3 ms·

To answer the question of why even use them at all, it's because on paper they still compare very well to the other options. Specific impulse is basically an e

by joshAg 12y ago

To answer the question of why even use them at all, it's because on paper they still compare very well to the other options. Specific impulse is basically an efficiency measure, like mpg for cars. The higher it is, the more efficient the rocket engine is. SpaceX's Merlin 1d has a thrust-to-weight ratio (twr) of 150, and a specific impulse (I_sp) at sea level of 2.73 km/s. I doubt this was an option for orbital sciences to use in their rockets. The NK33, which is the soviet engine that was used, has a twr of 137 and an I_sp at sea level of 2.91 km/s. The atlas V rockets by boeing and lockheed use an rd180, also russian built btw, which has a twr of 78.44 and an I_sp at sea level of 3.05 km/s. The delta IV rockets by boeing us an rs68 or rs68a have a twr of 51.2 and an I_sp of 3.58 km/s. I'm not sure if these are available for commercial use, though, since boeing shut down the commercial delta IV launch program. The US military still uses them though, so they are still being produced. Also, these use liquid hydrogen instead of RP1 (rocket grade kerosine) like all the rest of the engines here. The Saturn V used five F1 engines (also 1960s tech, but no longer available). Each of those have a twr of 82.27 and an I_sp of 2.58 km/s.

jessriedel 12y ago
Isn't this much more a question of reliability and track record than 10% performance differences?
- avmich 12y ago
  Both reliability and performance. And also reliability is something which you estimate, calculate basing on some models - it's probability, not a guarantee. How would you estimate the reliability of an engine which run over 3 hours on a test stand? Of course you might prefer a well known fuels, industry-proven range of pressures and other engineering solutions, and you test them on Earth first - but after that you have to fly. Track records are tricky for rocket systems. Since the first Sputnik, over the 57 years the humanity had may be - roughly - 4000 launches of space rockets of all kinds, with many different fuels, engines, sizes etc. It's relatively rare when you have a good track record which is measured by many tens of flights - let alone hundreds. When a system flew 20 times it's already considered rather mature - double so if those were all successful flights. So it's quite usual to use a new engine in a new design of rocket - in fact it's rare when you have many different rockets with the same engine type or many different engines used on the same rocket. Rockets and engines are usually optimized to each other - particularly rockets are built around engines, since margins are quite thin.
- joshAg 12y ago
  Most definitely that's a question that needs to be asked, but before you ask that, you need to ask what engines have performance specs worthy of further consideration. If you did it in reverse order, you're liable to get finalists like whatever they put in those volvos with a couple hundred thousand miles on them. :P It doesn't matter how reliable the engine is if it's not performant enough for the envisioned task. As to your question about reliability and track record, it's kinda hard to base the decision on past launch performance since most rockets-engines are built to a specific rocket and vice versa. These things aren't like a used car where you can look at thousands of unique datapoints. And it generally cost prohibitive to do a full dry-run with only a cargo full of, say, bricks, because a single launch costs well north of a hundred million dollars. Instead you have to run a lot of ground based tests and figure a bunch of probabilities. These guys won a one point something billion dollar contract from NASA to deliver shit to the ISS. NASA's not exactly well funded and not exactly known for flying by the seat of their pants, even by government agency standards. In order to win that contract they needed to prove to NASA's satisfaction that there was a high probability that their rocket would get to ISS without a major problem, let alone one as catastrophic and uncorrectable as exploding a few hundred feet above the pad.
- joshAg 12y ago
  Most definitely that's a question that needs to be asked, but before you ask that, you need to ask what engines have performance specs worthy of further consideration. If you did it in reverse order, you're liable to get finalists like whatever they put in those volvos with a couple hundred thousand miles on them. :P It doesn't matter how reliable the engine is if it's not performant enough for the envisioned task. As to your question about reliability and track record, it's kinda hard to base the decision on past launch performance since most rockets-engines are built to a specific rocket and vice versa. These things aren't like a used car where you can look at thousands of unique datapoints, or use the same engine in the silverado that you use in the corvette (yeah, this is actually true. different ecu settings, though). And it generally cost prohibitive to do a full dry-run with only a cargo full of, say, bricks, because a single launch costs well north of a hundred million dollars. Instead you have to run a lot of ground based tests and figure a bunch of probabilities. These guys won a one point something billion dollar contract from NASA to deliver shit to the ISS. NASA's not exactly well funded and not exactly known for flying by the seat of their pants, even by government agency standards. In order to win that contract they needed to prove to NASA's satisfaction that there was a high probability that their rocket would get to ISS without a major problem, let alone one as catastrophic and uncorrectable as exploding a few hundred feet above the pad.
riffraff 12y ago
how does the Vulcain2 compare?

