# Tag Info

52

Monopropellant systems such as catalyzed hydrazine thrusters are attractive at very small sizes, where the simplicity of a single propellant tank outweighs their relatively low performance. According to Wikipedia, Cavea-B requires a small amount of UDMH or a similar hypergolic to begin ignition -- every time you want to fire it, which can be a frequent ...

40

Previously posted comments are correct: in free space (assumed free of any other bodies' gravity fields) there is no way to convert the reaction wheels' angular motion to translational motion. There is one tongue-in-cheek way: throw a reaction wheel off the spacecraft in the direction opposite the direction of the desired delta-V! ;-) If you abandon the ...

35

To answer your title question: By using its engines. However you seems to be quite puzzled by the fact that velocity of an object can decrease and increase over the course of an orbit. If the orbit is perfectly circular, the speed will always remain the same (until thrusters are used). However, as is the case with Chandrayaan-2, most orbits are ...

21

John D Clark, Ignition!, p.165, on the testing of a Cavea B motor: “Well, through a combination of this and that, the motor blew on startup. We never discovered whether or not the [detonation] traps worked —we couldn't find enough fragments to find out. The fragments from the injector just short-circuited the traps, smashed into the tank, and set off the ...

17

I do suspect there's an obvious way to work it out from the specific impulse, which I know is Ns/kg.. but I'm not quite sure how I use that information. Does it mean that an engine with a specific impulse of 12000Ns/kg, can put out 1N of force for 12000 seconds, using 1kg of propellant? Exactly so. "Specific impulse" is short for "mass-specific impulse", ...

15

From what I've been able to find, the "fluff" is that the Epstein Drive electromagnetically accelerates the exhaust, improving propellant consumption over a "standard" inertially-confined fusion drive. Electromagnetically accelerating plasma is certainly physically plausible...real world ion and plasma thruster designs do just that. The problem is that ...

14

The VASMIR 200 is listed as having a thrust of 5.4 newtons, and you need 9.8 newtons to lift 1kg against earth's gravity. So 700 tonnes is going to need more than a million engines and be consuming more than 254 GW of electricity. So even if the engines are weightless this is not lifting off from earth without co-opting the power generation of a sizable ...

13

You can't directly convert them; they're two different measures. As Organic Marble points out, thrust is loosely analogous to a car's horsepower; specific impulse is loosely analogous to a car's gas mileage. Thrust in newtons is equal to the propellant mass exhaust flow rate in kg/s times the specific impulse in N·s/kg (which is dimensionally equivalent to ...

13

Unfortunately steam rockets would not be practical because they're not very efficient. I don't know how much you know about rocket propulsion so I'll begin with a discussion of one of the most important metrics: exhaust velocity, the speed of the gas coming out of the engine's nozzle. The higher that speed, generally the more efficient the engine. There ...

13

The properties of the fictional Epstein Drive are: Very high ISP with an exhaust velocity at least several percent of the speed of light Very high thrust while achieving that ISP or near to it, enough for 5+ g burns. Fusion powered. Based on the first two parameters, the thrust power is insanely high - this is not a fission sail, NTR, or low-power fusion ...

10

Conceptually, staging is getting rid of hardware we no longer need. Keeping useless hardware attached to the rocket is expensive, since added mass reduces acceleration. Ideally, we would want to get rid of hardware as soon as it gets useless, instead of piling it up to a "batch", which is essentially what the question boils down to. Rockets are very simple*...

10

I think this has to be evaluated for each propellant combination and situation individually. I was involved in a horizontal ground test program at AeroAstro in the mid 90's for a LOX/RP-1 (kerosene) engine that was designed to be LOX lead to start. After a very short burn was computer aborted due to an accidental trigger of the "high thrust" alarm, the ...

10

The energy efficiency isn't terribly useful when applied to rocket engines for space exploration. Using this definition of engine efficiency a cold gas thruster is more efficient than an ion engine$^{*}$ It's concerning the efficiency of with which the energy extracted from the fuel is converted to a force on the vehicle. It basically says if your exhaust is ...

8

This question turns out to be surprisingly involved to answer. 1. "Brachistochrone" is a 17th-century term for a particular physics problem. The term appears to have originated in 1694[src] (or several years later, by source), with Johann Bernoulli, in the course of originating (or at least popularizing) the Brachistochrone Problem (whose solution is the ...

8

The Everyday Astronaut just released an hour long video investigating this question. Some of the main points are: Aerospikes are especially advantageous to single stage to orbit vehicles, and current space companies are not building those. There isn't really an advantage in SSTOs compared to multi stage rockets. The efficiency advantage of aerospikes isn'...

