Author Topic: Human rating the BFS  (Read 21831 times)

Offline sevenperforce

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Re: Human rating the BFS
« Reply #160 on: 03/15/2018 05:54 PM »
Thank you very much for your excellent scenario. Perhaps there is a reason why Spacex did not add a vertical tail creating yaw stability. Instead of performing a post stall manouver like a Pugachev's Cobra, they might plan a flat 180 yaw turn.

If the BFR develops an unwanted yaw angle exceeding the RCS authority, yaw can also be corrected by differential use of split flaps on winglet.  if yaw exceeds amount that can be handled by RCS and split flaps, yaw can be corrected by rotating rocket 90 degrees on the longitudinal axis and use the wing to righten the rocket.

Mars airship May have a CoM near the center, but both tanker and BFS used to launch satellites will likely have a light nose resulting in CoM closer to the engines.
The BFS spaceship has very little aerodynamic yaw damping at high AoA, which should allow it to do the nose-down entry shown in the simulation with the RCS-assisted yaw roll, but it has superb yaw damping at low AoA because the center of pressure is pulled so far back as soon as the winglets start to develop lift. I doubt that the BFS can do a 180-degree yaw maneuver on approach, for this reason.

Interestingly enough, the nose-down entry solves the S-curve problem faced by the Shuttle; you're still flying at a high AoA, but you're flying upside-down, so your lift vector is negative and you can control your descent rate accordingly. I imagine that flying hypersonic at a right-side-up high AoA would rapidly shuttlecock those winglets, far beyond their roll compensation capabilities (winglet roll authority at high AoA requires some degree of aerodynamic lift, which isn't really possible with winglets that small in a right-side-up high-AoA entry). Those winglets would stall out if it came in like a Shuttle.

Offline IainMcClatchie

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Re: Human rating the BFS
« Reply #161 on: 03/15/2018 06:37 PM »
What S-curve problem faced by the Shuttle?

And by negative lift vector, do you mean the lift vector points towards the ground?  I understand that's wanted after the initial part of an interplanetary reentry, but once you are below orbital speed, wouldn't you want the lift vector to point away from the ground, to keep the vehicle in thinner air longer, thus reducing maximum G and peak heating?

Offline speedevil

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Re: Human rating the BFS
« Reply #162 on: 03/15/2018 06:57 PM »
What S-curve problem faced by the Shuttle?

And by negative lift vector, do you mean the lift vector points towards the ground?  I understand that's wanted after the initial part of an interplanetary reentry, but once you are below orbital speed, wouldn't you want the lift vector to point away from the ground, to keep the vehicle in thinner air longer, thus reducing maximum G and peak heating?

Interesting to note that to a first order, the acceleration needed,  density of the atmosphere and the velocity of LEO entry, and interplanetary Mars entry are pretty much the same.
It's just that with Mars, you are pushing down to keep in the atmosphere, not up to keep out of it.

Offline sevenperforce

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Re: Human rating the BFS
« Reply #163 on: 03/15/2018 07:46 PM »
What S-curve problem faced by the Shuttle?
The Shuttle had to fly at a very high AoA to keep plasma impingement only on the thermal tiles. At this AoA, however, the Shuttle generated so much lift that it would descend more slowly than it lost speed, which would cause the control surfaces to stall out and put it into a tumble. In order to descend more rapidly, the Shuttle had to roll over, so that the lift vector pointed sideways rather than up. But this pulled it off-course, so it had to repeat in the opposite direction.

Quote
And by negative lift vector, do you mean the lift vector points towards the ground?  I understand that's wanted after the initial part of an interplanetary reentry, but once you are below orbital speed, wouldn't you want the lift vector to point away from the ground, to keep the vehicle in thinner air longer, thus reducing maximum G and peak heating?
A big vehicle like the BFS or the Shuttle depends on its aerodynamic control surfaces to maintain the right attitude, since its passively aerodynamic stable mode (nose-first) has a distinct tendency to blow up. It needs a precise attitude in order to keep its heat shield oriented into the plasma stream.

