# What's the Isp of a model rocket “D-size” engine, compared to the Isp of engines that do reach space?

Now that I've learned a bit about real rockets, I wondered how a model rocket D-size engine fits in quantitatively. The Wikipedia article Model rocket lists thrust in Newtons, and Total Impulse in Newton-seconds, but one would need to know the mass of the propellant to calculate mass-specific impulse $I_{SP}$ and that's not listed.

A good, quantitatively reasoned educated guess or a number for a similar propellant type and design would be sufficient.

And of course this kind of engine will probably not get your rocket to the Karman line, but I'm looking for a number to better understand how rockets that do make it to space compare to a handy, historical reference point.

• Did you google "specific impulse model rocket engine?" en.m.wikipedia.org/wiki/Specific_impulse#Model_rocketry – Russell Borogove Jul 31 '17 at 8:57
• @RussellBorogove I hadn't realized that the needed data was in the $I_{SP}$ article but not the model rocket article. Putting information from both articles together, I was able to make a direct comparison between them and real, space-worthy engines in the answer below. Having this in one place now allows me to refer back to this when writing things like this answer. – uhoh Jul 31 '17 at 11:08
• Not only is the specific impulse of typical model rocket engines abysmal, but their mass fraction is also pretty terrible - often less than half the mass of the motor (let alone the rocket) is propellant. This is due to cost and safety. – pericynthion Aug 1 '17 at 20:28

The comment by @RussellBorogove is correct and while the information is not found in Wikipedia's model rocket page, it can be found on the specific impulse page, where $$I_{SP}$$'s for space-worthy rocket engines are also kept.
So for a D12-3 engine, $$I_{SP}$$ is about 82 seconds. The "12" in D12-3 means an average thrust of about 12N, so the total burn time is only about 1.7 seconds. After that, you have to eject the engine or the whole stage and ignite a new one.
The shuttle-type SRB's have an $$I_{SP}$$ of about 250 seconds, and a good LOX/LH2 engine can reach 450 seconds. Stages using those tend to be about 90% propellant mass and burn for on the order of 100 or 200 seconds.