Can a Jet Engine Remove drag from the Hyperloop tunnel?

[physicsforumsfan]

[Moderator: link removed]

Can a jet engine the size of the tunnel mean that you can have an effectively have drag-free vehicle? Any lorry tethered to the jet engine would be basically be drag free? Is this correct?

Thanks

 

[mfb]

What is your payload and where does it go?

Engines still have drag.

 

[Rive]

If I understand correctly, that would be like a moving stairway. If you stay on that you don't need to spend energy to move - but the energy spent on moving the stairway itself is still there and actually far higher than when you just walk the same distance.

 

[russ_watters]

Is this true? Can a jet engine the size of the tunnel mean that you can have an effectively have drag-free vehicle? Any lorry tethered to the jet engine would be basically be drag free? Is this correct?

Engines still have drag.

I've heard propulsion engineers describe what happens at the inlet to be "thrust" which I don't think is technically true, but rather is just pointing out that the ram effect is a feature, not a bug.

What is your payload and where does it go?

I'm picturing a train wrapped around a very long jet engine. I guess it would "work", but:
1. Isn't Hyperloop supposed to be green/electric?
2. How do you avoid cooking and asphyxiating the passengers with the jet exhaust from the train ahead of you?

 

[anorlunda]

I guess the idea is to reduce air pressure ahead of the jet and increase the pressure behind it. But a jet is an air breathing engine.

The lower the air pressure in front, the more work the compressor in the jet needs. The efficiency of compressor blades is nonlinear, so actual numbers would be needed to say what the balance between drag and compressor power is.

How many km of tunnel ahead of the vehicle need to be reduced in pressure? How many km aft of the train pressurized? There would be a transient effect and pressure gradients close to the train because air has inertia. But then the train is moving through that, so it depends on train speed. Once again, there can be no meaningful answer without numbers.

But the biggest issue is what @russ_watters mentioned. You're filling the tunnel with toxic gasses. On the following train, that kills the passengers and starves the jet for oxygen.

 

[russ_watters]

Reading the wiki article on the hyperloop, the original concept did in fact include a ducted compressor at the front, which would shunt some air out the bottom for levitation and the rest out the back. In other words, a standard hovercraft. But that roughly matches the idea in the OP.

The obvious problem I see is that since the vehicle operates at mach 1 and the system is self contained, this technique cannot be used to "eliminate drag" or pump-out the tube.

 

[boneh3ad]

I mean, if you want to eliminate drag, you would have to evacuate the tube and run the whole thing like a maglev in a vacuum. But that would make too much sense.

 

[LURCH]

I mean, if you want to eliminate drag, you would have to evacuate the tube and run the whole thing like a maglev in a vacuum. But that would make too much sense.

As I understand it, that’s exactly what the Hyperloop is. Notice the Wiki article uses the term “vactrain”, which refers to a train that runs in a vacuum. Personally, I have serious concerns regarding the safety of such a system.

 

[boneh3ad]

As I understand it, that’s exactly what the Hyperloop is. Notice the Wiki article uses the term “vactrain”, which refers to a train that runs in a vacuum. Personally, I have serious concerns regarding the safety of such a system.

If I recall, the tubes are evacuated but not entirely. It does raise safety issues, particularly of a train breaks down and you have to offload passengers, what do you do?

 

[mfb]

Push the train slowly to a station using another train (needs wheels or similar for slow speeds anyway). If that is not possible or would take too long: Fill the tube segment with air. How often do trains break down without any external influence?

 

[trurle]

[Moderator: link removed]

Can a jet engine the size of the tunnel mean that you can have an effectively have drag-free vehicle? Any lorry tethered to the jet engine would be basically be drag free? Is this correct?

Thanks

Not exactly drag-free, but significant drag reduction is possible with fixed tunnel diameter and fixed frontal area of vehicle. With stagnant shock-wave in front, the drag is roughly proportional to 4th power of speed. If you (effectively) remove front air through jet engine or large enough gap on the sides of vehicle, drag become roughly proportional to square of speed.
Hyperloop idea is actually not to reduce drag, but to create vehicle/tube in manner suitable for reduction of building costs - it mean the minimal possible tunnel diameter. The drag reduction happens because the central jet engine drag is smaller than the drag of same-area annular gap around vehicle, because of jet engine duct having smaller surface area with same frontal area.
The internal layout of vehicle become very constrained if you aim for drag reduction though - therefore you can always trade the higher drag for smaller central duct and less cramped internal layout.

 

[Baluncore]

The drag reduction happens because the central jet engine drag is smaller than the drag of same-area annular gap around vehicle, because of jet engine duct having smaller surface area with same frontal area.

But the velocity through the jet duct is proportionally higher, with drag being proportional to velocity squared, you cannot win.