Control surface authority depends on airflow over those control surfaces. In order to maintain control authority, you have to have a minimum amount of airflow. Ideally, you start out high in the atmosphere at really high speeds; there's not much air up there, but you're moving so fast that you still have enough airflow for control. As you descend, the air gets thicker, and you start to move slower, and so airflow is maintained.

The problem is when you slow down (due to drag) more rapidly than you descend. If you lose airspeed but you don't lose altitude, the air is still thin but you're getting less and less of it, and so your control authority drops, and eventually you stall, and tumble, and you have a Bad Day.

The BFS doesn't have to have the same subsonic glide ratio as the Shuttle, and its large tanks make it substantially fluffier, and it will have better heat shielding. As a result, it can do some tricks that the Shuttle could not -- like coming in upside-down to keep its lift vector pointed down and control its rate of descent more intentionally.

Offline Space Ghost 1962

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Re: Human rating the BFS
« Reply #164 on: 03/15/2018 08:18 PM »
Fairly accurate. One also had to do energy management coupled with the GNC, so that not only did the TPS not get overloaded, but the descent was managed in such a way that the runway could be acquired at the right altitude/approach/heading, managing a rather large state space at the time, so that the landing (or abort to alternative site) would be maintained.

For BFS, a similar energy management would need to be done.

Offline sevenperforce

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Re: Human rating the BFS
« Reply #165 on: 03/15/2018 08:24 PM »
Fairly accurate. One also had to do energy management coupled with the GNC, so that not only did the TPS not get overloaded, but the descent was managed in such a way that the runway could be acquired at the right altitude/approach/heading, managing a rather large state space at the time, so that the landing (or abort to alternative site) would be maintained.

For BFS, a similar energy management would need to be done.
And GNC is equally (or more) important with BFS, because you have a rather poorer glide ratio and a much smaller landing site.

If the BFS can control descent rate during entry with yaw alone, rather than using rolls and S-curves, it will be able to have much more control over its GNC than the Shuttle did. It has a wider survivable AoA range, which helps a lot.

Offline acsawdey

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Re: Human rating the BFS
« Reply #166 on: 03/15/2018 08:38 PM »
For BFS, a similar energy management would need to be done.

Fortunately SpaceX have got a guy with some previous experience in such things:

http://www.larsblackmore.com/losslessconvexification.htm

Admittedly, his previous work seems to have been with landing systems that are mostly propulsive (mars landers, Xombie) but it seems like it could be generalized to include aerosurfaces and maneuvers.

Offline Lar

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Re: Human rating the BFS
« Reply #167 on: 03/15/2018 11:02 PM »
I am just loving all this reentry discussion, the contrasts between BFS and Shuttle are fascinating.
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Offline rakaydos

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Re: Human rating the BFS
« Reply #168 on: 03/15/2018 11:20 PM »
A big vehicle like the BFS or the Shuttle depends on its aerodynamic control surfaces to maintain the right attitude, since its passively aerodynamic stable mode (nose-first) has a distinct tendency to blow up. It needs a precise attitude in order to keep its heat shield oriented into the plasma stream.


Why does the BFS need a passive aerodynamic mode nose first? As far as I'm aware, the only times it's actively traveling nose first, there is engine gimbal to keep it that way. It doesnt glide like the shuttle, the fins just balance out the weight of engines when doing a capsule-style entry on it's belly.

I would expect the passive stable mode to be tail first, if anything, to support vertical landing.

Offline speedevil

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Re: Human rating the BFS
« Reply #169 on: 03/15/2018 11:33 PM »
Why does the BFS need a passive aerodynamic mode nose first? As far as I'm aware, the only times it's actively traveling nose first, there is engine gimbal to keep it that way. It doesnt glide like the shuttle, the fins just balance out the weight of engines when doing a capsule-style entry on it's belly.

