What's wrong with this math: A lot of thrust and almost no fuel?

In summary, the conversation discusses the possibility of using a small amount of fuel ejected through a nozzle to provide enough force to lift a human and an ultra-light craft with a velocity of 200 m/s. The required pressure for this is 15 MPa and it is possible to store this pressure in cylinders from Alibaba. However, the conversation also raises questions about the use of a 1cm^2 nozzle and the calculation of effective velocity without combustion. The mass loss rate may also be negative if pressure is increased. Overall, the conversation highlights the need for further calculations and clarifications on the assumptions being made.
  • #1
gggnano
43
3
TL;DR Summary
Using basic calculator it seems as if a rocket can produce a lot of thrust and use little fuel if you increase pressure within practically possible range:10-20MPA?
ccv.png
So according to this if you need a force that can lift a human and even an extremely ultra-light craft (1500 N) and have a velocity of 200 m/s you can eject extremely small amount of fuel through the nozzle: 0.0007 or less kg per second for as long as you're using 15 MPa pressure which is doable and even storage cylinders on alibaba use it. Another calculator shows that hydrogen stored at the same pressure will have just 18.5 kg weight per cubic meter? So this gives over 7 hours flight time with 720km/h??

Several things I don't understand: is 1cm^2 nozzle with 15.1 Mpa pressure possible? How is the effective velocity calculated especially if you aren't combusting but just open cylinder to release pressure? Why is the mass loss rate negative if you increase pressure? I should've solved this as a polynomial calculation since the calculator gives odd values unless you reload. Thank you.
 
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  • #2
The tool is assuming combustion, not a high pressure reservoir so your assumptions about what things mean is off.
 
  • #3
^ Yeah...that's what I was suspecting...
 
  • #4
[A trolling follow-up post by the OP has been deleted, and this thread will now be closed]
 
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1. What is the problem with having a lot of thrust and almost no fuel in math?

The main issue with this is that it goes against the laws of physics. In order for an object to have a lot of thrust, it needs to have a significant amount of fuel to create that thrust.

2. Can't the thrust be generated by something other than fuel?

While there are alternative methods for generating thrust, such as electric propulsion, they still require some form of energy source. Without fuel, there is no way to create the necessary energy for thrust.

3. What if the object is already in motion and doesn't need fuel for thrust?

This scenario may be possible if the object is already in motion due to a previous source of thrust. However, in order to maintain that motion, some form of energy is still required. Without fuel, the object will eventually slow down and come to a stop.

4. Is it possible for an object to have too much fuel and not enough thrust?

Yes, this is also a problem in physics. Having too much fuel and not enough thrust can cause an object to become too heavy and unable to generate enough thrust to lift off or maintain its motion.

5. Are there any real-life examples of this issue in math?

Yes, in rocket science, engineers must carefully calculate the amount of fuel needed for a specific amount of thrust. If there is not enough fuel, the rocket will not be able to reach its intended destination. Similarly, in aviation, planes need a specific amount of fuel to generate enough thrust to take off and maintain flight.

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