46
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Accepted
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, ...
- 18.2k
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 ...
- 167k
18
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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.
...
- 167k
16
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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.)
...
- 54.9k
15
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Is this a correct understanding of Tsiolkovsky's rocket equation?
Your question is about the behavior of the Tsiolkovsky rocket equation itself, in the limit of very small final mass (dry mass). Roughly: "is there any limit to delta-v in theory?"
Using MathJax:
$$
...
- 149k
15
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Is this a correct understanding of Tsiolkovsky's rocket equation?
The implication of the rocket equation is that linear increases in ∆v require exponential increases in mass ratio for a single stage.
There's not strictly a maximum delta-v -- if you redo your plot ...
- 167k
15
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Accepted
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
...
- 2,262
14
votes
Accepted
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 ...
- 10.1k
14
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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 ...
- 19.4k
14
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Accepted
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 ...
- 167k
13
votes
Accepted
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$...
- 2,040
13
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Accepted
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,...
- 54.9k
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 $...
- 54.9k
12
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Accepted
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 ...
- 167k
12
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Accepted
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 ...
- 167k
12
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Accepted
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 ...
- 167k
12
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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/...
- 3,095
11
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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}$$
Which is the same as optimising for exhaust velocity.
With no constraints on thrust, particle accelerations can ...
11
votes
Why is specific impulse equivalent to effective exhaust velocity?
Rockets produce thrust by ejecting reaction mass at some velocity. The fundamental quantities involved are mass flow rate and exhaust velocity, thrust is the consequence of these.
It's no coincidence ...
- 10.7k
10
votes
Which currently operational orbital launch vehicle achieves highest system-specific impulse?
The current launcher with the highest overall system specific impulse is almost certainly the Delta IV Heavy.
As far as I can tell from the list of current orbital launchers on Spaceflight 101, it is ...
- 167k
9
votes
Accepted
What is the "specific impulse"?
In simplest terms it is just the thrust produced divided by the propellant flow rate. "How much thrust am I getting for the propellant I am expending?"
So bigger is better - you are getting more ...
- 178k
9
votes
Accepted
Averaging Specific Impulse for combined propulsion
$$I_{sp}=\dfrac{I_{sp1}\dot{m}_1+I_{sp2}\dot{m}_2+{...}}{\dot{m}_1+\dot{m}_2+{...}}$$
So each $I_{sp}$ is simply weighted by its fraction of the total mass flow rate. This extends to any number of $...
- 58.5k
9
votes
Accepted
Apollo SM RCS specific impulse?
Some resources (Wikipedia, Astronautix) give 312 seconds for the R-4D, but I think that's for the large-nozzle modern version.
Marquardt's "Apollo SM-LM RCS Engine Development Program Summary Report"...
- 167k
8
votes
Is liquid rocket engine more or less efficient when throttled?
Corroborating Russell Borogove's answer, some Stennis test data I have from 1987 on three different SSMEs shows a small drop in Isp with power level. From 109% to 100% the Isp dropped about 0.08%. I ...
- 178k
8
votes
Is liquid rocket engine more or less efficient when throttled?
It's surprisingly difficult to find a good answer to this question.
Generally, the rated full power level is where the engine is going to be most efficient.
According to Sutton's "Rocket Propulsion ...
- 167k
8
votes
Is ISP all there is to engine efficiency?
Is there another measure of efficiency that's thrust/mass flow? Or does ISP take that into account
Specific impulse (more fully, mass-specific impulse, or impulse
per unit mass) is, in fact, equal to ...
- 167k
8
votes
Why do chemical rockets encompass a weird shape on this thrust vs. Isp graph?
The historical NASA document "SPACE HANDBOOK: ASTRONAUTICS AND ITS APPLICATIONS" has a useful table, which I will partially reproduce here:
TABLE 1.-Specific impulse of some typical ...
- 1,857
8
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If specific impulse is directly related to exhaust velocity, would a ion post-accelerator improve the Isp of a propulsion system?
Since specific impulse and exhaust velocity are directly related via $$I_{SP}=\frac{V_e}{g_0}$$ anything that increases the exhaust velocity necessarily increases the specific impulse.
The issue is: ...
- 18.2k
7
votes
Deriving specific impulse from exhaust velocities?
The conversion factor isn't the gravitational pull of an arbitrary body, but rather the conversion between pounds as a unit of weight and pounds as a unit of mass -- which is a convention established ...
- 167k
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, ...
- 73.2k
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