Project Soyuz 7K-T Custom

diogom

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So after some further research (mainly TMA SUS training manual), turns out that as early as Soyuz-TM there is in fact active control during re-entry. It checks against a pre-programmed (one of several) "lost velocity / time" curve every 200 m/s, and adjusts the angle depending on if it's reached that speed too soon or late. In fact, it overcorrects to compensate the period when it was too shallow/steep.

Big question is, how much of this capability would 7k have already. Far as I can tell, there's no instrumentation output to the crew on the trajectory, nor the control inputs that later Soyuz has, so I would say the options are either "regular SUS" or ballistic, no manual mode. As for the normal re-entry, I think I identified the manual input to pick which way it would roll at the start of re-entry (left/right), which of course affects the lateral trajectory. Other than that, don't really see any of the versatility in speed/time curves modern Soyuzes have, especially since it relies on digital computers introduced after 7k. I could see though 7k having one single "hardwired" re-entry profile, generating analog signals proportional to the difference from the expected curve to feed into the roll channel. @Urwumpe did you ever find any specific info on early SUS?

There is of course the fact I wouldn't have said curve to implement anyway, but I think I could at least approximate one from TMA and hope it's good enough.
 

Urwumpe

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Nothing more specific than you should have. Yes, very likely hardwired analog logic, possibly based on an integrating accelerometers. There is also no way for the crew to change the landing site or to review the coordinates that the guidance uses (beyond a fixed lead angle in the globe instrument), so its likely only a "semiguided" reentry in the sense, that it limits the maximum amount of lateral velocity from the initial ground track by the output signal of an other integrating acceleration sensor, that is preloaded with the velocity change expected during reentry (inertial EI velocity - inertial velocity at end of guidance). Which means: The slower the Soyuz capsule gets, the less difference from the original groundtrack is allowed. I doubt that the early Soyuz already had the guidance packages installed, that allowed a skip reentry at lunar speeds with a guided landing in the USSR. This was expected to use a more sophisticated computer.
 

diogom

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Hm, so "worry" more about keeping it in a certain horizontal corridor, and the vertical path is what it is? Though in that case the solution is to hold roll at 0 degrees always. I guess IRL there might be some perturbations, but as far as Orbiter goes it's quite stable. Or still have some control of vertical speed, if anything just an initial estimation for roll at entry interface, but minimize the horizontal drift, maybe with a roll reversal at some preset velocity?
 

Urwumpe

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Hm, so "worry" more about keeping it in a certain horizontal corridor, and the vertical path is what it is? Though in that case the solution is to hold roll at 0 degrees always. I guess IRL there might be some perturbations, but as far as Orbiter goes it's quite stable. Or still have some control of vertical speed, if anything just an initial estimation for roll at entry interface, but minimize the horizontal drift, maybe with a roll reversal at some preset velocity?

Well, no, the plan was to use a gliding reentry, so its similar to Gemini, Apollo or the Space Shuttle. But likely much less sophisticated as for Gemini:

By rolling the spacecraft, you indirectly control the descent rate and by controlling the descent rate, you can control the deceleration (g-load) on the crew. But if you would always bank to the same side, you would fly a curve away from your ground track. So, you need to do roll reversals. This could be triggered depending on your flight path, high altitude winds and the target coordinates.... or much simpler for simple Cosmonants: By the ratio between lateral velocity and current velocity. If accumulated lateral velocity exceeds a certain (small) threshold, you need to reverse, the lower your speed is, the smaller the threshold has to be. A too low threshold would be sickening, as the spacecraft would be wildly rolling around without actually being able to steer itself. So you need to start at a high threshold velocity and gradually reduce it to stay close to your intended corridor. At the end of the reentry, you want to null it completely and go either into a steep dive or into a full lift configuration.

I think for Apollo, they went into a controlled dive at the end of reentry, but I am not sure about Soyuz there.
 

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magazine article describing atmospheric reentry (1980 should refer to Soyuz-7K)

May I rant a little? Why in Blakes name are Russian technical reports so unstructured?! This one almost reads like what we call a "media kit" for journalists in the west.

