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62

The whole point to the throat is to increase the exhaust velocity. But not just increase it a little bit -- a rocket nozzle is designed so that the nozzle chokes. This is another way of saying that the flow accelerates so much that it reaches sonic conditions at the throat. This choking is important. Because it means the flow is sonic at the throat, no ...


61

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 $$ ...


41

Start by considering what is seen by the people watching you from the Earth. Nothing can travel faster than the speed of light, $c$, so the quickest you could get to Kepler 186f would be if you were travelling at $c$ in which case it would take 490 years. In practice it would take longer than this because you have to accelerate from rest when you leave the ...


38

In space you don't just "go somewhere". You have to match orbits, while not wasting too much fuel. If you're in a low circular orbit, and you want to get to a high circular orbit, it takes two tangential burns, one to elongate your orbit into an ellipse, and another at the high point of the ellipse to make it circular again. This is called a Hohman ...


30

Ok David asked me to bring the rain. Here we go. Indeed it is very feesible and very efficient to use an electromagnetic accelerator to launch something into orbit, but first a look at our alternative: Space Elevator: we don't have the tech Rockets: You spend most of the energy carrying the fuel, and the machinery is complicated, dangerous, and it cannot ...


22

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 ...


18

At constant 1 g acceleration half-way through, then constant 1 g deceleration the remaining half, it takes 7 years in rocket time, 38 years in Earth time: http://www.cthreepo.com/lab/math1.shtml Scroll down to Long Relativistic Journeys and enter your data. To the Andromeda Galaxy (2.5 mil ly) it's 29 years in rocket time! :)


15

It's a great way to get gyroscopic stability. NASA has been using this technique for a long time. For instance, the Pioneer spacecraft used this method. Another example is the Juno spacecraft as well. I hope that answers your question sufficiently.


15

Aircraft rely on lift generated by interacting with the atmosphere and on using atmospheric oxygen to burn with fuel they carry. Orbits aren't stable until you are high enough that there isn't enough atmosphere to interact with, and long before that the oxygen content drops too low to be useful. So, to get to a stable orbit, you will need rockets ...


14

When swinging my comfy hammock, I travel all day even up to 0.99 $c$, some days even more, depending on what particles are passing me by and measuring my exorbitant speeds with their atomic clocks and photons..!


13

Nowadays, rockets use a Gimbaled Thrust System. The rocket nozzles are gimbaled (An appliance that allows an object such as a ship's compass, to remain horizontal even as its support tips) so they can vector the thrust to direct the rocket. In a gimbaled thrust system, the exhaust nozzle of the rocket can be swivelled from side to side. As the nozzle is ...


13

Here is a visualization: Momentum is mass times velocity, so draw it as the area of a rectangle: If we change the mass and velocity a little, we change the momentum: The total change in the momentum is the sum of green, blue, and purple rectangles. Their sizes are just length times width, so overall we have $\Delta p = m\Delta v + v\Delta m + \Delta ...


12

Deriving the relativistic equations for constant acceleration would be a formidable problem for most non-physicists. If you want to see how it's done then look at Gravitation by Misner, Thorne and Wheeler, chapter 6. For most of us the best option is just to look at John Baez's excellent article on the relativistic rocket. The relevant equation is: $$ d = ...


12

According to Wikipedia, Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine (The picture they provide is the same or very similar to that in the question). So, maybe Crazy Buddy is right.


11

Edited a little now that I better understand your question. Short In a multi-stage, the weight of the parts that are dropped along the ride compensates for the fact that the extra engines make it heavier in the beginning. Partially because a rocket's engine isn't that heavy compared to the fuel tank. The engine mostly just ignites and controls the ...


10

Maximum velocity attained by the Apollo spacecraft was 39,897 km/h which is $3.6\times 10^{-5}$ times the speed of light...


10

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 ...


10

The link you posted refers to a design which would supposedly make use of faster than light travel. After reading this, I immediately stopped reading, since this is not possible. I don't care if it's a NASA-affiliated person who says this, it is simply misguided! Before continuing, I should draw your attention to the fact that I will not be discussing the ...


9

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 ...


9

At this stage, does the rocket still accelerate the craft? If by "velocity of the exhaust" we are talking about its velocity measured in the frame of the rocket, then Yes. Let $\mathbf u$ be the exhaust velocity as measured in the rocket frame, then in free space, the non-relativistic rocket equation is \begin{align} \frac{d\mathbf v}{dt} = ...


8

This is a hugely open ended question and a very big subject, it's a bit like asking "what automobile is best for use on Earth?" I'll try to fill in some details. There's the unique environment of space: zero-g, potentially very cold temperatures, vacuum, potentially long travel times (months or years), varying delta V requirements, etc. Each of those need ...


8

Rather than leaving a brief comment on this topic, let me just point at this wikipedia page which is very comprehensive: http://en.wikipedia.org/wiki/Interstellar_travel My own comments: Once we learn to control fusion, that would be an attractive candidate for the engine. The nice thing is that there might be no need to convert the reactor's energy into ...


8

In a lot of ways this is a technology---rather than physics---question, but lets look at some limits imposed by physics. For rockets there are two numbers that matter: the velocity relative the spacecraft with which the fuel can be expelled (called the specific impulse) and the fraction of the original mass that is fuel. For very high mass fractions the ...


8

Space Shuttle Discovery re-boosted the orbit of Hubble during STS-82 (in 1997) and in the process reached 620 km altitude, which is higher than any other Shuttle flight.


8

I wrote a semi-popular book on this subject a couple of years ago: http://www.amazon.com/Can-Star-Systems-Be-Explored/dp/9812706178/ref=sr_1_3?ie=UTF8&s=books&qid=1297567209&sr=8-3 A lot of this was worked out. I illustrate the relativistic rocket and the photon sail. The photon sail is clearly the most reasonable of these two. I also ...


8

Stabilization. Example: Pioneer Equalize heating (barbecue mode). Example: Apollo Deploy antennas & booms (via centripetal force). Example: IMAGE Maintain tension in a solar sail. Example: Cosmos 1 Test general relativity. Example: LAGEOS Create artificial gravity. Example: Gemini Simplify or reduce weight of sensors (e.g. star trackers). ...


7

The vertical part is relatively easy, but to be in orbit you need to be going fast enough horizontally, that's around 8km/s in low earth orbit. The balloon does nothing to help with that. There are launch vehicles that carry a rocket up to altitude underneath a plane and then fire that from 50,000ft. It means you don't need to use the rocket to get through ...


7

There is good research on railguns at the University of Texas at Austin, led by Ian McNab. See, e.g., I.R. McNab. "Progress on Hypervelocity Railgun Research for Launch to Space." IEEE Trans. Mag. 45: 381-388, 2009. There is a list of his publications describing his team's work. The funding comes from the US Army, as there are applications in long-range ...


7

There is no single possible travel duration between two planets at a fixed epoch. You can chose a trajectory according to different criteria and then you can compute the transfer orbit that satisfies your constraints. Once you know the time t1 you would like to launch at and the time t2 at which you would like to reach your destination, you solve the Lambert ...



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