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Consider this graph of engine thrust versus specific impulse (from https://dawn.jpl.nasa.gov/mission/ion_prop.html):

enter image description here

Most propulsion technologies encompass roughly rectangular regions on the graph. Electric propulsion is the union of two rectangular regions (ion and magnetic). However, chemical propulsion has an irregular shape.

Why does chemical propulsion have this irregular shape?

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  • $\begingroup$ Acheivable Isp is smaller for low thrust chemical engines than for larger ones. Different fuel/oxidizer combinations are used for small, medium and large thrust engines. But I would like to see this graph extended to MN (meganewtons). The F-1 engine of Saturn V had about 7 MN, the J-2 engine of second and third stage 1 MN. $\endgroup$ – Uwe Nov 14 '18 at 14:47
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    $\begingroup$ Speculation: small engines have relatively higher thermal losses than large ones. $\endgroup$ – Hobbes Nov 14 '18 at 19:54
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    $\begingroup$ @Uwe it'd roughly just extend farther right. Low 300's (solid) to mid 400's (hydralox) is still the performance range for really big chemical engines. $\endgroup$ – Dan is Fiddling by Firelight Nov 14 '18 at 21:49
  • $\begingroup$ ISP is a measure of the average kinetic energy of the exhaust products. All this is telling you is that for low thrust chemical engines it is difficult to produce very high exhaust velocities. Only when the engines get above a certain size can they accommodate the infrastructure required to produce the highest chemical exhaust velocities. For smaller engines to achieve such high ISP would require adding too much weight to the designs, eating up any delta-V the higher ISP might add, so they are simply not made. $\endgroup$ – J... Nov 14 '18 at 23:44
  • $\begingroup$ I speculate that the shape is wierd because the graph is logarithmic. On a non-logarithmic scale, it's probably just a diagonal parabola. $\endgroup$ – Mooing Duck Nov 15 '18 at 1:05
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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 due to historical accident.

10N is quite small for a chemical rocket engine. Such units are mainly used for attitude control of small spacecraft rather than making significant maneuvers, so reliability, simplicity, and light weight are more critical than specific impulse. I'm not sure if it's not actually possible to make high-Isp small thrusters, or if it's just that no one bothers.

In particular, I note that Aerojet's catalog of bipropellant (MMH+NTO) thrusters (which get around 300 seconds Isp) extends down to 22N; below that, they offer catalyzed hydrazine monoprop thrusters (around 220 seconds) down to 1N; the engineering simplicity of requiring only a single propellant tank pays for the loss in specific impulse (and it's probably tricky to get good bipropellant mixing in such a small combustion chamber). These two categories of thruster contribute to the left two-thirds of the chemical engine envelope in the plot.

Further up and to the right, small hydrogen-oxygen engines stake out the high end of the Isp envelope: the Chinese YF-73 at 44kN and 420 seconds, then a bunch of engines in the neighborhood of the US RL10: 65-100 kN and 440-460 seconds. That's the high-water specific impulse mark for production chemical engines, the space shuttle main engine RS-25 is off the right hand side of the chart at 2200kN and 452 seconds. Again, I'm not sure whether it's possible to make high-Isp hydrogen-oxygen engines smaller than ~40kN or whether it's just not done.

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  • $\begingroup$ Would be interesting indeed whether small high-Isp engines are possible, with modern 3D-printing techniques and whatnot. I'd see considerable interest for such engines in the near future. $\endgroup$ – leftaroundabout Nov 14 '18 at 23:18
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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 chemical propellants

Low-energy monopropellants________________________ 160 to 190.

High-energy monopropellants: Nitromethane_______________________________ 190 to 230

Bipropellants (liquid): Low-energy bipropellants___________________________ 200 to 230.

Medium-energy bipropellants________________________ 230 to 260.

High-energy bipropellants___________________________ 250 to 270.

Very high-energy bipropellants_______________________ 270 to 330.

Super high-energy bipropellants_______________________ 300 to 385.

Boron metal components and oxidant____________________ 200 to 250.

Lithium metal components and oxidant___________________ 200 to 250.

It seems very likely to me that the "Chemical" category encompasses several different fuels, and probably both solid and liquid fuels, which is why it's not a uniform shape.

Someone more expert might be able to guess which specific fuels compose the various areas.

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    $\begingroup$ That table is extremely dated (ca 1958 maybe?) -- hydrogen/oxygen isn't even listed, but the RL10 was flying a 425+ sec specific impulse on hydrogen/oxygen by 1962. $\endgroup$ – Russell Borogove Nov 14 '18 at 19:41
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The "Chemical" shape exists because of two clear regimes. This is a bit by chance because of the propellants involved but basically its this:

  • engines on the left are pressure fed or solid
  • engines on the right are various types of pump fed (expander cycle/gas generator/staged combustion/electric)

As an aside the diagram doesn't really clarify the whole range of posiblities for electrically augmented hydrazine monopropellant, e.g. - power augmented catalytic hydrazine thruster (Isp = 250 - 280) - hydrazine arcjet (Isp 550 - 600)

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  • $\begingroup$ Aren’t arcjets covered by “electrothermal” in the plot? $\endgroup$ – Russell Borogove Nov 15 '18 at 1:05
  • $\begingroup$ They might be, I actually found that particular bubble on the plot hard to understand. N2H4 decomposes, I think, to H2, N2 and NH3, the presence of most of those in the bubble made me wonder what the labels meant. $\endgroup$ – Puffin Nov 15 '18 at 23:17
  • $\begingroup$ I read that as "electrothermal thrusters using hydrogen, hydrazine, or ammonia as reaction mass". $\endgroup$ – Russell Borogove Nov 15 '18 at 23:25

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