I would expect the passive stable mode to be tail first, if anything, to support vertical landing.
I was assuming that was meant to be 'has a' not 'needs a'.
BFS on maximum reentry weight for P2P, or Mars entry has around 100 tons of total mass in the structure and fuel, and 150 tons in the cargo area, which is obviously well forward.
« Last Edit: 03/15/2018 11:34 PM by speedevil »

Offline sevenperforce

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Re: Human rating the BFS
« Reply #170 on: 03/16/2018 02:19 AM »
Why does the BFS need a passive aerodynamic mode nose first? As far as I'm aware, the only times it's actively traveling nose first, there is engine gimbal to keep it that way. It doesnt glide like the shuttle, the fins just balance out the weight of engines when doing a capsule-style entry on it's belly.

I would expect the passive stable mode to be tail first, if anything, to support vertical landing.
I was assuming that was meant to be 'has a' not 'needs a'.
BFS on maximum reentry weight for P2P, or Mars entry has around 100 tons of total mass in the structure and fuel, and 150 tons in the cargo area, which is obviously well forward.
Right. The BFS very clearly has a nose-first aerodynamic stability mode; if you have any sort of atmosphere in play, those winglets are going to shuttlecock. Moreover, it does need to glide for landing approach, as shown in the wireframe video in the 2017 IAC presentation.

Online AncientU

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Re: Human rating the BFS
« Reply #171 on: 03/16/2018 03:01 PM »
Why does the BFS need a passive aerodynamic mode nose first? As far as I'm aware, the only times it's actively traveling nose first, there is engine gimbal to keep it that way. It doesnt glide like the shuttle, the fins just balance out the weight of engines when doing a capsule-style entry on it's belly.

I would expect the passive stable mode to be tail first, if anything, to support vertical landing.
I was assuming that was meant to be 'has a' not 'needs a'.
BFS on maximum reentry weight for P2P, or Mars entry has around 100 tons of total mass in the structure and fuel, and 150 tons in the cargo area, which is obviously well forward.
Right. The BFS very clearly has a nose-first aerodynamic stability mode; if you have any sort of atmosphere in play, those winglets are going to shuttlecock. Moreover, it does need to glide for landing approach, as shown in the wireframe video in the 2017 IAC presentation.

I don't believe that any part of the EDL could be characterized as a 'glide' -- the landing approach is actually tail first... otherwise, belly first, nose down then up.
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Offline sevenperforce

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Re: Human rating the BFS
« Reply #172 on: 03/16/2018 03:55 PM »
Right. The BFS very clearly has a nose-first aerodynamic stability mode; if you have any sort of atmosphere in play, those winglets are going to shuttlecock. Moreover, it does need to glide for landing approach, as shown in the wireframe video in the 2017 IAC presentation.

I don't believe that any part of the EDL could be characterized as a 'glide' -- the landing approach is actually tail first... otherwise, belly first, nose down then up.
I disagree. Take a look at this video again:

https://www.youtube.com/watch?v=tdUX3ypDVwI?t=35m31s

Note the altitude-vs-velocity graph on the right.

From 7 km/s down to 3 km/s, the BFS enters at a high inverted AoA (about 50 degrees); body lift vector points toward the surface. At 3 km/s and 10 km altitude (35:45), the BFS initiates a yaw maneuver around the prograde axis lasting 45 seconds realtime; this yaw maneuver concludes at approximately 1.8 km/s and 5 km altitude (35:48) and the descent rapidly reverses, with the vehicle rising back to 10 km altitude at an airspeed of 900 m/s in about 90 seconds, with an AoA of about 60 degrees.

Over the next seventy seconds, the vehicle begins to pitch forward to avoid a stall, then suddenly pitches backward and stalls, with RCS pointing the vehicle just below retrograde for about eight seconds before landing engine ignition at Mach 2.4 or thereabouts. Engine gimbal completes the attitude correction to retrograde and completes the 40-second landing burn.

So you definitely have a glide period of a little over two minutes. It's a very high AoA glide (lowest AoA is about 45 degrees) but definitely a glide.

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