The article describes a bit of things and the key subsystems of all spacecraft involved in the Salyut program, but quickly jumps from topic to topic, without any main thread or storyline.
 

diogom

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magazine article describing atmospheric reentry (1980 should refer to Soyuz-7K)

Interesting stuff in there, thanks. The detail of the 10000 km of separation after launch, for example. A lot more detail on Salyut itself than I expected, actually. And it seems the SA's thrusters are actually 5 kgf higher than what I'm using.

May I rant a little? Why in Blakes name are Russian technical reports so unstructured?! This one almost reads like what we call a "media kit" for journalists in the west.

The article describes a bit of things and the key subsystems of all spacecraft involved in the Salyut program, but quickly jumps from topic to topic, without any main thread or storyline.

Rant away, it is a bit chaotic.

Alright, Gemini makes sense, and I agree it would probably look similar on Soyuz (thanks so far btw). I reckon I can work up a "lateral velocity accumulator" if I think about it long enough. On this aspect, TMA and co. seem to do only one roll reversal per entry, which is why I'm wondering if the earlier models already did that, or there were indeed several reversals as needed and something in the modernisation made only one enough. The former way would probably more accurate, the latter seems to be described as "close enough". Either way, the ratio of lateral / longitudinal sounds like a good idea.

As for vertical guidance, from my quick read up Gemini seems like it was a step above Soyuz, with the "fancy" inertial guidance. That is, from all I've heard so far it doesn't seem that Soyuz knew where it was landing and thus where to aim with lift, as you say too, it was just given a time to retro-burn and followed the program: orient, inertial hold and burn, separate at 140 km, SUS on, etc. Unless there's something that can be uploaded to the ship from the ground (but not by the cosmonauts?). In fact even TMA and co. don't seem to be that sentient, they just determine and feed it the most appropriate speed/time curve to follow to reach the landing site, and it "blindly" follows the curve.

The solution I'm seeing with all the input so far, as far as implementation, is a fixed "speed change / time" curve. So start counting when re-entry starts, have integrating accelerometers measuring both lost speed and lateral motion (drag and something else), and periodically (or continuously?) check if more or less lift is needed, and roll accordingly. When the lateral / longitudinal threshold is reached, roll reversal. This would be a curve I'd estimate from one of the TMA ones, knowing the overall shape and using 7200 m/s as the end of reentry, not the first time I'd fire up Matlab and MacGyver some curve fitting 'till it's good enough. As for the end portion, the way of TMA as I understand it is at 7200 m/s, reentry is considered done, it holds the current roll angle and at a certain altitude, engages the ballistic mode until the parachutes are deployed.

Sounds realistic? Still not sure if I'm complicating too much or not.
 

diogom

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Since it's the low hanging fruit, had a start on Ballistic:


Had to change the roll thrusters: previously they were purely roll (Orbiter Z axis), and during ballistic reentry that just meant that lift always remained positive, even on 180º roll. The "roll" should instead be around the velocity vector (opposite of red Drag in the video) while in a balanced orientation during reentry, not the capsule's own longitudinal axis. In theory I think the capsule should rebalance itself just from aero loads, but I don't know how rotation would factor into it.

So I noticed from pics the roll thrusters on the SA aren't actually parallel to the bottom edge, they're somewhat angled "inward" towards the hatch, as they are on the mesh as well: this I'm assuming fits like a glove into their thrust vector being orthogonal to the velocity vector. From photos it seems certain to me they're at an angle, I just haven't found confirmation that that's the purpose. But I angled them around 20º in, and from the capsule's point of view, roll is no longer purely roll, but it works out during reentry to keep the angle of attack and balanced position more or less constant.
 

