5 ms·

There are two other reasons which SpaceX has mentioned before: - When you're landing a rocket, you need to be able to throttle down quite low. Even a single M

There are two other reasons which SpaceX has mentioned before: - When you're landing a rocket, you need to be able to throttle down quite low. Even a single Merlin 1-D engine, throttled down, is too much thrust to be able to hover with a nearly empty booster. It's really hard to get stable combustion at very low throttle settings. Having only one engine out of nine running for landing makes this much more manageable. - There are economies of scale and reliability when you're building large numbers of something. Ariane 5 only launched 6 times in 2017. It uses one first stage engine, so they're only building one engine every 2 months. Falcon 9 launched 18 times in 2017, with 9 first stage engines per launch, that's roughly an engine every 2 days. More continuous construction, better economies of scale, more repeatable.

vannevar 9y ago
There are economies of scale and reliability when you're building large numbers of something. Yes. One of the biggest predictors of reliability in aerospace systems in general is time in operation---the longer you've actually run something, the closer it approaches the upper limit of reliability for that component. Running 27 small copies gives you 27x the time in operation vs a single big-ass engine. The downside is complexity, and in particular unknown failure interactions which could bring down the entire system in a cascade. As long as many of the components are identical, though, an improvement in reliability of that component should translate directly into lower probability of failure interactions since there are fewer failures, period.
- bunderbunder 9y ago
  How does one quantify the risk of cascading failure? My initial instinct, informed by computing, is to say that it's easier to avoid cascading failures in the system that is composed of more, smaller parts. All other things being equal, in a rocket with 5 engines, if one of them fails, then each of the remaining ones needs to pick up 1/4 of the slack to compensate. In a rocket with 9 engines, not only would each of the remaining engines need to pick up only 1/8 of the slack, but the total size of the gap would be a little over half as much, too. (This is obvs ignoring the possibility that the failure in question is catastrophic.)
  - deleted 9y ago
    [deleted]
  - physcab 9y ago
    The difference between aerospace and computing is that aerospace is heavily materials dependent. Getting a bad pixel in a monitor is rare but when it happens it’s not catastrophic. When you’re producing a ton of parts you have to also maintain quality. Not doing so will introduce impurities which will alter the material properties. An alloy with different material properties could potentially burn, fracture, expand etc more differently than represented in their acceptable failure models.
  - neltnerb 9y ago
    To answer the first question, you use a fault tree analysis to predict potential failure starting points (like a broken component) and then describe how those failures will propagate through the system. https://en.wikipedia.org/wiki/Fault_tree_analysis https://en.wikipedia.org/wiki/Fault_tree_analysis For an example, say I'm building a system that needs to hold a block of aluminum at 550C, 99% of the time. Okay, so you add a thermocouple and a heater to it, easy. What if the thermocouple fails? Well, if the thermocouple fails open then the temperature will read infinity and the heater will shut down and probably produce a non-catastrophic failure. If the thermocouple fails closed, the temperature will read room temp and the heater will blast full on until the aluminum melts at 660C, which is a catastrophic failure. If the relay in the temperature controller fails, the furnace probably turns off but theoretically could fail on if the relay switch gets fused. Okay, so I can see that there is an unlikely but possible chain of events that could cause a catastrophic failure. So I add a second thermocouple to act as a safety shutoff using a second redundant relay and controller if it reads a temperature above 600C. Total probability then is estimated by either using real world performance