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thekalki

Earth might be little too thick for this but should work great on Moon, Mars.

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If you don't launch to escape velocity, you launch to an elliptical orbit that intercepts the launch altitude.

That's why you basically always need a rocket engine anyway.

...thus the small rocket contained in every spin launch.

The initial velocity just replaces a rocket's booster stage, not a whole rocket.

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.

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.

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.

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)

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.

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.

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?

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)

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.

> 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.

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.

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.

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?

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.

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