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The NPR News item What's Iran Up To With Recent Rocket Launch Attempt? by NPR's Geoff Brumfiel, about a recent launch test in Iran includes the following:

Earlier this week, Iran attempted to launch a rocket carrying a satellite into space. The Trump administration says their goal is really to develop long-range weapons. NPR's Geoff Brumfiel looks into what Iran is up to.

[...]

Markus Schiller is the founder of ST Analytics, an independent consultancy in Germany. He's spent a lot of time looking at Iran's space program, and he says there are links to the military. For example, the engines Iran uses on its space rockets have a military origin.

MARKUS SCHILLER: It's actually a missile engine.

BRUMFIEL/NPR: But Schiller says it's not a very good missile engine. It's an old design from the Soviet Union, picked up by the North Koreans and later transferred to Iran. It's clunky and inefficient. To get even a small payload into space requires the rocket to be huge. It takes weeks to set up.

Also, Schiller says, based on photos, Iran's space rocket can't work as a missile. The second stage is just too small. To him, the launch looks nonthreatening.

Question: What actual engine are they talking about, and in what way is it "not a very good missile engine"? Why would this engine require "the rocket to be huge"?

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    $\begingroup$ taking weeks to set up would make it a very poor missile engine -- the Thor missiles the UK bought from the US were criticised for their 15 minute launch time. $\endgroup$ – JCRM Jan 20 at 9:49
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    $\begingroup$ an inefficient engine requires more fuel, requiring exponentially larger fuel tanks because of the rocket equation. $\endgroup$ – JCRM Jan 20 at 10:33
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    $\begingroup$ your source said it was inefficient, not undersized. Insufficient thrust can be worked around by using multiple engines or a kick stage $\endgroup$ – JCRM Jan 20 at 11:55
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    $\begingroup$ @jamesqf it goes into the trade space because an engine with bad Isp causes the vehicle to get larger for a given mission. So the missiles' launchers, fueling systems, etc, etc all get bigger. But which is cheaper, building a big launch trailer or developing a better engine? $\endgroup$ – Organic Marble Jan 20 at 22:57
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    $\begingroup$ Wouldn't it be more threatening if you're testing an "orbital" rocket design with an engine that has a very low hope of getting anything useful into space? That sounds like a passable IRBM then. $\endgroup$ – Nick T Jan 21 at 2:01
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What actual engine are they talking about

Organic Marble has identified it as the S5.2/9D21.

in what way is it "not a very good missile engine"?

It uses a propellant combination, kerosene/IRFNA, common for military missiles in the 1950s-1960s, but now considered obsolete, which yields a poor specific impulse (233 seconds at sea level, or effective exhaust velocity of 2285 m/s).

Why would this engine require "the rocket to be huge"?

Rearranging the rocket equation:

$$\Delta v = v_\text{e} \ln \frac {m_0} {m_f}$$

to

$$e ^ \frac {\Delta v} { v_\text{e} } = \frac {m_0} {m_f}$$

shows that the mass of a rocket is proportional to the exponential of (delta-v divided by effective exhaust velocity). Small changes in the exhaust velocity can yield large changes in mass.

Assuming ~9400 m/s delta-v required to reach orbit, a 233 second engine requires an initial-mass-to-final-mass ratio of around 60; a good modern kerosene/LOX engine like the RD-180 of Atlas V has a specific impulse of 311 seconds at sea level, which requires a mass ratio of around 20 -- a 3x lighter rocket from a 33% more efficient engine.

The statement "not a very good missile engine" needs qualification; it's not a very good long-range missile engine (which, to be fair, is an important question about Iranian engines if you're a German analyst) because of the low specific impulse. The Scud series the engine is derived from is a ~300km range tactical missile; for such a short flight the required ∆v is much lower; it's thus in a shallower part of the exponential curve and the penalty for poor specific impulse is much smaller.

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  • $\begingroup$ tag-teams? is the gamification of SE moving to a new level? ;-) $\endgroup$ – uhoh Jan 20 at 15:11
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    $\begingroup$ @uhoh now you can edit all three together and post the perfect answer you seek! $\endgroup$ – Organic Marble Jan 20 at 15:16
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    $\begingroup$ You know I can't pass up a chance to link the rocket equation and specific impulse WP pages. $\endgroup$ – Russell Borogove Jan 20 at 16:37
  • $\begingroup$ Your last sentence provides a lot of insight. $\endgroup$ – uhoh Jan 20 at 23:29
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    $\begingroup$ I said the propellant combo was obsolete, but, I mean, yeah? Russia fields solid-fueled rockets in the short-range ballistic role these days. SS-21, SS-23, SS-26 for example. Solids are much safer to operate. $\endgroup$ – Russell Borogove Jan 21 at 4:09
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Supplemental answer: Scud missiles were powered by the S5.2/9D21 engine, burning inhibited red fuming nitric acid and kerosene.

Thrust is 132 kN, Isp is "2285 Nskg" or ~233 seconds.

enter image description here

enter image description here

Sources: 1,2,3

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    $\begingroup$ Ns/kg is newton-seconds per kilogram, the “correct” SI unit for specific impulse, equivalent to meters per second. Divide by 9.806 to get ISP in seconds. $\endgroup$ – Russell Borogove Jan 20 at 14:23
  • $\begingroup$ Thanks. I was just looking at the wikipedia article and found that in there as well. $\endgroup$ – Organic Marble Jan 20 at 14:25
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    $\begingroup$ And a hell of an oxidizer to handle in the field! Wow. $\endgroup$ – Organic Marble Jan 20 at 14:26
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    $\begingroup$ adding that even under ideal conditions that ISP could only get about 1.5% of launch mass to orbit (and this isn't an ideal rocket) would make this a complete answer. $\endgroup$ – uhoh Jan 20 at 14:51
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    $\begingroup$ I rolled back the edit because the units on Isp are exactly what was in the source document, hence the quote marks. $\endgroup$ – Organic Marble Jan 20 at 14:54
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According to Gunter's web site, this launcher originated in the Shahab-5 IRBM, which is a continuation of the Scud-D / No-dong-A, No-dong-B and Shahab-5 / 6 / Taep’o-dong-2B / Unha-2

And Scud-D is originated in Soviet R-17 military missle

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    $\begingroup$ Is it possible to name the engine itself or at least say something about its capability in terms of orbital applications? $\endgroup$ – uhoh Jan 20 at 12:18

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