# Tag Info

56

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  ...

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 ...

27

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 ...

20

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

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 ...

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.

12

Nowadays, rockets use 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 swiveled from side to side. As the nozzle is moved, ...

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 ...

11

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.

10

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 $\Delta p = m\Delta v + v\Delta m + \Delta ... 9 At this stage, does the rocket still accelerate the craft? If by "velocity of the exhaust" we are talking about its velocity measured in the frame of the rocket, then Yes. Let$\mathbf ube 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 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 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 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 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). ... 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 ... 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 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 ... 6 For what it's worth, even though a rocket starts its flight going straight up, once it has traveled through most of the atmosphere it soon starts to change its direction so that it spends most of its flight accelerating in the "around the earth" direction (i.e. basically horizontal). Also, to reach orbit a vehicle either has to reach a high enough speed, or ... 6 It isn't needed in a rocket, however if you are going to the effort of sending something up outside the atmosphere (or even just high up within the atmosphere) you might as well try and get some useful data out of it. This might even help you get sponsorship for your rocket, as data from climbs through altitude is useful to a number of academic institutions. ... 6 There are two concepts here that may be getting mixed together, namely, conservation of linear momentum and conservation of angular momentum. Newtons laws state that an object in motion will say in motion unless acted upon by an external force. So unless interstellar friction is a problem, the spaceship will keep travelling linearly in the same direction ... 6 The intensity will not increase (of course you have to be careful in the definition of intensity because it's trivial that intensity depends on the speed of shuttle since it's, in its simplest form, defined as the rate of flow of photons per unit time), but the frequency will change, and all the photons from the Cosmic Microwave Background (CMB) will become ... 5 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 ... 5 It would be an extremely cumbersome and inefficient way to do it. Already one uses the acceleration of ions in ion propulsion systems in space: An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either ... 5 Launch weight would be lower if you had a fixed fuel load, and only one engine and fuel tank. However the specific impulse applied to the payload would be lower. The problem is that even when the fuel is say 90% exhausted, the rocket is still trying to accelerate the now grossly oversized fuel tank and engine. So the trick is to try to reduce the deadweight ... 5M$is reducing. Thus,$\mathrm dM$has a negative value. In contrast, in the above equations, you can see an$M-\mathrm dm$term. Here, we can see that$M$will reduce only if$\mathrm dm$has a positive value. In other words, when time goes forwards, the mass that got thrown out ($m$) is increasing, thus$\mathrm dm$is positive. In contrast,$M\$ ...

5

Since you're doing the thought experiment anyway, there are at least two important engineering considerations that limit this more severely than what accelerations humans can stand (which is trivially solvable with robots anyway): (1) The energy needs would be huge. What percentage of the total mass would have to be fuel in order to supply enough energy to ...

5

A 747 - can get you to around 35,000 feet. Still very much within the atmosphere. So what do you do then? Launching a rocket from that point still requires an awful lot of kit, so while you have reduced your propellant requirements a little, the 747 still has to carry a launch platform, so you're not really getting anything out of this. New technologies, ...

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