 

[trurle]

But the velocity through the jet duct is proportionally higher, with drag being proportional to velocity squared, you cannot win.

If area of jet duct is the same as are of annular gap, the inlet air speed would be the same. Also, for high-bypass fan the increase of speed at outlet (and related increase of drag at rear part of duct) is not big, therefore net drag reduction is still possible.

 

[Baluncore]

If area of jet duct is the same as are of annular gap, the inlet air speed would be the same.

Annular clearance is still required. There would still be the wetted area of the skin in the annular gap.
The velocity of the air in the jet duct would be greater than vehicle speed. That increases the drag within the duct. Air inside the duct would need to flow faster than the vehicle or be denser than the air in the tunnel, or the duct section would fill the vehicle.
What is the speed of sound in the rarified tunnel air?

 

[trurle]

Annular clearance is still required. There would still be the wetted area of the skin in the annular gap.

Flow of air in annular gap in hyperloop can be low - limited by engineering tolerances/clearances, not by physical constraints. I think significant flow in at least 2 strips of vehicle skin located top-left and top-right with outer surface (plus active "hover" control on bottom) would be good for passive stability provided by Bernoulli effect, although you can in theory stabilize vehicle completely by active means, with flow around vehicle reduced to few percents of flow through jet engine.

The velocity of the air in the jet duct would be greater than vehicle speed. That increases the drag within the duct. Air inside the duct would need to flow faster than the vehicle or be denser than the air in the tunnel, or the duct section would fill the vehicle.

Same applies for (small) annular gap. As i mentioned before, the point of hyperloop is to have fixed drag in as small tunnel as possible, therefore annular gap area is constrained by same amount as jet duct area.

What is the speed of sound in the rarified tunnel air?

Speed of sound in first approximation is not dependent on air density. Inside duct, speed of sound is a bit higher because adiabatically compressed air is warmer.

 

[mfb]

As far as I know no group follows the original concept with an air cushion any more. They all went to rails with wheels or magnetic levitation. Once you don’t need the air any more you can make a better vacuum so drag won’t be such a big issue.

 

[Baluncore]

Once you don’t need the air any more you can make a better vacuum so drag won’t be such a big issue.

That effectively precludes the use of a jet engine.

 

[mfb]

Yes, you don't need it any more.

 

[LURCH]

I don’t honk it also removes concern about the speed of sound in the tunnel, right? Since no shockwave of any consequence can be generated in such a thin medium, I mean.

 

[mfb]

If the vacuum is good enough then the speed of sound is not an important parameter any more, sure.

You need a really straight track to reach it without excessive g-forces, however. Even with extreme banking: 0.5 g at 340 m/s gives you a curvature radius of 23 km. 0.2 g gives a curvature radius of about 50 km.

 

[essenmein]

If the vacuum is good enough then the speed of sound is not an important parameter any more, sure.

You need a really straight track to reach it without excessive g-forces, however. Even with extreme banking: 0.5 g at 340 m/s gives you a curvature radius of 23 km. 0.2 g gives a curvature radius of about 50 km.

Its in a tube so I would assume it would just rotate in there relative to the horizon to keep the total acceleration vector normal to the "floor", no banking needed. So if they then hit a 11km radius corner at mach 1, the total "g force" experienced by a passenger would be 1.41G (towards the floor), not sure if that's too much or not. I know its crazy, but they could also slow for corners...

 

[russ_watters]

Its in a tube so I would assume it would just rotate in there relative to the horizon to keep the total acceleration vector normal to the "floor", no banking needed.

Definitely.

So if they then hit a 11km radius corner at mach 1, the total "g force" experienced by a passenger would be 1.41G (towards the floor), not sure if that's too much or not...

I'm not seeing that -- @mfb 's numbers look right -- could you post your math?

 

[essenmein]

Definitely.

I'm not seeing that -- @mfb 's numbers look right -- could you post your math?

Same numbers as mfb, 23km rad for 0.5G, 11.5km rad for 1G, now you have 1G horizontal with 1G vertical (from Earth's mass), resulting in net accel vector of sqrt(2) G at 45deg.

(edit: corrected exact numbers lol)

 

[russ_watters]

Same numbers as mfb, 23km rad for 0.5G, 11.5km rad for 1G, now you have 1G horizontal with 1G vertical (from Earth's mass), resulting in net accel vector of sqrt(2) G at 45deg.

(edit: corrected exact numbers lol)

Yep, got it; missed the sqrt 2 and was thrown by the smaller radius you used.

 

[LURCH]

Its in a tube so I would assume it would just rotate in there relative to the horizon to keep the total acceleration vector normal to the "floor", no banking needed.

Although the train is in a tube, the track is embedded in the floor. The current plans are to bank the track at the curves portions, in order to achieve exactly the effect you have proposed. Different technique, same result.