6 ms·

It makes sense. If BFR works, it will make all the rockets obsolete, by simply being fully reusable. Hence the only sensible thing to do is to concentrate 100%

It makes sense. If BFR works, it will make all the rockets obsolete, by simply being fully reusable. Hence the only sensible thing to do is to concentrate 100% of the efforts on it.

cma 8y ago
What all is different about the BFR heat shield compared to the shuttle? I assume there is much newer tech since the 70s, is it easier to refurbish?
- NikolaeVarius 8y ago
  Newer material. But big issue with shuttle was that each panel was unique and had to always be placed in a specific spot
- bufferoverflow 8y ago
  The shuttle didn't do any re-entry engine burns. It fully relied on the friction.
  - Retric 8y ago
    It's not really friction that's slows stuff on reentry. The heat is from compressing the air. Which is why the heat spikes away from the vehicle. Anyway, the rocket equation means if you wanted to slow down before reentry you would effectively need to accelerate to twice the final speed 0 to X then X to 0. That's simply not going to happen.
    - foota 8y ago
      Why not? Isn't that what they currently do for the falcon 9s? Sure, the rocket equation makes it more difficult since it's an exponential increase in fuel, but that's not impossible, right?
      - balfirevic 8y ago
        > Isn't that what they currently do for the falcon 9s? It's not (not even close). Falcon 9 first stage isn't reentering from orbital velocity. It's essentially impossible, and if it weren't it would be terribly wasteful. Orbital velocity is going to be primarily shed by atmosphere.
    - LeifCarrotson 8y ago
      It takes much, much less energy to slow down a nearly-empty shell than one that was full of fuel and oxidizer.
      - manicdee 8y ago
        You have to take the slowing-down fuel with you on the speeding-up part of the trip though, which means it takes much more energy to speed up in the first place.
        twtw 8y ago
        Yes. SpaceX surely knows this, and reckons that the cost is worth full reusability with reduced refurbishment cost.
        Retric 8y ago
        No, they are using a heat shield which is unnecessary if you slow down before reentry. What confuses people is you need to bleed off a little energy to start decent, and another tiny amount to land. But, between those points 99% of the energy is bleed off via air.
        manicdee 8y ago
        You can compare the weight of the heat shield required for atmospheric reentry braking to the weight of the fuel you need for propulsive reentry braking: if the fuel weighs more, use the heat shield instead. This could be the difference, for example, between 100t of fuel versus 15t of heatshield.
        ygra 8y ago
        It eats into the payload, of course. That's why they have different landing trajectories (RTLS, barge in the ocean) and sometimes don't land at all, depending on the payload mass. The propellant itself doesn't cost much (Elon once noted that fully fueling a Falcon 9 is about 200k, which is nothing compared to the 62M that a launch costs).
  - jessriedel 8y ago
    I don't think BFR is going to do any substantial re-entry burns. The reason is that it's completely impractical to slow down from orbital or interplanetary speeds with rockets, so you have to rely on your heat shield. BFR will only do very small de-orbit burns in order to bring it's trajectory into the atmosphere, just like the shuttle did. In contrast, the Falcon 9 first stages are at suborbial speeds and don't have a heat shield, so they need to do a substantial re-entry burns to bring down their speed before they hit the atmosphere to avoid damage.
    - Retric 8y ago
      The falcon 9 first stage is not just suborbital, it’s vastly slower than LEO at just Mach 10. It’s also still in very thin atmosphere at 80 km, it’s also spending more time in very thin air as it’s sperates while going up, which is why they don’t need heat shields. It’s true they fire the engines briefly at the top, but doing so to slow slightly from Mach 10 is very different vs slowing from Mach 10 to zero or Mach 22.7 to Mach 10. And this still significantly reduces the cargo they can take to LEO. PS: Remeber kenetic energy is velocity ^ 2 so your bleeding off 1/5 the energy. Further, adiabatic heating is from the compression of gas, lower speeds means lower compression.