Urwumpe

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During reentry, you also use the pitch and yaw thrusters to stabilize the spacecraft near the desired AOA (with a relatively large deadband). Not sure if Soyuz was able to do that during ballistic reentry, I assume not. Apollo and Gemini did that, but both also degraded to a pure rotating ballistic reentry, once they had no reliable data about the orientation of the spacecraft.
 

diogom

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During reentry, you also use the pitch and yaw thrusters to stabilize the spacecraft near the desired AOA (with a relatively large deadband). Not sure if Soyuz was able to do that during ballistic reentry, I assume not. Apollo and Gemini did that, but both also degraded to a pure rotating ballistic reentry, once they had no reliable data about the orientation of the spacecraft.
Supposedly, for TMA at least, the limits to get into ballistic are 173º in roll and 54º in yaw. Though interestingly the gyro suspensions seem to be aligned to the velocity vector in the balanced position, not the body axes. Same deal for the descent BDUS, seems like I have some realigning to do, can't fully reuse the existing code for the orbital portion. As for pitch and yaw during ballistic, the back-up ballistic section mentions pitch and yaw being damped, though as I understand it the threshold is 3.5 deg/s (in that mode, with a dedicated BDUS set, possibly lower in regular ballistic). Edit: had the realisation that the yaw thrusters are conveniently placed and oriented to give "perfect" yaw (no unintended pitch or roll rates) in this velocity vector coordinate system. While measuring yaw relative to the body axes, every way I think of it they will always introduce some roll and sometimes pitch. So one more thing to adjust, and it seems like the whole thruster system was designed around that 20º rotated coordinate system for reentry.

For interest's sake:
1660074602358.png
Gamma is roll and Psi is yaw, relative to velocity axes, x and z are switched relative to Orbiter's coordinate system.

Meanwhile, took some time to update the main chute:
1660075434513.png

Normals pending. Still working out how I'm gonna do the pilot (I understand there are two in quick succession?) + drogue sequence, in terms of which objects are created and animated, but drogue at least is gonna be a must for this release.
 
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diogom

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As far as re-entry guidance for the ballistic mode, I'd say that's pretty much complete. Behaviour is looking good and stable, also added in the respective indicator lights and for any descent, the venting of all propellant when the heatshield jettisons.

I do need to have a look at the aerodynamics. I had to reduce the rotation drags, since at "max-q" the roll thrusters were having trouble keeping up the roll rate. Modern Soyuz has an extra set of roll thrusters precisely for this reason, so maybe I overcorrected, though I haven't been able to confirm their presence in 7K yet, there's some contradiction. For example, Soyuz 10:
1660860696801.png

And Soyuz 30:
1660860738401.png

Also, as far as documentation of the real thing goes, the (passive) stabilisation in pitch should be finished at around 95 km, yet it's still going (slow and wide swings) at entry interface in Orbiter, so I'm having to manually kill rotation at the correct pitch. Also, the entry interface defined as losing 25.6 m/s in speed is happening around 72 km, while it should be around 80 km. Could either be a thinner atmosphere or lower vessel drag than expected.

One other factor is the centre of mass should be offset 85 mm vertically, but the centre of pressure still on the symmetry axis, but when I tried that, nothing I changed could get the balance point at 20-25º AoA, it was more like 35º, sounds a bit too much to settle for. Currently it's shifted 0.5 mm, maybe the much smaller difference between centre of mass and centre of pressure could slow down the process of reaching the balance position? It did stabilise quicker when I had 85 mm. Maybe I missed something in the vessel configuration which could make that offset work.

I'm wondering how much this is my cross sections / drags / CG, but also Orbiter's atmosphere having some differences from reality. I'm using the values from Shipedit (3.8 m^2 cross-sections, give or take, seems about right for the heatshield side at least), but while the L/D is correct (0.3) throughout entry, I don't really know what drag values to expect realistically. Same goes for the airfoils, not sure what chord length and wing area would be for a capsule.
 

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Hi diogom,
i was just wondering if you released a new version after this file "7k_T_dll_13062022" or if it is still the latest ?
thanks.
 

diogom

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Hi diogom,
i was just wondering if you released a new version after this file "7k_T_dll_13062022" or if it is still the latest ?
thanks.
Nope, that's the latest. Still a couple of things to wrap up for the next one, but it's on hold for the next few weeks.
 
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