7

No, it's a textbook case of conservation of the linear momentum vector in the absence of any external forces. Linear momentum of a system Sum(mv) is a conserved quantity even if individual parts are allowed to change their momentum vectors. Actually reaction wheels also conserve angular momentum of the total system (ship + wheel) as well! But that's Okay ...

7

@tfb is correct: this is another form of ion propulsion, or generally, electric propulsion. A good general reference for electric propulsion is Fundamentals of Electric Propulsion: Ion and Hall Thrusters from JPL's DESCANSO series. The problem with electric propulsion using ultra-high exhaust velocities is the power required to drive the exhaust beam. If ...

7

There have been a number of different QAs on this site over the years discussing various methods of giving a launcher some initial advantage: What benefits can be gained from launching below ground? Could fuel be pumped from the ground to a launcher? What technological hurdles prevent the development of a space gun? What are the biggest challenges for high ...

7

Yes. Thrust is additive. If you want more thrust, adding more engines or scaling up the size of your engines are both valid ways of doing it. There is a balancing act you have to make, however. At one extreme, single large engines can have unstable exhausts where the combustion products 'stick' to one side of the nozzle, to a first degree of ...

6

tl;dr: The problem is that this idea relies on a fundamental misunderstanding of Special Relativity. $$+$$ Newton's second law of $F = ma$ doesn't work, exactly, in Special Relativity. $$+$$ But if you instead write down Newton's second law as $F = dp/dt$, where $p$ is momentum, this does work exactly (and equally for all observers), even in ...

6

After reading all your answers I'd like to summarise the situation. The black circles are the circular orbits and the red ellipse is the transfer orbit. Consider a spacecraft in the elliptical orbit. At the point P the velocity is greater than the circular orbital velocity, and that's why the distance from the centre increases. And at the point A the ...

6

It appears to be unworkable. If Burns had properly accounted for the total momentum of the box+ring system, which must include the energy/momentum of the applied fields and forces required to accelerate the individual components (like the ring) inside the box, he would have noted that the total momentum never changes, even under relativistic transformations ...

6

Ruling out nuclear propulsion in the comments pretty much rules out electric thrusters of all flavours as well since the Juno mission is notable for pushing the limits of solar power, and only needs enough to operate sensors and radio link and is still 1/5 solar panels by dry weight (340 kg making 486 W, where electric demands for thrusters are in kW). For ...

6

It would have more TWR, the same efficiency, but the matter is almost irrelevant anyway. (If you're trying to build a weapon, not a thruster, the laser is better because it is collimated in a narrow beam and the lamp isn't.) Lasers are inefficient... and their inefficiency means they output heat. Which can only be disposed of in space by radiation. So if ...

5

Apparently there isn't much xenon, from any source, in the moon's "atmosphere". I put "atmosphere" in quotation marks because it's actually an exosphere, where the distance an atom or molecule travels between collisions with other atoms or molecules is greater than the scale height. In the moon's case, it is much greater. Overwhelmingly, particles (atoms or ...

5

The amount of propellant required to achieve a certain delta-V is dependent on the ratio between the starting and ending mass of the spacecraft, according to the Tsiolkovsky rocket equation; a given thruster and fuel supply will get you more delta-V on a smaller spacecraft and less delta-V on a larger one. That is, 0.058 km/s per kg is not an inherent ...

5

The method described proposes a propulsive force based on following the gradient of the vacuum energy density. However, I see two problems There is no evidence given in the question showing that a gradient in vacuum energy exists. Even if one did exist (some location has a lower vacuum energy than another location) there is no evidence given in the ...

5

According to various bits of information from Apollo Flight Journals (Apollo- 8, 10 to 13 and 15): LH2 was dumped propulsively through Continuous Vent System (very small delta-v). LOX and pressurizing helium were propulsively dumped through the J-2 engine bell. in addition to the above, Auxiliary Propulsion System (two ullage motors) were mentioned to be ...

5

I managed to reproduce your error. When it says "^1/2" in the equation, you are supposed to take the square root of the quantity in the parentheses. You are not doing this. You are dividing by 2 instead. 0.0906 / 169.34 = 0.000535 365659 / 38.64 = 9463 .000535 * 9463 = 5.063 5.063 / 2 = 2.53 It should be .000535 * (9463)^(1/2) = 0.052 .000535 * 97....

5

That was a good description of a magnetic sail, which is a concept that is out there but hasn't been tested yet. It can change the direction of thrust by changing the orientation of the magnetic field. It has a theoretical advantage over solar sails in that the weight increases with the circumference of the sail rather than with the area, but I guess I ...

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