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Questions tagged [specific-impulse]

Questions about specific impulse (Isp), which is a comparative measure of efficiency of rocket, jet engines and used propellants, measured either in seconds of thrust per unit weight of propellants, or effective exhaust velocity (thrust per unit mass of propellants) as a measure of launcher system's specific energy efficiency.

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If specific impulse is directly related to exhaust velocity, would a ion post-accelerator improve the Isp of a propulsion system?

I understand that ion- and hall thrusters resemble a particle accelerator, and that led me to wonder if a post-accelerator (particle accelerator) used to boost the velocity of the ions would improve ...
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Why do chemical rockets encompass a weird shape on this thrust vs. Isp graph?

Consider this graph of engine thrust versus specific impulse (from https://dawn.jpl.nasa.gov/mission/ion_prop.html): Most propulsion technologies encompass roughly rectangular regions on the graph. ...
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What would be the specific impulse of a continuous nuclear fusion drive?

Let's assume problems of running sustained nuclear fusion are overcome (be it by making the mechanism highly energy-positive, or just supplying all the energy deficit externally, like beamed power.) ...
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When does $I_{sp}$ become more of a concern than thrust?

In an orbital launch from Earth, the priority in the initial phase of the launch is high thrust, so as to actually get off the ground, and minimise losses due to "gravity drag" and air resistance. ...
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Optimal thrust vs isp for variable isp rockets (VASIMR) when taking off/orbiting

This question was initially inspired through playing Kerbal Space Program, however I am seeking a more general answer. I've been playing the excellent Near Future Propulsion mod for KSP, using ...
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Which chemical propellant combination has the highest theoretical isp?

That is disregarding practical factors like making a bomb rather than a rocket in the case of using acetylene+ozone. I know that the highest isp achieved was 542 with lithium, hydrogen, and fluorine. ...
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How do I get Specific impulse in (s) from (N * s/kg)?

Gross mass 19.32 tonnes Thrust 1,260 kN Specific impulse 2314 N·s/kg How do I get Specific impulse in (s) ?
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How do you calculate total Thrust and Specific Impulse for a rocket stage?

Given the stage has the following configuration: Thrust 741.4 kN (Main) 47.1 kN (Vernier) Specific impulse 297.999 s (Main) 288.999 s (Vernier) How ...
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How can i design a finite thrust maneuver to approximate an impulse thrust maneuver?

An impulse thrust maneuver is a good approximation of a true orbital maneuver. Of course, a true impulse maneuver in reality would require an infinite amount of force in an infinitely small amount of ...
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Understanding Von Braun's Math: Theoretical Isp

I've just read Wernher Von Braun's Project Mars: A Technical Tale, and I'm trying to understand the math in the back. I can follow some of what he's doing, but other parts I just can't tell where his ...
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What's the Isp of a model rocket “D-size” engine, compared to the Isp of engines that do reach space?

Now that I've learned a bit about real rockets, I wondered how a model rocket D-size engine fits in quantitatively. The Wikipedia article Model rocket lists thrust in Newtons, and Total Impulse in ...
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What is the “specific impulse”? [duplicate]

One of the most important values when talking about rocket engines is the so-called specific impulse. I have read the Wikipedia article but still have trouble understanding what it really is and what ...
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Deriving specific impulse from exhaust velocities? [duplicate]

I can't remember where I heard it but I thought that dividing the exhaust velocity by the gravitational pull of a body, it is possible to find the specific impulse of the rocket engine. However, what ...
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What's the specific impulse of a human farts/urination?

I was curious about how effective farting/urination would be as a propulsion system, but I couldn't find any data on the specific impulse of human farts/urination. Could you actually get a couple of ...
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Was a large-nozzle variant of the J-2 ever considered?

The J-2 engine used on the upper stages of the Saturn launchers has a specific impulse in vacuum of 421 seconds. This is substantially lower than that of the smaller RL10, 440-460 seconds depending on ...
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Why use Kerosene?

According to SpaceX's website, the Falcon 9 uses kerosene as fuel. According to Wikipedia, diesel fuel beats kerosene in energy density in terms mass by 2.8 MJ/Kg and in volume by 5.6MJ/L, so why ...
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Comparison between solid rocket fuels. Which are better by performance and cost? [closed]

I am interested for a comparison of solid rocket fuels by their performance and cost. Comparison by specific impulse (Isp), energy that gives (which one is more energitic), density, cost to ...
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Is there a maximum Isp for “exothermic chemical reaction rockets”?

The question Is there a maximum $\text{I}_{sp}$? reminded me that I once read somewhere that the maximum possible $\text{I}_{sp}$ for a rocket engine based on expansion driven by exothermic, ...
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Is there a maximum ISp?

With ISp defined as ${v_e}\over{g_0}$, the naive approach would tell me, the maximum possible specific impulse, with propellant moving at speed of light relative to the craft, is $c\over{g_0}$ or 30,...
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What specific impulse can I expect from iron powder produced from lunar regolith?

Getting rocket propellant from other sources than Earth is by many viewed as an important step towards further exploration of space. One such source could be our closest neighbour, the Moon. The ...
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Averaging Specific Impulse for combined propulsion

How would one go about calculating ISp of a rocket with combined propulsion? E.g. the Space Shuttle during the phase of launch, with both SRBs and SSME active? Of course there's the simple "...
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Is this a correct understanding of Tsiolkovsky's rocket equation?

When I graph the rocket equation, substituting arbitrary values for v(exhaust) and m1, so m0 because m1 - m0, the graph implies that increasing propellant mass past a certain point does not increase ...
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Is liquid rocket engine more or less efficient when throttled?

How does the efficiency of the rocket engine using liquid propellants depend on the throttling setting? (Is it more or less efficient while working on eg.: 85% of its maximum thrust?). The rocket ...
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Why do we use the gravitational acceleration at the Earth's surface when calculating the specific impulse of an engine?

So one of the ways of calculating Isp is: Isp = Ve/g0 You divide the effective exhaust velocity by acceleration at earth's surface. This gives you a number in seconds, and is generally considered ...
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Rocket Propulsion Elements: Total Impulse Problem

I figured this is probably an okay place to post this question. I managed to get (a), (c) and (d) (quite simply), but I have absolutely no idea why I can't work out (b). I used $$ I_{sp}=\frac{I_t}{...
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Relationship between part dimensions and performance of ion electrospray thruster

I got the relationship between power, thrust and specific impulse for an ion electrospray thruster as follows: $$P/T= 1/2 \times g_o \times I_{\mathrm{sp}}$$ Where: $P$ = power required (in Watts), $...
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Most energy efficient mass ratio of a rocket, or exhaust velocity

In the process of thinking about some hard science fiction, I need to substitute some mass fractions that will minimize the amount of energy used in order to be maximally generous to the author. So ...
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Why do lower molar mass rocket exhaust products give higher specific impulse?

I tried to work this out. My thinking is as follows. If the same amount of energy is given to a low mass particle and a high mass particle, the low mass particle is faster. A particle with a mass of 1 ...
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Which currently operational orbital launch vehicle achieves highest system-specific impulse?

I'm primarily looking for comparison of currently operational orbital launch vehicles by system-specific impulse, or Issp, and to get a fair impression of achievable rocket propellant performance and ...
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Nuclear thermal rocket specific impulse calculation uses 1 amu, is that wrong?

Following the parameters in the Wikipedia nuclear thermal rocket article, it seems to paint somewhat of a shaky world view. Consider these quotes: Current (2010) 25,000 pound-thrust reference ...