Astronauts say riding Falcon 9 rocket was different from space shuttle
spaceflightnow.com
spaceflightnow.com
Shuttle possibly being much to heavy for that & the final orbit insertion burns being done by the OMS, which was quite low acceleration given the mass of the shuttle.
Perhaps the best resource I know to compare the accelerations is flightclub.io, I ran two simulations for STS-129 [0] and DM-2 [1] so you can compare their profiles.
These are in no way authoritative, but it does seem like Crew Dragon would have higher acceleration (as high as 3.7G) compared to shuttle (remaining under 3G the whole time).
Note also that in the article says that Hurley described the ride on the Merlin Vacuum upper stage engine as "kind of like driving fast on a gravel road" which suggests that the proximity of the engines and vibrations may have been more important than raw acceleration.
[0] https://flightclub.io/result/2d?id=bb67b508-71e1-4e4d-8cfc-a...
[1] https://flightclub.io/result/2d?id=4bd73964-9a16-4918-867c-c...
Of the 35.1 MN of maximum total shuttle launch thrust, about 84% came from the solid boosters, only 16% from the somewhat misleadingly named "main" engines.
https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Boo...
And shuttle took a much more aggressive path. SpaceX rockets aim very high for longer burn times (more up, less sideways on first stage). It is technically a less efficient approach but allows for the recovery trickery.
There can be other effects as well --- the Saturn V had significant issues with "pogo oscillations", which are just what they sound like: strong periodic variations in vertical acceleration. Falcon 2nd stage issues might be similar.
>The sideways shaking was unbelievable. The vibration was so intense, you couldn't see the instrument panel. I thought we'd had it...
https://youtu.be/kqAzLyFS_pk?t=41
They said that the Titan II, which launched the Gemini missions was incredibly smooth, just very powerful and loud.
The first stage was smoother, because as you said "liquid smoother than solid rockets", but the second stage was noticeably rougher than the shuttle (specifically the stage separation/transition, which the shuttle did not suffer from due to the SSMEs continously firing even during SRB separation).
The article is quoting comments from a press event, but I remember on the livestream one of Bob/Doug specifically commented on how rough the ride was as one of the first things they said after SECO/docking (can't remember exactly which). I'm thinking it must have been pretty dang rough if it was the first thing on their mind.
It's unsourced, but I've seen other comments on the internet remark that certain satellite payloads actually can't use the Falcon 9 due to its vibration profile being too rough compared to others rockets.
https://youtu.be/zNklfC6jgBs?t=14653
It was Bob. Rough transcript:
"We were surprised a little bit at how smooth things were off the pad. The space shuttle is a pretty rough ride heading into orbit with the SRBs, and our expectation was as we continued with the flight into second stage that things would basically get a lot smoother than the space shuttle did, but Dragon was huffing and puffing all the way into orbit and we were definitely driving and riding a dragon all the way up, and so it was not quite the same ride, the smooth ride, as the space shuttle was, up to MECO. A little bit less Gs, but a little bit more alive is probably the best way I would describe it."
Edit: Just noticed that that description is in the original article, too, fwiw
How can this be interpreted as a negative thing, if anything if I were a school boy watching that it that would make me want to get involved and want to experience it myself! An astronaut talking about taking a Capsule (dragon) all the way to Space, and describing it that way... what could be cooler than that?
Also worth noting that no one in involved in Space is expected to do a comfy job, much less being an Astronaut as it requires decades of rigorous physical and mental conditioning, training and exercise and psychological evaluation to just be a candidate in order to prepare you to live in an environment where even going to the bathroom, sneezing, or coughing could all kill you and potentially everyone in your crew in Space.
Yi So-yeon did an interview at MIT earlier this year with Mar's Society [1] describing this exact situation, and even when having almost died upon re-entry due to a faulty heat-shield malfunction that messed up the trajectory of the landing that almost killer her and her whole crew she was the first to sign up for another mission!
That's what an Astronaut is, they're cut from the same clothe as protype Fighter Jet pilots, where death was the most likely outcome; they're not 'normal' People and this doesn't phase them when confronted to do it again for a bigger purpose. Its why they're such exceptional Human beings and why they should be treated as such.
Its not about astronauts playing lift off buckaroo, its about developing viable travel and transport systems.
I guess that is true, Yi made the same argument about sending construction based equipment and the dilemma of having to make it more robust than necessary tolerances and specs thus increasing the weight of the payloads.
But SpaceX hasn't had a satellite failure to date, with the exception of the secret spy satellite that was 'lost,' or the facebook one that exploded on the launchpad (THANKS, ELON!).
I wonder what payloads specifically that would include?
Iirc, this is some combination of a the “SpaceX press event” that they did as well as some words exchanged upon arrival. (I’m not 100% on that, though I do feel like all the quotes came from videos from their first two days in the shuttle.)
The Falcon also stages really early (this allows you to land the first stage), so the 2nd stage MVac engine is more powerful than is really ideal. As fuel burns away, I understand the MVac has to throttle down a lot to keep g-loads under control. I believe it can throttle down to something like 35%. Rocket combustion tends to get less stable at lower throttle settings, so this probably is part of what they're feeling.
The third stage also starts before the second stage is completely depleted; this is why there's an "open grate" visible in photos of the rocket (about half way down the stack, see [1]), gives the exhaust somewhere to go before the previous stage is jettisoned. Hot staging is advantageous as you don't need extra ullage motors to deal with propellant being in the wrong place inside the tanks in microgravity. The Soyuz (and other R-7 variants) are brilliantly engineered; There's a reason they're still flying nearly 70 years on.
[1] https://upload.wikimedia.org/wikipedia/commons/thumb/9/9a/So...
On the other hand you also don't want to turn your crew into pancakes or even make your upper stage look like a squashed can. You have a couple options:
- throttle the engine
- design payload weight, propellant load and trajectory to make g loads bearable till burnout
And one more thing related to stage 2 burnout - precise orbit insertion. Rocket engines used to be pretty imprecise at shutdown, delivering a slightly lower/higher impulse at shutdown at random. Due to this many older rockets have also some small & precise engines to correct the orbit insertion once the main one shuts down - many Chinese rockets have that. As far as I know, the Falcon 9 upper stage engine (Merlin 1D Vacuum) is precise enough so this is not needed.
We'll have to wait for the next mission, which will have an astronaut who has also ridden a Soyuz (and a Shuttle).
Seems like such a vital part of our history.
It's bumpy as hell of a ride due to all the vibration from the engine placement but otherwise it's not too bad.
The total mass boosted by the shuttle was higher, but much of that was the orbiter itself, not usable payload.