they estimate that such things would take several decades to reach commercial market. I think we can consider some radical ideas in the meantime to keep ourselves amused:-). Let's treat this thread as an academic exercise. Assume that professor 'X' has asked student 'Y' to solve the hypersonic tunnel transportation problem. All Y has to do is come up with the best solution possible.

    Some of issues with supersonic aircraft are listed here: http://en.wikipedia.org/wiki/Supersonic_transport#Challenges_of_supersonic_passenger_flight

    It appears to me that tunnel based flight is the only alternative for overland supersonic/hypersonic aircraft because of the sonic boom issue. This is the primary reason for tunnels to be considered seriously. This do creates a massive problem of shock waves inside the tunnel. My major concern is how to contain it. Few advantages of using a tunnel,which are not present in earths atmosphere, are specified below.

    Aircraft can go supersonic as soon as possible because climbing to cruising altitude and reaching the ocean is not required. Aerodynamics can be optimized for a single flight regime as the aircraft is effectively flying on a single altitude. As discussed earlier, there is no turbulence, side shear, moisture etc. because of the containment of the tunnel. We can choose the gases that gives the best lift to drag ratio. We can choose the pressure of these gases for optimizing lift and drag. As the flight times would be short we can use rocket engines and exotic engines don’t have to developed.

    I understand that is a very complex problem to quantify for amateurs like most of us here. There are hundreds of variables, many of which are unknown and interact with each other in unpredictable ways. So, It would be great if someone can provide a quantitative bound on this problem with variables of their own choosing(you can begin with x-15 as a test case). If anyone finds a fundamental flaw please mention it and try to provide a workaround if possible.
  9. Oct 15, 2011 #8
    A vacuum pulled train is an interesting idea. Set the center of gravity so that the train could freely orient "down" with one G turns. Fun! Perfect for the next Sci-Fi movie. Then again, it would probably be cheaper to rebuild the old steam locomotives and run them on the surface. (That's opinion, not calculated fact.)
    Now, if we want to go underground, assuming we can figure out how to go really deep, we could just use gravity to pull us to the next destination.
  10. Oct 15, 2011 #9


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  11. Oct 15, 2011 #10


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    Leave us not take Professor X's name in vain. He's my hero. I consider myself a scrawnier, but much nastier, version of Wolverine.
  12. Oct 15, 2011 #11


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    Sorry, call me a thicky.

    You've got a tunnel. An enclosed space, necessarily as small diameter as possible, since its atmo requires a lot of maintenance for pressure and purity.

    Why fly planes?
    The only advantage of flying craft is they have a whole sky to go where they wish. They do this a huge cost (fuel that goes into lift, safety, must be lightesight, etc).

    Take a plane and it in an enclosed space and you've removed its entire advantage, while not only retaining its worst aspects, but dramatically increasing those aspects by imperfectly navigating thing so near to things it can crash into.

    What's wrong with a rail? You're right next to the surface, you don't have to worry about control surfaces or lift or flight path drift or crashing into anything.

    As far as I can see, there is no point to the whole flight aspect. What you've got there is the all worst aspects of flying vehicles and rail vehicles.
    Last edited: Oct 15, 2011
  13. Oct 17, 2011 #12


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    Wall effects will be very great until you have a tunnel many times the diameter of your object. Evacuating this chamber will cost an extraordinary amount of money, time, and energy, and filling it with any combination of gases is really not going to gain you much over just flying in the atmosphere if anything at all. Moving air out of the way is not the main source of drag for an object at this speed. The largest source is wave drag created by the shock and associated changes in entropy and total pressure. That isn't going to depend on fluid density, but instead on other fluid properties. There is also no such thing as a gas with a good lift-to-drag ratio. That depends on the shape of the aircraft.

    As mentioned before, this is not a trivial problem. Flying at such high Mach numbers means you will have very little time to react and the forces involved mean any slight error in steering could be catastrophic. In fact, the control issues are one of the major barriers that still remain to hypersonic flight in the atmosphere. You are incorrect in assuming that SAM systems perform better than this. SAMs fly usually no more than ~Mach 3. You are talking about exceeding that by quite a bit. You would also have plenty of turbulence. I am unfamiliar with what you mean by side shear.

    Your tube would have to be huge to mitigate ground effects, which throw a huge wrench in everything we know about designing low-speed aircraft. To my knowledge, very-high-speed aircraft haven't even been investigated that close to the ground.

    What about shocks is unknown? What are you wanting to know? You can't get rid of them. They cause drag. They interact with surfaces. You have to live with it.

    The key thing here is that both the physics and the practicality of this problem mean that it is just flawed to the core and generally a bad idea. Outside the box, sure, but still a bad overall idea.

