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46 votes
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What is the story behind specific impulse being expressed in seconds?

It's even simpler than a German-American disagreement. It's use of ambiguous units. The term "specific X" means the amount of X you can get from a unit mass of something. For instance, in batteries, ...
Tom Spilker's user avatar
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21 votes
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Why use Kerosene?

The hydrogen side of this question has been addressed here: Combination of liquid hydrogen and liquid oxygen -- short answer is that a hydrogen first stage would be lighter, loaded, but much more ...
Russell Borogove's user avatar
18 votes
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Have there been rocket engines designed for diesel? What ISP theoretically possible?

Apparently, at least one OTRAG rocket test used diesel. OTRAG's intended fuel was kerosene with a nitric acid/$N_2O_4$ blend for oxidizer, so I would guess they used a similar oxidizer with diesel. ...
Russell Borogove's user avatar
16 votes

What is the "specific impulse"?

If you want to understand how the 'seconds' value fits the greater image, there's this rather contrived definition (which nobody uses because it's contrived and mostly useless but evocative enough.) ...
SF.'s user avatar
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15 votes
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What is the best chemical rocket fuel from a purely specific impulse standpoint?

From the historical NASA document Space Handbook: Astronautics and Its Applications, by Robert W. Buchheim (1959). Super high-energy bipropellants 300 to 385, ISP Fluorine-Hydrogen Fluorine-Ammonia ...
tckosvic's user avatar
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14 votes
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Is ISP all there is to engine efficiency?

ISP is really just the effective exhaust velocity of the engine normalized by a gravitational constant. It does take into account thrust and mass flow and is a first-order measure of an engine's ...
Erik's user avatar
  • 10.1k
14 votes
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What would be the specific impulse of a continuous nuclear fusion drive?

The easiest way is just to think in terms of energy. Using numbers from wikipedia, the mass of a deuterium nucleus is 2.014 daltons, that of a tritium nucleus is 3.016, helium 4 is 4.0026 and a ...
Steve Linton's user avatar
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14 votes
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Why do chemical rockets encompass a weird shape on this thrust vs. Isp graph?

I'm guessing that the chemical rocket envelope in the plot encompasses points representing actually-built rocket engines, rather than theoretical ones, hence some of the irregularity of the shape is ...
Russell Borogove's user avatar
13 votes
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Understanding Von Braun's Math: Theoretical Isp

I guess that it goes approximately like this: assume that the enthalpy change (I'll denote it $\Delta H$) is fully converted to the kinetic energy of the exhaust and the exhaust moves with velocity $v$...
Litho's user avatar
  • 2,050
13 votes
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Which theoretical propulsion system has the highest specific impulse?

With respect to specific impulse and nothing else? Simple, the photon drive, $c\over g_0$ or $3.057×10^7$ seconds (almost a year). It's pretty damn simple though - any kind of directional light source,...
SF.'s user avatar
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12 votes

Is there a maximum Isp for "exothermic chemical reaction rockets"?

The theoretical limit is set by the specific energy of the reaction of combustion of the propellant. Knowing specific energy $e$ of given substance, we can put a cap on obtainable specific impulse $...
SF.'s user avatar
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12 votes
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Is there a maximum Isp for "exothermic chemical reaction rockets"?

450-455s Isp is typical of H2/O2; according to the Huzel and Huang data, a hydrogen-beryllium mix combusted with oxygen can hit ~540s. The numbers in that table are for moderate chamber pressure and ...
Russell Borogove's user avatar
12 votes
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What's the specific impulse of a human farts/urination?

From the ballistics, I estimate the "exhaust velocity" of a urination stream at about 0.5-1.0 m/s2 (i.e. 0.05-0.1 sec specific impulse) though a number of factors can influence that. Assuming a liter ...
Russell Borogove's user avatar
12 votes
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When does $I_{sp}$ become more of a concern than thrust?

Ideally, thrust and specific impulse would gradually trade off during flight, but that isn’t generally achievable. Instead, large changes in thrust and ISP are done during staging. It's pretty common ...
Russell Borogove's user avatar
12 votes

What is the best chemical rocket fuel from a purely specific impulse standpoint?

