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Comment by evv | Hacker News Reader

The behavior of the booster landing looks very different than SpaceX's.

Blue Origin's booster seems to switch to a different control logic at ~30ft up, from a rapid descent to a slow controlled touchdown procedure. Meanwhile, SpaceX has a very smooth touchdown that seems to use a single control logic. I'm sure that the SpaceX approach is better for fuel consumption, but the slower Blue Origin landings seem better for spectators!

I love watching the physical effects of somebody's code, and I'd love to hear more about the decisions that led to this behavior.

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F9/FH cannot throttle down enough to do a hover landing, so they have to do a suicide burn (i.e., fire the engine just so the speed goes to 0 m/s at 0 m altitude.) The engine is too powerful and the body is too light to hover.

Blue Origin might eventually encounter just that problem with New Glenn.

I don't know where you're getting this idea. The grasshopper hovered all the time in tests and uses the same single-engine as the F9 full thrust.

https://www.google.com/search?q=spacex+grasshopper+video&num...

I don't think it was exactly the same engine, and Grasshopper didn't have 9 engines, it had one. The engines are a surprisingly large mass fraction of a dry first stage.

They probably just put extra weight (fuel) in the grasshopper for easier testing. The F9 on the other hand is basically an empty soda can with engines at the bottom and small fins at the top when it’s landing so even when firing just a single engine at minimum power it will generate too much thrust to hover.

SpaceX is forced to do the "hoverslam" landings (where the velocity reaches zero exactly as altitude reaches zero) because their engines can't actually throttle low enough to hover when the booster is almost empty. At the lowest power it would still accelerate upwards.

Blue Origin's booster, as far as I know, doesn't have this limitation (not that it seems to cause SpaceX too much trouble).

It is probably fair to say that initially SpaceX was "forced" to perform hover slams due to thrust/weight ratios, but they seem to have embraced that option and turned it into a strength. For example, some of their landings now use three engines instead of one, getting slightly better fuel efficiency and "slamming" even more dramatically. If they were given the option today to do a slow landing like New Shepherd, I don't think they would take it. It will be interesting to see whether Blue goes to a faster landing profile when they start making orbital rockets that are actually performance sensitive.

SpaceX's Merlin engines can't throttle down low enough to achieve a thrust-to-weight ratio of 1 (hover) or less than 1 (controlled descent), therefore they have to do a "suicide burn" where they synchronize engine startup during descent such that speed is "exactly" 0 m/s just as the stage is touching down on the landing zone/barge.

It seems like BO's engines are smaller and/or have deeper throttle capabilities, so they actually do hovering and controlled descent.

> It seems like BO's engines are smaller and/or have deeper throttle capabilities, so they actually do hovering and controlled descent.

Blue Origin only uses one engine for New Shepard, so it's actually proportionately much larger than the Merlin engines on Falcon 9. The reason they can hover is that New Shepard is fundamentally a MUCH lower performance vehicle than Falcon 9. This allows them to land with a larger fraction of their launch mass, decreasing the amount of throttling required.

The single BE-3 engine used by New Shepard can throttle from 490KN to 89KN, about 18%. The 9 Merlin 1D engines on a Falcon 9 first stage can throttle to 39%, but 8 of them can be shut down during landing for a total throttle capacity of 4.33%. The fact that New Shepard can hover with 18% throttle but Falcon 9 can only do a suicide burn with 4.33% should tell you everything you need to know about the relative performance (and use cases!) of the two rockets.

They are both great solutions.

Blue Origin could design the most elegant solution to their problem without a lot of compromises.

SpaceX always intended reuse but originally they were going to recover boosters with parachutes. They already had a full flight manifest depending on engines designed for mass production and thrust to weight, not throttle ability. So they had to work with what they had which was a rocket that could not hover. They turned the limitations into a benefit because the hoverslam is more fuel efficient which is far more critical for their service than it would be for Blue Origin.

>SpaceX always intended reuse but originally they were going to recover boosters with parachutes

I know that engineers have obviously done that math on this, but I still find it incredibly counterintuitive that they are not at least shaving off some velocity with any chutes or other aerodynamic surfaces (except of course there are the grid fins, but afaik those are primarily for steering the ballistic trajectory in the upper atmosphere at hypersonic speeds, and don't significantly alter the terminal velocity relevant to how much fuel you need for a hover burn).

SpaceX totally 'flies' the first stage, but as a cylinder with gridfins it can't shed that much velocity due to aerodynamics.

If you watch the first stage telemetry as it comes back down on the livestream (they don't always show you it, sadly) you can see that actually a huge proportion of the velocity is lost through atmospheric drag. I was shocked the first time I saw it.

Obviously the terminal velocity of the stage is still pretty fast but compared to the speed it starts at it's much smaller.

Yea, atmospheric drag does most of the work to slow the boostet rocket. Which makes it all the more counterintuitive to my brain that the economics and engineering doesn't make sense for deploying some kind of chutes to shave off velocity.

Every kg in fuel you need to burn to land, you need to lift off the pad. From what I found through some Googling, a F9 booster uses a little under 1,000kgs to do the hover burn. I would imagine some lightweight aero surfaces would be more weight-efficient at decelerating the rocket than the 999th, 997th, 996th, (nth) kg of fuel.

Parachutes hinder re-usability. You can't deploy a chute, land, then expect to take off again quickly without manual refurbishment. SpaceX's ultimate goal is zero refurb re-usability.

A parachute would also be next to worthless for landing a fully-loaded BFR on Mars.

SpaceX takes the iterative approach and is designing their vehicle for Mars landings now to get practice, at the expense of being able to carry a bit less to high-energy orbits (which they've largely negated with Falcon Heavy and Falcon 9 upgrades) here on Earth.

I noticed the same thing when the first New Shepard video was released. Here’s a more detailed explanation of what’s going on:

https://news.ycombinator.com/item?id=10620730

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