Why was the engine of the launch vehicle recently tested in Iran "not a very good missile engine"?

Question

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"?

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. — , Jan 20 '19 at 09:49
an inefficient engine requires more fuel, requiring exponentially larger fuel tanks because of the rocket equation. — , Jan 20 '19 at 10:33
@JCRM less likely, but it could also be reasonably efficient but have insufficient thrust, therefore accelerate slower and spend too much time fighting gravity. Let's identify the engine, see what was inadequate about it, and find out why this ended up being the engine used in the launch test. — uhoh, Jan 20 '19 at 10:40
your source said it was inefficient, not undersized. Insufficient thrust can be worked around by using multiple engines or a kick stage — , Jan 20 '19 at 11:55
@JCRM it's an interview for a non-technical audience, it could be hand-waving or a wrong-wordism. You're probably right, but let's not do the Iranian Space Agency's job for them ;-) — uhoh, Jan 20 '19 at 11:59
I have to wonder, though, whether efficiency is really a prime criterion for a missile engine - as long as it is sufficient for the job, of course. — jamesqf, Jan 20 '19 at 17:51
@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? — Organic Marble, Jan 20 '19 at 22:57
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. — Nick T, Jan 21 '19 at 02:01
@Organic Marble: My point is that, unlike satellite launches, you (especially if you are a regime like Iran, North Korea, &c) only get launch a significant number of the missiles once, at mostt. Afterwards you are not likely to be in a position to launch more. — jamesqf, Jan 21 '19 at 03:45
@JCRM: Not if it were meant for a first-strike weapon - you can take as long as you want to set it up when you don't have to worry about there already being incoming missiles. — Vikki, Jan 21 '19 at 03:49
@JCRM Is the "set up" here talking about launching? I would have guessed that "set up" refers to installing the missile in its silo (or whatever it's launched from) and getting it ready to launch, not the actual last-minute launch sequence once the missile is in position and ready to go. — reirab, Jan 21 '19 at 16:37

Russell Borogove · Accepted Answer · 2020-04-23T15:45:36.003

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"?

The old Soviet engine on the Scud-B 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). Somewhere in the development through the North Korean Nodong series to the Iranian missile, it appears the combination changed to TM-185 (20% gasoline, 80% kerosene) fuel with AK-27I (27% N2O4, 70% HNO3) oxidizer, with similar specific impulse.

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.

tag-teams? is the gamification of SE moving to a new level? ;-) — uhoh, Jan 20 '19 at 15:11
@uhoh now you can edit all three together and post the perfect answer you seek! — Organic Marble, Jan 20 '19 at 15:16
You know I can't pass up a chance to link the rocket equation and specific impulse WP pages. — Russell Borogove, Jan 20 '19 at 16:37
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. — Russell Borogove, Jan 21 '19 at 04:09
In reality the difference is even greater than 3x because you also have to contain that fuel in something. — Loren Pechtel, Jan 21 '19 at 23:42

Organic Marble · Answer 2 · 2019-01-20T14:54:23.827

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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.

Sources: 1,2,3

edited Jan 20 '19 at 14:54

answered Jan 20 '19 at 14:14

Organic Marble

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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. – Russell Borogove Jan 20 '19 at 14:23
Thanks. I was just looking at the wikipedia article and found that in there as well. – Organic Marble Jan 20 '19 at 14:25
Kerosene/IRFNA at 233 sec is poor specific impulse indeed. – Russell Borogove Jan 20 '19 at 14:25
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And a hell of an oxidizer to handle in the field! Wow. – Organic Marble Jan 20 '19 at 14:26
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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. – uhoh Jan 20 '19 at 14:51
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I rolled back the edit because the units on Isp are exactly what was in the source document, hence the quote marks. – Organic Marble Jan 20 '19 at 14:54

score 7 · Answer 3 · answered Jan 20 '19 at 12:01

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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

answered Jan 20 '19 at 12:01

Pavel Bernshtam

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Is it possible to name the engine itself or at least say something about its capability in terms of orbital applications? – uhoh Jan 20 '19 at 12:18

Why was the engine of the launch vehicle recently tested in Iran "not a very good missile engine"?

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