      - jessriedel 8y ago
        Everything you've said is agreeing with me except maybe the clarification that the Falcon 9 first stage doesn't quite cross the the Karman line, right? In particular, I think "sub-orbital velocity" is commonly taken to include trajectories that are substantially below orbital velocity https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight like the Mercury-Redstone mission Freedom 7 that achieved a peak speed of ~5,100 mph, less than Mach 7. https://en.m.wikipedia.org/wiki/Mercury-Redstone_3 https://en.m.wikipedia.org/wiki/Mercury-Redstone_3 Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage. https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosphere-on-their-return-to-earth https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosp... (You don't explicitly say otherwise, but one could misread your comment as suggesting that the re-entry burns are optional or negligible.) My main point was that the re-entry burns are about reducing the velocity significantly (30%, or whatever), not just for steering the trajectory into the atmosphere like for de-orbit burns.
- iknowstuff 8y ago
  Everyday Astronaut explained this pretty well: https://www.youtube.com/watch?v=SCCw_M8MAU0&t=110s https://www.youtube.com/watch?v=SCCw_M8MAU0&t=110s The Shuttle's heatshield had to deflect significantly more energy because of the different landing approach.
  - Retric 8y ago
    The shuttles largest issue was it's weight. It's the old square cube law. You need to remove more energy based on the mass of the object, but you remove it over what amounts to a 2d area. Unfortunately, part of the design requirements was to return a large amount of mass from orbit which means it needed to be designed to dump a rather obscene amount of energy into the upper atmosphere. This meant it could not do a rapid decent and because it would overheat, and thus needed to stay in the upper atmosphere for a long time. Which meant it's thermal tiles needed to operate for much longer time periods requiring extreme amounts of thermal protection. PS: The wings also added even more weight issues.
    - cultus 8y ago
      The Shuttle was one of the finest examples of MI complex waste and negligence there has ever been.
      - canhascodez 8y ago
        The Shuttle was designed for a use-case that evaporated, and it's not necessarily inaccurate to suggest that it was manufactured so as to spread "pork" to as many Congressional districts as possible. However, wasting money on space is not necessarily a waste, particularly if no one else is doing it. And for all their faults, the Shuttles flew high, and accomplished much.
holmberd 8y ago
well it is all fun and games until you put actual people in it.
zackbloom 8y ago
There is a counter argument, if you continually shift your focus you will never allow your products to get to the point where they're really making money. Reusable stage two might be a year away, BFR five. That's four years of lost revenue.
- coryrc 8y ago
  They're already making money; reusable second stage would only increase profit a little.
- Empact 8y ago
  Thankfully, Falcon 9 is already making money. Reusing stage 2 is an optimization that represents an allocation of capital - capital should generally go to the highest-expected-return uses.
- eloff 8y ago
  Weighed against the investment to get a reusable second stage. It may still be logical.
wongarsu 8y ago
Maintanance, fuel costs and other operating costs would probably still leave small enough rockets competitive (because of the rocket equation rockets get exponentially smaller as the payload gets lighter). But I agree in your point the Falcon 9 will become obsolete once the BFR is proven to be reliable and manages a sufficient launch frequency.
- larkeith 8y ago
  For small, specialized launches there are already other companies filling the niche: https://en.wikipedia.org/wiki/Electron_(rocket) https://en.wikipedia.org/wiki/Electron_(rocket)
- Obi_Juan_Kenobi 8y ago
  Rockets get exponentially larger as delta-v requirements increase. As payload increases or decreases, rockets merely need to scale linearly. Conceptually, this is quite simple to understand: simply launching two identical rockets will result in double the payload. The mass part of Tsiolkovsky's equation is simply mass-initial / mass-final. Halve both the numerator and denominator, and you have the same fraction.

