6 ms·

The problem is drag and heating. The booster phase of a rocket moves slowest in the thickest part of the atmosphere so it mostly avoids this. Usually at the end

The problem is drag and heating. The booster phase of a rocket moves slowest in the thickest part of the atmosphere so it mostly avoids this. Usually at the end of the booster phase a rocket is going 2-4 km/s but it's also at 60 km above sea level. (All these numbers vary somewhat by launch vehicle.) To travel at that speed even at jumbo jet heights, ~15km, would generate insane amounts of heat and drag; the front of the craft would be white-hot and there would be massive loss of energy to friction. And it would still need to have the fuel and engines to accelerate another 5 km/s to stay in orbit. It's physically possible but the engineering is insanely difficult, much more so than a traditional rocket, which is already no cakewalk.

Symmetry 4y ago
Oh yes, the casing to let it go through the atmosphere at hypersonic velocity is definetly a significant engineering challenge. But it's a solvable one, unlike trying to get through troposphere at orbital velocities.
adgjlsfhk1 4y ago
You know, they do the math in the video. You aren't in the lower atmosphere to loose a significant amount of your initial velocity. I have lots of other questions about the feasibility and cost competitiveness, but this just isn't a credible one.
- ericbarrett 4y ago
  Any craft which starts from sea level (and the top of a mountain or a rail system is basically sea level) will indeed lose a significant amount of its energy to heat. Check out the Sprint missile, which was chemically rather than kinetically launched but faced a similar challenge: https://en.m.wikipedia.org/wiki/Sprint_(missile) https://en.m.wikipedia.org/wiki/Sprint_(missile) I'm not saying it's impossible, just that to pull this off while still having a craft capable of reaching orbital speeds is an immense challenge.
  - adgjlsfhk1 4y ago
    From a quick skim of the article, I don't believe deceleration or energy loss calculations are given. TFA gives a figure of 150 m/s of aerodynamic losses which is fairly tiny compared to the roughly 2km/s launch or the 1km/s gravity losses.
    - ericbarrett 4y ago
      150 m/s seems insanely low, only slightly higher than a regular rocket launch which moves much more slowly through the lower atmosphere. I don't think a vehicle which still has the mass to reach orbital velocity could attain that. There are smarter people than I working on this, but I remain a skeptic. Also, economic viability: per the Tsiolkovsky rocket equation, for a 100kg payload (almost microsat territory) and an orbital-insertion engine with a generous ISP of 320, you'd still need over 500kg of propellant to accelerate the "second stage" 5km/s to orbital velocity. This ignores the mass of the tank and engine and electronics (which have to survive 10,000g!). If you remove the latter from the payload you're looking at just a few kg remaining for the sat itself and you've massively raised the cost for the upper stage due to the exotic materials required to hold up under such acceleration. Then even if SpinLaunch or another team pulls off said engineering, you have to compete against a traditional rocket which could launch ~100 equivalent satellites to low-Earth orbit in one shot—at a known cost, with high reliability—using only materials and design widely in use today. Edit: The same numbers with a 1,000kg payload are ~6,000kg at launch. How are you going to design a 6,000kg craft which can survive 10,000g?
      - adgjlsfhk1 4y ago
        I think the main flaw with this analysis is assuming that 320 is the upper bound ISP. There are plenty of upper stage engines that are getting over 450 (eg RL10). Obviously making a high quality second stage that is small and durable enough isn't easy, but it's not obvious to me that it's impossible to make cost competitive. (that said they obviously are something like 95% likely to fail)
        4y ago
chasil 4y ago
The video covers this. The nose is copper and aluminum that do melt under these temperatures, but this is prevented by two elements. Only the first five seconds of the launch will deliver the majority of heat, and this heat can be dissipated by thermal conductivity. I think they have this covered.
- DoctorOetker 4y ago
  is the total heat delivered as a function of height for each design known publically? could it make sense to replace the bulk of the mass with water, turning into steam to partially recuperate thrust? like a continuous steam explosion to keep the nosecone cool while providing thrust?
DoctorOetker 4y ago
not a criticism or a confirmation, just a remark: > [...] the front of the craft would be white-hot [...] To the extent heating is inevitably experienced, a conventional way is to get rid of it by radiative cooling. Complaints about the visual appearance of a radiative cooling subsystem visibly doing its job is a bit weird.

