Friday, September 4, 2009

Specific Impulse Explained

Specific Impulse, Isp , is a term frequently bandied about in rocketry, but rarely fully explained. I've used twice, here and here, both times in reference to high-Isp Blue Thunder blends.

I finally found a sensible explanation of what exactly the specific impulse of a specific propellant is. It's the specific impulse per unit mass. For example, just over one newton (102g) of Black Jack propellant in the G75J reload generates 155Ns of total impulse; the specific impulse of Black Jack is thus around 150 seconds. On the other hand, the same mass produces 220Ns in the H220T, so the specific impulse of that formulation of Blue Thunder is 212 or so.

A sampling of Isp s for several types of propellant:

White Lightning (W): 205 (J570); 200 (G79) 186 (N2000); 192 (K550); 210 (D10W)

Blue Thunder (T): 213 (D21); 225 (F32T); 212 (H220T); 222 (J460); 231 (M2500)

Black Jack (J): 143 (E11); 149 (G75); 147 (I154)

Black Max (FJ): 143 (F23); 156 (G53); 152 (I225); 154 (K513); 151 (M1800)

Redline (R): 182 (F27); 209 (H165); 192 (J420); 194 (K1275) 188 (M2000)

Mojave Green (G): 201 (G76); 203 (H250); 203 (J500); 205 (L1390); 202 (M2100)

Warp-9 (N): 225 (G69 Endburner); 234 (G339 Coreburner); 228 (J1999 Coreburner); 219 (K1999N Coreburner); 210 (L339N Endburner)

Cesaroni Skidmark: 178 (G69); 168 (I297); 171 (J250); 180 (K675); 171 (l1410); 176 (L3400)

Blue Thunder and Warp-9 are roughly tied for the highest (most efficient); White Lightning, Redline, and Mojave Green are tied for second; Skidmarks a bit below average, and the Black Jack / Max motors the lowest.

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