    Far from it. There is plenty of work going on in hypersonics. The problem is that it is an expensive area to investigate, particularly in up-front cost, and the primary (if not exclusive) interested parties are governments, which have to divvy up funding for a multiptude of projects. Hypersonics is usually one of the first things on the chopping block, and as such, there has been a pretty repeatable roller coaster pattern to how much research is going on in the area ever since the 50s. There is still plenty of research going on in the area, however. I assure you of that. Right now we are at the top of one of those spikes but seemingly starting to head back down as a result of the major economic downturn of these past few years.

    The concept of "tunnel-based flight" is just an unbelievably dangerous and terrible idea. Tunnel-based supersonic maglevs or rail cars is a much more feasible idea. There is a lot of work going on right now on quiet supersonic transports for atmospheric flight though, so this whole perceived problem is well on its way to becoming moot.

    But not hitting the walls both during "take off" and "landing" are.

    Not effectively; it is flying at a single altitude. However, flying in tunnels is not only dangerous, but less versatile than flying in the open atmosphere as well as more expensive since you have to build the tunnels, somehow develop an all-new concept for the vehicles and then operate the system.

    There will likely be plenty of turbulence. I don't know what you mean by side shear.

    Already addressed. No such concept that is innate to a gas.

    Again, this will have a fairly small effect on overall drag given that in atmospheric flight we already have very low pressures at the altitudes in question.

    One of the major reasons for developing said engines is precisely to obviate the need for rocket engines. Rocket engines are expensive and very difficult to reuse. Solid rocket engines are not reusable, while liquid rocket engines are much more prone to disaster. Using engines such as scramjets is much more efficient and cheaper, and they are getting closer to reality (see the X-51).

    You still haven't said what it is that you even want to quantify. If you have a Mach number, the gas properties and the shape of the vehicle, you can get some reasonable drag estimates fairly easily if that is what you are looking at (see: Newtonian Impact Theory, Method of Characteristics). The problem is, you haven't been clear at all about what you want to quantify. Additionally, the idea just seems to be bad right down to the fundamental concept.

    There aren't that many variables as long as you know which ones are important. The problem is there are just so many fundamental problems with the idea. Thinking outside the box is a great thing, but this just wouldn't work. It would be much safer and easier to do a train of some sort propelled by a rocket, but even that would not be as preferable as using an air-breathing propulsion system, which necessitates an oxygen-rich atmosphere in the tunnel.

    I couldn't agree more.
  14. Oct 17, 2011 #13


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    Jeez, my post was written in a hurry. Lots of errors. Sorry.

    In a nutshell, my question still stands: once you've got a tunnel, what purpose does flight serve at all? No up side at all, while the down sides are legion.
  15. Oct 17, 2011 #14


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    Don't worry, your point was still clear.
  16. Oct 17, 2011 #15
    Hi pdcpdcpdc-
    Why don't you design a tunnel from Los Angeles to New York using brettjor's idea in #8. The tunnel will go underground in an arc, and gravity will accelerate the vehicle at the beginning, and coast to a stop at the end. No added energy is used. Assume vacuum all the way. We can correct for drag later (I know it is large).

    Now, what is the shape of the arc for minimum transit time? There has to be an arc that has shorter transit time than any other route. Is it a half circle, or parabola, or half sine wave, or something else? If the straight line distance from Los Angeles to New York is 5000 km, what is the minimum transit time assuming vacuum?
  17. Oct 17, 2011 #16


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    It seems to me that it would be just about as practical to hitch your vehicle to a team of gophers and let them dig your way for you.
  18. Oct 18, 2011 #17
    The challenge I posted in post #15 is a very interesting problem, which Isaac Newton solved in 1697. It does have a unique solution. Any takers?
  19. Oct 18, 2011 #18
    Wow, this thread is heating up. I am feeling the love already.:wink:

    I am working on a quantitative argument. It will revolve around 'Bloodhound SSC'. You can check it out for yourself in the meantime. I will try to answer all the queries then.:cool:
  20. Oct 18, 2011 #19
    Hello brettjor and Bob S

    If my understanding of your concept is correct, it will involve a lot of digging. In a nutshell, it would be several orders of magnitude more expensive than surface transport of any sort. Take a look at this article http://en.wikipedia.org/wiki/Channel_Tunnel.

    Do let me know if I am missing something.
  21. Oct 18, 2011 #20


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    Belay the in-depth answer for now. Just answer the one central question:

    If you're set on a tunnel, name a single advantage of flight over rail that comes close to offsetting the myriad disadvantages (and not something lane like 'reduced friction').
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