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80

The problem is what Konstantin Tsiolkovsky discovered 100 years ago: as speed increases, the mass required (in fuel) increases exponentially. This relation, specifically, is $$ \Delta v=v_e\ln\left(\frac{m_i}{m_f}\right) $$ where $v_e$ is the exhaust velocity, $m_i$ the initial mass and $m_f$ the final mass. The above can be rearranged to get $$ ...


29

TL;DR: This answer arrives at roughly the same conclusion as Kyle Kanos', i.e. in addition to payload considerations, the difficulty lies in stuffing a small rocket with a mass of fuel exceeding to the mass of the rocket itself. This answer, however, is more rigorous in how the $\Delta v$ budget is treated. Developing a relationship between rocket and ...


13

The first thing that should jump out to anyone is the following excerpt from the abstract of the recent NASA paper: Thrust was observed on both test articles, even though one of the test articles was designed with the expectation that it would not produce thrust. Specifically, one test article contained internal physical modifications that were designed ...


13

One of my mentors likes to say, "Nothing resembles a new effect quite so much as a mistake." Conservation of momentum is a fundamental principle of mechanics supported by hundreds of years of experimental evidence since the language needed to discuss it was codified by Newton. Certainly it's the case that electromagnetic radiation carries momentum and can ...


12

Why can't a space ship accelerate infinitely? Because a space ship needs to carry fuel, and because that fuel needs to be contained in a fuel tank. That need to carry the fuel needed to make the spacecraft accelerate leads to the very nasty ideal rocket equation, $$\Delta v = v_e \ln \left( \frac {m_{\text{initial}}} {m_{\text{final}}} \right)$$ The ...


11

You want a gas so you don't need to expend energy vaporising the propellant. You also want the gas to be as dense as possible so you can get as much impulse per unit volume of propellant as possible. It's also nice if the gas is inert and non-corrosive so you don't need to worry about it degrading or corroding whatever you're storing it in. Finally it's nice ...


11

The primary factor that determines the ability of an aircraft to takeoff is having a speed exceeding that of the liftoff speed: that is the minimum (air) speed of the aircraft to generate sufficient lift by its wings to counteract the gravitational pull from the earth. Large passenger planes at takeoff often change the wing configuration (lowers flaps etc) ...


7

It is possible to generate thrust using EM radiation such as a laser or microwaves. Discussed in this XKCD blog. However this relies on momentum being transferred from the photons of the EM radiation to the object being propelled. This is not possible if the microwaves are completely sealed within a container as indicated in the article. This paper will ...


7

My guess is that the exhaust is not perfectly in line with the balloon's centre of mass. In other words, the exhaust is slightly off-centre. That will cause the thrust of the escaping gas to push the nozzle slightly to the side, causing the balloon to rotate sightly as it travels forward. If this deflection stays constant you will get a perfect circle. If ...


6

The maximum theoretical speed that a spaceship can reach isn't limited by anything (except the speed of light of course). However for a practical spaceship with a finite amount of fuel, the speed of the exhaust will set a practical maximum on the speed of the spaceship. This is because in order to accelerate to a higher speed, the spaceship would have to ...


5

Probably possible, rockets have been built that used kerosene and even Hydrogen peroxide. Liquid natural gas stores a lot of energy per volume, but hydrogen is very light and so offers a huge amount of energy per kg. And in trying to accelerate your rocket vertically upward at several 'g' it's kg that matter. Note: data from ...


5

Noble gases have the advantage of being chemically inert, so that they are less likely to react with atoms in the electrostatic grids. Since ion thruster to date have been deployed only on unmanned transports, regular maintainance is not an option. Because of that, Noble gases are favoured over, say, hydrogen One reason to pick Xenon over Argon though, ...


4

Rather than beating your student over the head with facts, try to approach the problem the way scientists did in the first place, by following the scientific method. Your student should come up with a hypothesis, and use known theory to make a prediction (calculate the momentum transfer in some idealized model), and then build a model to test the prediction. ...


4

The principle of relativity says that we can analyze a physical situation from any reference frame, as long as it moves with some constant speed relative to a known inertial frame. Thus, the ion drive does not find it more difficult to accelerate the ship when the ship is "going fast" because the ion drive cannot physically distinguish going fast from going ...


4

You should consider yourself a 160 lb bag of water. The criterion for lifting you up is that the up-momentum per second in the water is roughly larger than the up-momentum you need. From the height of 200 ft, 60 m, you know the velocity is more or less $\sqrt{2gh}=35 m/s$, and to lift you up at 100kg requires transferring $mg=1000$ units of up-momentum per ...