TL;DR: Pentaborane or LH2/LOX From a purely specific impulse standpoint, pentaborane is perhaps one of the most potent rocket fuels, providing excellent specific impulse (about 340/365 seconds in atm/...
WarpPrime's user avatar
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11 votes
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Do lower pressure pressure-fed rockets get more Delta-V?

First of all: great observation! This is indeed the reason why pressure fed rocket engines are limited in possible chamber pressure, the added weight from the tanks isn't worth it at a certain point. ...
Ruben's user avatar
  • 1,154
11 votes

Which theoretical propulsion system has the highest specific impulse?

Optimising for Isp only is problematic, as it's simply: $$I_{sp} = \frac{v_e}{g}$$ Which is the same as optimising for exhaust velocity. With no constraints on thrust, particle accelerations can ...
SE - stop firing the good guys's user avatar
11 votes

Why is specific impulse equivalent to effective exhaust velocity?

Rockets produce thrust by ejecting reaction mass at some velocity. The fundamental quantities involved are mass flow rate and exhaust velocity, thrust is the consequence of these. It's no coincidence ...
Christopher James Huff's user avatar
10 votes

Which currently operational orbital launch vehicle achieves highest system-specific impulse?

The current launcher with the highest overall system specific impulse is almost certainly the Delta IV Heavy. As far as I can tell from the list of current orbital launchers on Spaceflight 101, it is ...
Russell Borogove's user avatar
9 votes
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What is the "specific impulse"?

In simplest terms it is just the thrust produced divided by the propellant flow rate. "How much thrust am I getting for the propellant I am expending?" So bigger is better - you are getting more ...
Organic Marble's user avatar
9 votes
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Averaging Specific Impulse for combined propulsion

$$I_{sp}=\dfrac{I_{sp1}\dot{m}_1+I_{sp2}\dot{m}_2+{...}}{\dot{m}_1+\dot{m}_2+{...}}$$ So each $I_{sp}$ is simply weighted by its fraction of the total mass flow rate. This extends to any number of $...
Mark Adler's user avatar
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9 votes
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Apollo SM RCS specific impulse?

Some resources (Wikipedia, Astronautix) give 312 seconds for the R-4D, but I think that's for the large-nozzle modern version. Marquardt's "Apollo SM-LM RCS Engine Development Program Summary Report"...
Russell Borogove's user avatar
8 votes

Is ISP all there is to engine efficiency?

Is there another measure of efficiency that's thrust/mass flow? Or does ISP take that into account Specific impulse (more fully, mass-specific impulse, or impulse per unit mass) is, in fact, equal to ...
Russell Borogove's user avatar
8 votes

Why do chemical rockets encompass a weird shape on this thrust vs. Isp graph?

The historical NASA document "SPACE HANDBOOK: ASTRONAUTICS AND ITS APPLICATIONS" has a useful table, which I will partially reproduce here: TABLE 1.-Specific impulse of some typical ...
Roger's user avatar
  • 1,867
8 votes

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

Since specific impulse and exhaust velocity are directly related via $$I_{SP}=\frac{V_e}{g_0}$$ anything that increases the exhaust velocity necessarily increases the specific impulse. The issue is: ...
Tom Spilker's user avatar
  • 18.3k
8 votes

Rocket engine with lowest Isp to reach orbit

The only French orbital rocket ever flown was the Diamant. In the first configuration (Diamant A), the first stage only had, according to Wikipedia and a document produced by the engine manufacturer, ...
Xx_DrkLeo_xX's user avatar
7 votes
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Is there a maximum ISp?

A photon rocket should have a Ve of c, hence ~30 megasecond Isp. The rocket equation would be tricky to apply, of course -- if you have a magic matter-energy reaction that perfectly converts "fuel" ...
Russell Borogove's user avatar
7 votes
Accepted

What specific impulse can I expect from iron powder produced from lunar regolith?

Figure 5 on page 23 of this paper shows the Isp of tri-propellants including iron, where the curve goes out to 95% Fe and 5% H2. Just eyeballing it, the curves might converge at around 190 seconds. On ...
Mark Adler's user avatar
  • 58.2k
7 votes

Why use Kerosene?

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 would they use kerosene instead? First off, that's wikipedia for you, ...
David Hammen's user avatar

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