3 ms·

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

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)
    - ericbarrett 4y ago
      For that kind of ISP you need hydralox. Now you've got to accelerate cryogenically cooled hydrogen at 10,000g! Also if it ruptures during spin-up you've probably destroyed your launch mechanism (not that keralox would be much better). At least you wouldn't need much insulation on the tank since it only has to spend ~6-7 minutes in flight.
      - JumpCrisscross 4y ago
        > that kind of ISP you need hydralox Merlin 1D and Rutherford get close to 350 in vacuum with RP-1. Hydralox would need to be insulated during spin up, which probably makes it prohibitive. In any case, if anything goes wrong in spin-up or deployment, the entire facility is toast. Whether there's a bit of extra chemical energy added to the mix is likely a rounding error.
jkqwzsoo 4y ago
As discussed in the video, the SpinLaunch has the advantage that the acceleration to launch velocity is done in vacuum without reaction mass, so they can use a relatively heavy vehicle. The heavy vehicle obviates the issue you discuss, since there is more kinetic energy stored in the heavier vehicle than in a chemically-launched vehicle (which in this case needed to be light enough to be accelerated to velocity in 5 seconds). Thus, the fraction of energy lost as heat will be lower. Again, this is because the vehicle can be accelerated electrically under vacuum until it reaches the required velocity. The heavy mass (in the vehicle’s nose cone) is not part of the orbital vehicle.

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)
  - ericbarrett 4y ago
    For that kind of ISP you need hydralox. Now you've got to accelerate cryogenically cooled hydrogen at 10,000g! Also if it ruptures during spin-up you've probably destroyed your launch mechanism (not that keralox would be much better). At least you wouldn't need much insulation on the tank since it only has to spend ~6-7 minutes in flight.
    - JumpCrisscross 4y ago
      > that kind of ISP you need hydralox Merlin 1D and Rutherford get close to 350 in vacuum with RP-1. Hydralox would need to be insulated during spin up, which probably makes it prohibitive. In any case, if anything goes wrong in spin-up or deployment, the entire facility is toast. Whether there's a bit of extra chemical energy added to the mix is likely a rounding error.

jkqwzsoo 4y ago

As discussed in the video, the SpinLaunch has the advantage that the acceleration to launch velocity is done in vacuum without reaction mass, so they can use a relatively heavy vehicle. The heavy vehicle obviates the issue you discuss, since there is more kinetic energy stored in the heavier vehicle than in a chemically-launched vehicle (which in this case needed to be light enough to be accelerated to velocity in 5 seconds). Thus, the fraction of energy lost as heat will be lower. Again, this is because the vehicle can be accelerated electrically under vacuum until it reaches the required velocity. The heavy mass (in the vehicle’s nose cone) is not part of the orbital vehicle.