metrics or best-guesses. I'd say the odds of a thermocouple failing in 10 years of operation at 550C is nearly 100%, so this failure will nearly certainly occur. Or consider an LED array with 10 of them in parallel. If one blows open, the remaining 9 each get 10% more current so are more likely to fail. So your first branch of the tree might be that the odds are 10% that an LED will fail at design current within five years. That may well not qualify as a failure, especially since the other 9 LEDs are ~10% brighter due to the higher-than-spec current. But now your probability for the next failure is 20% within five years. So you do need to define different outcomes, usually by severity of impact and probability of outcome in event of a predicted possible failure point.
- sehugg 9y ago
  Yes, one of the poorly-understood problems with early NASA missions was "pogo", a propellant vibration problem which affects the entire vehicle: https://www.popsci.com/how-little-vibrations-break-big-rockets https://www.popsci.com/how-little-vibrations-break-big-rocke...
avmich 9y ago
> When you're landing a rocket, you need to be able to throttle down quite low. When R-7 was developed in 1950-s, as a weapon, one of requirements was the precision of attained speed at the moment of engine cut-off. It was solved by shutting down main chambers on the upper (second) stage, powered by RD-108 - 4 chambers about 20 tons of thrust each - and maintaining thrust from steering chamber, 4 of them, 3 tons of thrust each. This is another example of deep throttling.
- Gravityloss 9y ago
  That'd be quite something, improving the Soyuz to do powered landings. Talk about getting mileage from your design. On the other hand, it can't even stand on the pad fully fueled, it's hanging from an arm...
marktangotango 9y ago
The landing case makes me wonder about the wisdom of Blue Origins choice of going with a larger engine, the BE-4 with 550,000 lbf for the New Glen vehicle. Much more thrust than a Falcon 9 landing on a single M1D. Surely they've run the numbers and find it viable though.
- ygra 9y ago
  Thrust isn't that much of an issue. Yes, Falcon 9 cannot hover. But they don't want it to anyway, as it's too wasteful on fuel.
- Robotbeat 9y ago
  Blue Origin also has worse mass fraction on the booster stage than SpaceX does, which means they don't need to throttle down as much. If you look at New Shepard as a preview to New Glenn, there are a whole bunch of aerosurfaces which add dry mass, essentially ballast. New Glenn will add big side fins, which will help increase lift, drag, and provide more ballast. Also, the New Glenn booster will have 7 engines. Not so different from Falcon 9's 9 engines per booster. Overall, New Glenn is a good design (and will eventually have a reusable 2nd stage). If SpaceX stopped with Falcon 9 and Falcon Heavy, New Glenn could easily give SpaceX a run for their money. Luckily, SpaceX won't stop with Falcon Heavy. BFR, once they finish getting Block 5 out the door in a couple months and crew Dragon flown sometime by around the end of the year, will be almost their sole engineering/development effort (Starlink being kind of a separate division). Of course, Blue Origin also won't sit still with New Glenn. They'll make the upper stage reusable, then also add a hydrogen/oxygen kick stage for very high energy payloads, and then they'll be working on the New Armstrong monster. SpaceX is super far ahead, but luckily Bezos is so rich that I expect both companies now to deliver on their equally grand visions.
  - marktangotango 9y ago
    The thing about these reusable boosters that’s surprising to me when I think about is; they really don’t need very many of them. Like 10 or 50?
pjscott 9y ago
Another reason is that, because the Merlin 1-D is small enough, they can use a vacuum variant of it as the upper stage engine rather than designing another one.
philipwhiuk 9y ago
It's slightly ironic that SpaceX has brought economies of scale to the space industry and Telsa is struggling with scale in the automotive industry.
- greglindahl 9y ago
  Yes and no -- SpaceX has the greatest scale in the space industry, and Tesla has small scale in the car industry. That's mainly a statement that cars are a huge industry, and rocket launchers is a small one.