4

I think you are really asking "how can light deliver an impulse to the sail". The answer is that although light has no mass it does carry momentum. When light is reflected off the sail, conservation of momentum requires that the sail changes momentum by twice the momentum of the light. The extra kinetic energy of the sail comes from the red shift of the ...


4

I am an engineering student making an ionocraft as my master's project, these are my 2 cents: Initially, there is the problem of space charge saturation: simply put, there is an upper limit to the amount of ions that can coexist in a given space. This has been shown and modelled in papers concerning high voltage coronas. Another way to see this is as a ...


3

Because most payloads are quite heavy. I am not sure what kind of payloads you had in mind, I am no expert on this, but I think that most launches contain satellites, which might be heavier then you think, for instance the satellite in this BBC Documentary weighs 6000 kg. And according to Wikipedia, miniaturized satellites weigh less than 500 kg (so heavier ...


3

A problem I can see with this layout is that it generates a point (at the last upward curvature) for the plane to lift off. In general planes need different runway lengths depending on weight and type of the plane, as well as on external influences like wind. I therefore assume(!) that a typical flat layout is suited for a wider range of aircraft, and also ...


3

These quantum states were predicted already in 1979 by Berry and Balazs for photons (called Airy beams), shown experimentally in 2007 by Christodoulides et. al. and have had lots of applications since then. It is also a phenomena general to wave equations and not specific to the Schrödinger or Dirac equations. The authors find a solution to the Dirac ...


3

The engine isn't designed to use solar power. The thrust$^1$ is due to the motion of microwaves within a resonant cavity, and the microwaves are generated by a magnetron on the space ship. The power source for the magnetron would presumably be a nuclear generator or some other compact way of storing energy. The original 2006 paper describing the drive is ...


3

It seems like the previous answers were based on the abstract or third-party articles about the abstract, but I gather from this Wired article that the full paper provides more details (behind a paywall, so I can only speak for what the article says). The described tests were on the Cannae drive, the inventor of which believed required slots in the drive to ...


3

Well still is a little tricky to define in space, but assuming it isn't close enough to any other object to feel its gravity it will just stay there. Until you use its engines. Rocket engines don't need anything to push against, throwing the exhaust out of the back is what pushes it forward. Think of it as like the recoil of a gun when you fire a bullet ...


2

Specific impulse is usually defined as $I_{sp} = \frac{F_T}{\dot m ~ g_0} $ That's true only if you use standard metric units. With force expressed in pounds-force and mass flow rate expressed in pounds, one simply divides the force (in pounds-force) by the mass flow rate (in pounds/second) and voila! you have specific impulse in lbf·s/lb. For example, ...


2

The purpose of a propulsion system is to provide acceleration, but acceleration means the momentum of the accelerating object is changing. However momentum is a conserved quantity so if the object's momentum changes by $+p$ then the momentum of something else must change by $-p$ to keep the net change equal to zero. In this sense propulsion systems always ...


2

I think I understand your problem, but I think there is at least one mistake. If I work in the frame of reference of the spacecraft, its mass is constant. The Poynting vector intercepted from the photon beam and hence force exerted is diminished by a factor $\gamma^2 (1-v/c)^2$ (see below). Thus I think that $$ a = \frac{2P \gamma^2 (1 - v/c)^2}{mc} = ...


2

Disposal in outer space The objective of this option is to remove the radioactive waste from the Earth, for all time, by ejecting it into outer space. The waste would be packaged so that it would be likely to remain intact under most conceivable accident scenarios. A rocket or space shuttle would be used to launch the packaged waste into space. There ...


2

Delta-V is effectively just a change of speed so it has dimensions of $LT^{-1}$ e.g. metres per second. Suppose you have a rocket with mass $m$ that can generate a thrust (i.e. a force) $F$, then by Newton's first law the acceleration is simply: $$ a = \frac{F}{m} $$ Acceleration is $dv/dt$, so you get the change in velocity simply by integrating the ...


2

If you are particularly short on space, and you have a consistent type (so speed and weight) of aircraft - you can add a ski-jump to help convert the forward speed into vertical lift.


2

The acceleration you're describing is known as a gravity assist. You can get an idea of the maximum acceleration possible by working in your rest frame i.e. the frame in which you're stationary and the planet is approaching you at a speed $v$. If you can arrange to do a half loop round the planet and exit in the opposite direction then you'd leave the ...



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