Some questions about ramjets and scram jets...?

Question

1. What speed needs to be attained to render the above mentioned engines useful? [How fast do they need to go before they work?]

2. At what point can a ramjet be replaced with a scram jet?

3. How high in the atmosphere can both go before oxygen becomes to scarce for them to be efficient? [What is the highest attainable altitude and yet be an efficient air supply for both engines?]

4. If possible, I'd like an approximation of how large and heavy common scram jets and ramjets are.

5. How big would a gas tank have to be in order to reach the highest altitude attainable and go thrice around the world?

6. Exactly how much G's would the pilot have to endure in a scramjet at it's fastest speed?

7. What are their fastest speeds?

8. If 2 were attached to an object and in use, would the object go faster or reach the fastest speed quicker? Or both?

9. Liquid hydrogen is used, right?

10. Okay, I think this is the last I can think of:

If a ramjet [or a scram jet if possible] is vacuum sealed, creating inside of it a condition similar to the vacuum of space, and suddenly opened, would the air be sucked through fast enough, heated quick enough, and injected with fuel, would it work? [I hope that makes sense...??]

If at all ya'll can answer any or all of these questions, I thank u greatly. Thanx for your time, you all!

Answer 1

Answers:

1. Depends on the design.  Generally, ramjets are efficient above Mach 3.

2. About Mach 6.

3. Don't know.

4. Ramjets and scramjets are much lighter per unit of thrust, since they have no moving parts.

5. Pretty big.  Probably not possible to achieve that range.

6. At it's 'fastest speed', an airplane is no longer accelerating, so zero G's.

7. NASA's X-43 has sustained Mach 9.6, although it didn't take off under it's own power and used a rocket booster to get it up to speed.

8. Huh??

9.  A ramjet can be designed to use a variety of combustible fuels, including liquid hydrogen.

10. No, wouldn't work.  It might work for a brief spurt, but wouldn't lead to sustained combustion.

Answer 2

just one right now.

6. Acceleration has nothing to do with speed except to determine how long it takes to get there. The machines could be designed to subject the pilot to no more than 1G and still reach their top speeds. Or the designers could decide they want to get their faster and design for a higher acceleration.

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