Grasshopper Flies Again

It’s getting higher, but this flight is still a long way from the advertising video. This article also includes some information on Elon Musk’s kerfluffle over the New York Times’ review of Tesla.

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2 Responses to Grasshopper Flies Again

  1. Hop David says:

    I’ve watched Musk’s advertising video several times. A few things are unclear to me. Does the first stage impart horizontal velocity to the second stage? If so it must land downrange of the launch pad. Or if it lands on the same pad it took off from, it must cancel it’s horizontal velocity and then impart enough horizontal velocity the opposite direction to return to the launch pad. This adds to the lower stage’s delta V budget. If all the first stage does is achieve a higher altitude, the second stage still has an ~ 8 km/s delta V budget.

    The terminal velocity of a massive object like the lower stage will be high. So the soft vertical landing shown in the video will take more delta V and further boost the propellant mass fraction.

    The much lighter 2nd stage has a lower terminal velocity so the soft vertical landing is more plausible. But how does it re-enter intact? Shedding 8 km/s by aerobraking subjects it to extreme temperatures. Some say just slap some PICA-X on it like Musk did with the Dragon.

    Unlike the second stage, the Dragon doesn’t have an 8 km/s delta V budget. When you don’t have to meet a 95% propellant mass fraction, it’s much easier to slap on more structure and thermal protection. Recovery of the Dragon does nothing to demonstrate the 2nd stage can be recovered.

    For rapidly recovering and re-using the first stage, I give 2 to 1 odds Musk fails. For the 2nd stage, I bet against Musk 20 to 1 odds.

    • Tom says:

      Hey Hop! Great to hear from you. When I watched the video, it looked like the first stage did impart horizontal velocity, though it’s unclear how much. Without a significantly larger second stage, it would have to be about the same velocity applied by a non-reusable first stage. After separation, it turned around and three engines fired to get it headed back towards the launch site. While it ‘feels’ like that would take as much propellant as it took to get the booster to where it was, it only requires the same mass fraction of propellant. Given that the stage isn’t pushing the second stage, cargo, and original fuel load, the amount of fuel required is fairly low. The coast back to launch site doesn’t require any fuel, except to get oriented for the landing, then the center engine fires again for landing. There’s a lot going on there, and I’m hoping they’re going to get it down. These early tests, while cool, are just re-doing the DC-X flights. We’ll know things are getting serious when they do a supersonic downrange flight with multiple engine relights.

      I also agree that the second stage animal is a tough one to tame. My concept for a reusable vehicle had the second stage continue downrange for recovery in Africa or something, meaning you don’t have to deal with all the energy dissipation returning from orbit does. I also proposed using standard shipping containers for returning the stage to the launch site to cut down on costs. One thing the videos gloss over with the second stage is the amount of time required to re-align the 2nd stage’s orbit with the launch site. The shuttle with its huge wings had the capability to return to its launch site after one orbit, but the most you can do with a wingless second stage is offset the center of mass from the center of pressure. Without some sort of cross range control, it will take several orbits before the second stage’s orbital path passes right over the launch site, allowing a return.

      I don’t see how these flights are just for show, however. A large group of people at SpaceX apparently believe they headed in the right direction for reusability, and they’re putting a lot of money into it. They were wrong before with ocean recovery, and I’m hoping they’ve got it right this time.

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