vannevar 9y ago

There are economies of scale and reliability when you're building large numbers of something. Yes. One of the biggest predictors of reliability in aerospace systems in general is time in operation---the longer you've actually run something, the closer it approaches the upper limit of reliability for that component. Running 27 small copies gives you 27x the time in operation vs a single big-ass engine. The downside is complexity, and in particular unknown failure interactions which could bring down the entire system in a cascade. As long as many of the components are identical, though, an improvement in reliability of that component should translate directly into lower probability of failure interactions since there are fewer failures, period.

bunderbunder 9y ago
How does one quantify the risk of cascading failure? My initial instinct, informed by computing, is to say that it's easier to avoid cascading failures in the system that is composed of more, smaller parts. All other things being equal, in a rocket with 5 engines, if one of them fails, then each of the remaining ones needs to pick up 1/4 of the slack to compensate. In a rocket with 9 engines, not only would each of the remaining engines need to pick up only 1/8 of the slack, but the total size of the gap would be a little over half as much, too. (This is obvs ignoring the possibility that the failure in question is catastrophic.)
- deleted 9y ago
  [deleted]
- physcab 9y ago
  The difference between aerospace and computing is that aerospace is heavily materials dependent. Getting a bad pixel in a monitor is rare but when it happens it’s not catastrophic. When you’re producing a ton of parts you have to also maintain quality. Not doing so will introduce impurities which will alter the material properties. An alloy with different material properties could potentially burn, fracture, expand etc more differently than represented in their acceptable failure models.
- neltnerb 9y ago
  To answer the first question, you use a fault tree analysis to predict potential failure starting points (like a broken component) and then describe how those failures will propagate through the system. https://en.wikipedia.org/wiki/Fault_tree_analysis https://en.wikipedia.org/wiki/Fault_tree_analysis For an example, say I'm building a system that needs to hold a block of aluminum at 550C, 99% of the time. Okay, so you add a thermocouple and a heater to it, easy. What if the thermocouple fails? Well, if the thermocouple fails open then the temperature will read infinity and the heater will shut down and probably produce a non-catastrophic failure. If the thermocouple fails closed, the temperature will read room temp and the heater will blast full on until the aluminum melts at 660C, which is a catastrophic failure. If the relay in the temperature controller fails, the furnace probably turns off but theoretically could fail on if the relay switch gets fused. Okay, so I can see that there is an unlikely but possible chain of events that could cause a catastrophic failure. So I add a second thermocouple to act as a safety shutoff using a second redundant relay and controller if it reads a temperature above 600C. Total probability then is estimated by either using real world performance metrics or best-guesses. I'd say the odds of a thermocouple failing in 10 years of operation at 550C is nearly 100%, so this failure will nearly certainly occur. Or consider an LED array with 10 of them in parallel. If one blows open, the remaining 9 each get 10% more current so are more likely to fail. So your first branch of the tree might be that the odds are 10% that an LED will fail at design current within five years. That may well not qualify as a failure, especially since the other 9 LEDs are ~10% brighter due to the higher-than-spec current. But now your probability for the next failure is 20% within five years. So you do need to define different outcomes, usually by severity of impact and probability of outcome in event of a predicted possible failure point.
sehugg 9y ago
Yes, one of the poorly-understood problems with early NASA missions was "pogo", a propellant vibration problem which affects the entire vehicle: https://www.popsci.com/how-little-vibrations-break-big-rockets https://www.popsci.com/how-little-vibrations-break-big-rocke...

avmich 9y ago

> When you're landing a rocket, you need to be able to throttle down quite low. When R-7 was developed in 1950-s, as a weapon, one of requirements was the precision of attained speed at the moment of engine cut-off. It was solved by shutting down main chambers on the upper (second) stage, powered by RD-108 - 4 chambers about 20 tons of thrust each - and maintaining thrust from steering chamber, 4 of them, 3 tons of thrust each. This is another example of deep throttling.

Gravityloss 9y ago
That'd be quite something, improving the Soyuz to do powered landings. Talk about getting mileage from your design. On the other hand, it can't even stand on the pad fully fueled, it's hanging from an arm...

marktangotango 9y ago

The landing case makes me wonder about the wisdom of Blue Origins choice of going with a larger engine, the BE-4 with 550,000 lbf for the New Glen vehicle. Much more thrust than a Falcon 9 landing on a single M1D. Surely they've run the numbers and find it viable though.

ygra 9y ago
Thrust isn't that much of an issue. Yes, Falcon 9 cannot hover. But they don't want it to anyway, as it's too wasteful on fuel.
Robotbeat 9y ago
Blue Origin also has worse mass fraction on the booster stage than SpaceX does, which means they don't need to throttle down as much. If you look at New Shepard as a preview to New Glenn, there are a whole bunch of aerosurfaces which add dry mass, essentially ballast. New Glenn will add big side fins, which will help increase lift, drag, and provide more ballast. Also, the New Glenn booster will have 7 engines. Not so different from Falcon 9's 9 engines per booster. Overall, New Glenn is a good design (and will eventually have a reusable 2nd stage). If SpaceX stopped with Falcon 9 and Falcon Heavy, New Glenn could easily give SpaceX a run for their money. Luckily, SpaceX won't stop with Falcon Heavy. BFR, once they finish getting Block 5 out the door in a couple months and crew Dragon flown sometime by around the end of the year, will be almost their sole engineering/development effort (Starlink being kind of a separate division). Of course, Blue Origin also won't sit still with New Glenn. They'll make the upper stage reusable, then also add a hydrogen/oxygen kick stage for very high energy payloads, and then they'll be working on the New Armstrong monster. SpaceX is super far ahead, but luckily Bezos is so rich that I expect both companies now to deliver on their equally grand visions.
- marktangotango 9y ago
  The thing about these reusable boosters that’s surprising to me when I think about is; they really don’t need very many of them. Like 10 or 50?

pjscott 9y ago

Another reason is that, because the Merlin 1-D is small enough, they can use a vacuum variant of it as the upper stage engine rather than designing another one.

philipwhiuk 9y ago

It's slightly ironic that SpaceX has brought economies of scale to the space industry and Telsa is struggling with scale in the automotive industry.

greglindahl 9y ago
Yes and no -- SpaceX has the greatest scale in the space industry, and Tesla has small scale in the car industry. That's mainly a statement that cars are a huge industry, and rocket launchers is a small one.