pdcpdcpdc said:
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.
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.
pdcpdcpdc said:
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.
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.
pdcpdcpdc said:
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.
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.
pdcpdcpdc said:
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 ;-).
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.
pdcpdcpdc said:
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 :-)).
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.
pdcpdcpdc said:
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.
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.
pdcpdcpdc said:
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 Earth's atmosphere, are specified below.
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.
pdcpdcpdc said:
Aircraft can go supersonic as soon as possible because climbing to cruising altitude and reaching the ocean is not required.
But not hitting the walls both during "take off" and "landing" are.
pdcpdcpdc said:
Aerodynamics can be optimized for a single flight regime as the aircraft is effectively flying on a single altitude.
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.
pdcpdcpdc said:
As discussed earlier, there is no turbulence, side shear, moisture etc. because of the containment of the tunnel.
There will likely be plenty of turbulence. I don't know what you mean by side shear.
pdcpdcpdc said:
We can choose the gases that gives the best lift to drag ratio.
Already addressed. No such concept that is innate to a gas.
pdcpdcpdc said:
We can choose the pressure of these gases for optimizing lift and drag.
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.
pdcpdcpdc said:
As the flight times would be short we can use rocket engines and exotic engines don’t have to developed.
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).
pdcpdcpdc said:
I understand that is a very complex problem to quantify for amateurs like most of us here.
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.
pdcpdcpdc said:
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.
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.
DaveC426913 said:
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.
I couldn't agree more.
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.