by joshAg 12y ago

jessriedel 12y ago

Isn't this much more a question of reliability and track record than 10% performance differences?

avmich 12y ago
Both reliability and performance. And also reliability is something which you estimate, calculate basing on some models - it's probability, not a guarantee. How would you estimate the reliability of an engine which run over 3 hours on a test stand? Of course you might prefer a well known fuels, industry-proven range of pressures and other engineering solutions, and you test them on Earth first - but after that you have to fly. Track records are tricky for rocket systems. Since the first Sputnik, over the 57 years the humanity had may be - roughly - 4000 launches of space rockets of all kinds, with many different fuels, engines, sizes etc. It's relatively rare when you have a good track record which is measured by many tens of flights - let alone hundreds. When a system flew 20 times it's already considered rather mature - double so if those were all successful flights. So it's quite usual to use a new engine in a new design of rocket - in fact it's rare when you have many different rockets with the same engine type or many different engines used on the same rocket. Rockets and engines are usually optimized to each other - particularly rockets are built around engines, since margins are quite thin.
joshAg 12y ago
Most definitely that's a question that needs to be asked, but before you ask that, you need to ask what engines have performance specs worthy of further consideration. If you did it in reverse order, you're liable to get finalists like whatever they put in those volvos with a couple hundred thousand miles on them. :P It doesn't matter how reliable the engine is if it's not performant enough for the envisioned task. As to your question about reliability and track record, it's kinda hard to base the decision on past launch performance since most rockets-engines are built to a specific rocket and vice versa. These things aren't like a used car where you can look at thousands of unique datapoints. And it generally cost prohibitive to do a full dry-run with only a cargo full of, say, bricks, because a single launch costs well north of a hundred million dollars. Instead you have to run a lot of ground based tests and figure a bunch of probabilities. These guys won a one point something billion dollar contract from NASA to deliver shit to the ISS. NASA's not exactly well funded and not exactly known for flying by the seat of their pants, even by government agency standards. In order to win that contract they needed to prove to NASA's satisfaction that there was a high probability that their rocket would get to ISS without a major problem, let alone one as catastrophic and uncorrectable as exploding a few hundred feet above the pad.
joshAg 12y ago
Most definitely that's a question that needs to be asked, but before you ask that, you need to ask what engines have performance specs worthy of further consideration. If you did it in reverse order, you're liable to get finalists like whatever they put in those volvos with a couple hundred thousand miles on them. :P It doesn't matter how reliable the engine is if it's not performant enough for the envisioned task. As to your question about reliability and track record, it's kinda hard to base the decision on past launch performance since most rockets-engines are built to a specific rocket and vice versa. These things aren't like a used car where you can look at thousands of unique datapoints, or use the same engine in the silverado that you use in the corvette (yeah, this is actually true. different ecu settings, though). And it generally cost prohibitive to do a full dry-run with only a cargo full of, say, bricks, because a single launch costs well north of a hundred million dollars. Instead you have to run a lot of ground based tests and figure a bunch of probabilities. These guys won a one point something billion dollar contract from NASA to deliver shit to the ISS. NASA's not exactly well funded and not exactly known for flying by the seat of their pants, even by government agency standards. In order to win that contract they needed to prove to NASA's satisfaction that there was a high probability that their rocket would get to ISS without a major problem, let alone one as catastrophic and uncorrectable as exploding a few hundred feet above the pad.

riffraff 12y ago

how does the Vulcain2 compare?