It makes sense. If BFR works, it will make all the rockets obsolete, by simply being fully reusable. Hence the only sensible thing to do is to concentrate 100% of the efforts on it.

cma 8y ago

What all is different about the BFR heat shield compared to the shuttle? I assume there is much newer tech since the 70s, is it easier to refurbish?

NikolaeVarius 8y ago
Newer material. But big issue with shuttle was that each panel was unique and had to always be placed in a specific spot
bufferoverflow 8y ago
The shuttle didn't do any re-entry engine burns. It fully relied on the friction.
- Retric 8y ago
  It's not really friction that's slows stuff on reentry. The heat is from compressing the air. Which is why the heat spikes away from the vehicle. Anyway, the rocket equation means if you wanted to slow down before reentry you would effectively need to accelerate to twice the final speed 0 to X then X to 0. That's simply not going to happen.
  - foota 8y ago
    Why not? Isn't that what they currently do for the falcon 9s? Sure, the rocket equation makes it more difficult since it's an exponential increase in fuel, but that's not impossible, right?
    - balfirevic 8y ago
      > Isn't that what they currently do for the falcon 9s? It's not (not even close). Falcon 9 first stage isn't reentering from orbital velocity. It's essentially impossible, and if it weren't it would be terribly wasteful. Orbital velocity is going to be primarily shed by atmosphere.
  - LeifCarrotson 8y ago
    It takes much, much less energy to slow down a nearly-empty shell than one that was full of fuel and oxidizer.
    - manicdee 8y ago
      You have to take the slowing-down fuel with you on the speeding-up part of the trip though, which means it takes much more energy to speed up in the first place.
      - twtw 8y ago
        Yes. SpaceX surely knows this, and reckons that the cost is worth full reusability with reduced refurbishment cost.
        Retric 8y ago
        No, they are using a heat shield which is unnecessary if you slow down before reentry. What confuses people is you need to bleed off a little energy to start decent, and another tiny amount to land. But, between those points 99% of the energy is bleed off via air.
        manicdee 8y ago
        You can compare the weight of the heat shield required for atmospheric reentry braking to the weight of the fuel you need for propulsive reentry braking: if the fuel weighs more, use the heat shield instead. This could be the difference, for example, between 100t of fuel versus 15t of heatshield.
      - ygra 8y ago
        It eats into the payload, of course. That's why they have different landing trajectories (RTLS, barge in the ocean) and sometimes don't land at all, depending on the payload mass. The propellant itself doesn't cost much (Elon once noted that fully fueling a Falcon 9 is about 200k, which is nothing compared to the 62M that a launch costs).
- jessriedel 8y ago
  I don't think BFR is going to do any substantial re-entry burns. The reason is that it's completely impractical to slow down from orbital or interplanetary speeds with rockets, so you have to rely on your heat shield. BFR will only do very small de-orbit burns in order to bring it's trajectory into the atmosphere, just like the shuttle did. In contrast, the Falcon 9 first stages are at suborbial speeds and don't have a heat shield, so they need to do a substantial re-entry burns to bring down their speed before they hit the atmosphere to avoid damage.
  - Retric 8y ago
    The falcon 9 first stage is not just suborbital, it’s vastly slower than LEO at just Mach 10. It’s also still in very thin atmosphere at 80 km, it’s also spending more time in very thin air as it’s sperates while going up, which is why they don’t need heat shields. It’s true they fire the engines briefly at the top, but doing so to slow slightly from Mach 10 is very different vs slowing from Mach 10 to zero or Mach 22.7 to Mach 10. And this still significantly reduces the cargo they can take to LEO. PS: Remeber kenetic energy is velocity ^ 2 so your bleeding off 1/5 the energy. Further, adiabatic heating is from the compression of gas, lower speeds means lower compression.
    - jessriedel 8y ago
      Everything you've said is agreeing with me except maybe the clarification that the Falcon 9 first stage doesn't quite cross the the Karman line, right? In particular, I think "sub-orbital velocity" is commonly taken to include trajectories that are substantially below orbital velocity https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight like the Mercury-Redstone mission Freedom 7 that achieved a peak speed of ~5,100 mph, less than Mach 7. https://en.m.wikipedia.org/wiki/Mercury-Redstone_3 https://en.m.wikipedia.org/wiki/Mercury-Redstone_3 Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage. https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosphere-on-their-return-to-earth https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosp... (You don't explicitly say otherwise, but one could misread your comment as suggesting that the re-entry burns are optional or negligible.) My main point was that the re-entry burns are about reducing the velocity significantly (30%, or whatever), not just for steering the trajectory into the atmosphere like for de-orbit burns.
iknowstuff 8y ago
Everyday Astronaut explained this pretty well: https://www.youtube.com/watch?v=SCCw_M8MAU0&t=110s https://www.youtube.com/watch?v=SCCw_M8MAU0&t=110s The Shuttle's heatshield had to deflect significantly more energy because of the different landing approach.
- Retric 8y ago
  The shuttles largest issue was it's weight. It's the old square cube law. You need to remove more energy based on the mass of the object, but you remove it over what amounts to a 2d area. Unfortunately, part of the design requirements was to return a large amount of mass from orbit which means it needed to be designed to dump a rather obscene amount of energy into the upper atmosphere. This meant it could not do a rapid decent and because it would overheat, and thus needed to stay in the upper atmosphere for a long time. Which meant it's thermal tiles needed to operate for much longer time periods requiring extreme amounts of thermal protection. PS: The wings also added even more weight issues.
  - cultus 8y ago
    The Shuttle was one of the finest examples of MI complex waste and negligence there has ever been.
    - canhascodez 8y ago
      The Shuttle was designed for a use-case that evaporated, and it's not necessarily inaccurate to suggest that it was manufactured so as to spread "pork" to as many Congressional districts as possible. However, wasting money on space is not necessarily a waste, particularly if no one else is doing it. And for all their faults, the Shuttles flew high, and accomplished much.

holmberd 8y ago

well it is all fun and games until you put actual people in it.

zackbloom 8y ago

There is a counter argument, if you continually shift your focus you will never allow your products to get to the point where they're really making money. Reusable stage two might be a year away, BFR five. That's four years of lost revenue.

coryrc 8y ago
They're already making money; reusable second stage would only increase profit a little.
Empact 8y ago
Thankfully, Falcon 9 is already making money. Reusing stage 2 is an optimization that represents an allocation of capital - capital should generally go to the highest-expected-return uses.
eloff 8y ago
Weighed against the investment to get a reusable second stage. It may still be logical.

wongarsu 8y ago

Maintanance, fuel costs and other operating costs would probably still leave small enough rockets competitive (because of the rocket equation rockets get exponentially smaller as the payload gets lighter). But I agree in your point the Falcon 9 will become obsolete once the BFR is proven to be reliable and manages a sufficient launch frequency.

larkeith 8y ago
For small, specialized launches there are already other companies filling the niche: https://en.wikipedia.org/wiki/Electron_(rocket) https://en.wikipedia.org/wiki/Electron_(rocket)
Obi_Juan_Kenobi 8y ago
Rockets get exponentially larger as delta-v requirements increase. As payload increases or decreases, rockets merely need to scale linearly. Conceptually, this is quite simple to understand: simply launching two identical rockets will result in double the payload. The mass part of Tsiolkovsky's equation is simply mass-initial / mass-final. Halve both the numerator and denominator, and you have the same fraction.