Symmetry 4y ago

Oh yes, the casing to let it go through the atmosphere at hypersonic velocity is definetly a significant engineering challenge. But it's a solvable one, unlike trying to get through troposphere at orbital velocities.

adgjlsfhk1 4y ago

You know, they do the math in the video. You aren't in the lower atmosphere to loose a significant amount of your initial velocity. I have lots of other questions about the feasibility and cost competitiveness, but this just isn't a credible one.

ericbarrett 4y ago
Any craft which starts from sea level (and the top of a mountain or a rail system is basically sea level) will indeed lose a significant amount of its energy to heat. Check out the Sprint missile, which was chemically rather than kinetically launched but faced a similar challenge: https://en.m.wikipedia.org/wiki/Sprint_(missile) https://en.m.wikipedia.org/wiki/Sprint_(missile) I'm not saying it's impossible, just that to pull this off while still having a craft capable of reaching orbital speeds is an immense challenge.
- adgjlsfhk1 4y ago
  From a quick skim of the article, I don't believe deceleration or energy loss calculations are given. TFA gives a figure of 150 m/s of aerodynamic losses which is fairly tiny compared to the roughly 2km/s launch or the 1km/s gravity losses.
  - ericbarrett 4y ago
    150 m/s seems insanely low, only slightly higher than a regular rocket launch which moves much more slowly through the lower atmosphere. I don't think a vehicle which still has the mass to reach orbital velocity could attain that. There are smarter people than I working on this, but I remain a skeptic. Also, economic viability: per the Tsiolkovsky rocket equation, for a 100kg payload (almost microsat territory) and an orbital-insertion engine with a generous ISP of 320, you'd still need over 500kg of propellant to accelerate the "second stage" 5km/s to orbital velocity. This ignores the mass of the tank and engine and electronics (which have to survive 10,000g!). If you remove the latter from the payload you're looking at just a few kg remaining for the sat itself and you've massively raised the cost for the upper stage due to the exotic materials required to hold up under such acceleration. Then even if SpinLaunch or another team pulls off said engineering, you have to compete against a traditional rocket which could launch ~100 equivalent satellites to low-Earth orbit in one shot—at a known cost, with high reliability—using only materials and design widely in use today. Edit: The same numbers with a 1,000kg payload are ~6,000kg at launch. How are you going to design a 6,000kg craft which can survive 10,000g?
    - adgjlsfhk1 4y ago
      I think the main flaw with this analysis is assuming that 320 is the upper bound ISP. There are plenty of upper stage engines that are getting over 450 (eg RL10). Obviously making a high quality second stage that is small and durable enough isn't easy, but it's not obvious to me that it's impossible to make cost competitive. (that said they obviously are something like 95% likely to fail)
      - 4y ago

chasil 4y ago

The video covers this. The nose is copper and aluminum that do melt under these temperatures, but this is prevented by two elements. Only the first five seconds of the launch will deliver the majority of heat, and this heat can be dissipated by thermal conductivity. I think they have this covered.

DoctorOetker 4y ago
is the total heat delivered as a function of height for each design known publically? could it make sense to replace the bulk of the mass with water, turning into steam to partially recuperate thrust? like a continuous steam explosion to keep the nosecone cool while providing thrust?

DoctorOetker 4y ago

not a criticism or a confirmation, just a remark: > [...] the front of the craft would be white-hot [...] To the extent heating is inevitably experienced, a conventional way is to get rid of it by radiative cooling. Complaints about the visual appearance of a radiative cooling subsystem visibly doing its job is a bit weird.