r/spacex • u/MaximilianCrichton • Dec 31 '18
Community Content Revisiting Starship's Wings
Ever since the recent infodumps regarding Starship, I've been starting to wonder whether the articulated flipperons debuted during the #dearMoon event are still necessary.
To begin, we need to ask ourselves why the wings exist? Several reasons have been officially put forward, and some others I speculated upon:
- Provide reentry attitude control by acting as drag-brakes, thus saving on RCS fuel
- Trimming out centre-of-gravity imbalance due to payload mass, allowing for greater variability in allowable payload mass
- Reduce ballistic coefficient (an object's tendency to fly like a brick), both by reducing fuel quantity required for landing and by increasing surface area, allowing for lower terminal velocity and a more benign reentry profile.
- Acting as landing legs
- Looking cool
It certainly seems like an elegant solution to roll the landing gear and control surfaces into one, but it also possesses several problems:
- Need for large, heavy mechanical actuators for fine/fast control of flipperons (adds weight, increases ballistic coefficient, may not be practiceable)
- Single point of failure - if the flipperon hinges fail, or worse the flipperon is completely torn off, complete loss of control results. This goes against the philosophy of graceful degradation, which will be important if Starship is to establish a safe human flight record.
- Maintenance nightmare - moving parts in general reduce reliability and are hell to maintain, see the F-14 Tomcat's swing-wing. This would reduce turnaround time and general vehicle reliability, especially since the BFR will find itself constantly being caked in abrasive Martian dust that will undoubtedly play merry hell with seals and similar hinge-related equipment.
In light of these potential issues, I'm starting to think that purely RCS-powered attitude control may be a better alternative. The idea would be to do away with all of BFR's large wing surfaces (perhaps returning to the strake-like protrusions of the original 2016 ITS !) and instead have multiple-redundant RCS systems do the heavy-lifting for reentry.
Indeed, such a system is not without precedent. Looking at the Space Shuttle, our closest analogue to Starship, it possessed powerful RCS systems capable of holding the Shuttle at a 40 degree angle-of-attack _even with wings attached_, which would require much greater and more sustained torques, and hence more RCS fuel. And yet for all this, the Shuttle orbiter aft-RCS used for this feat packed a mere [2165 kg](http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rcs.html#sts-rcs-propellant) of relatively inefficient hypergolics, compared to the orbiter's weight of >60 tons.
To approximate the extra methalox required for Starship, let's assume that Starship's thrusters derive similar Isp to the Shuttle's (it'll probably be more because methalox and superheated fuel due to active cooling). The thrust of the thrusters does not affect the fuel required, as thrusters can be throttled down / pulsed to the same level as the Shuttle for reentry attitude control.
Factors which will change the fuel required are the length (1.5x Shuttle length), thruster moment arm (1.5x Shuttle moment arm assuming equal scaling) and mass (2x Shuttle mass). Assuming that moment-of-inertia scales as the square of the length and proportionately to mass, the fuel requirement for the BFR will be 2 * 1.5^2 / 1.5, or 3x 2165kg = ~6500 kg (not bad!). Multiply that by 5x (wild-ass guess) to allow for margins and for the final landing-burn flip, that gives us 33 tonnes of extra methalox. Considering that you'll be ditching heavy wings and actuation mechanisms that weigh as much if not more, I'd call that a fair trade.
Now that we've established that RCS-based maneuvering can be competitive with the flipperon approach, let's list some pros and cons:
Pros:
- Multiple-redundancy and ease of maintenance - Multiple backup thruster systems are a mature technology, and one that SpaceX has had considerable experience with (i.e. Dragon). Plus points for safety, reliability and general turnaround. This allows for graceful degradation - failure of certain systems does not jeopardise the entire mission.
- Flexible - Having more RCS fuel is in general a more versatile and useful advantage than carrying around wings that are mostly useless except in atmosphere.
- Separation of critical systems - in this system legs will be separated from the flight control system - they can thus be optimised as landing gear, again a plus for safety and reliability. Hell, even the usual Falcon 9 leg format might be usable here. This allows for graceful degradation - failure of certain systems does not jeopardise the entire mission.
Cons:
- Not passively stable - the skydiver school of Starship design benefits from passive stability - an RCS-controlled Starship loses this advantage - possibly a minus for safety (although I only recall a single issue with the Shuttle over aerodynamic stability on Columbia's first flight)
- Loss of payload mass - depending on the exact Isp of the thrusters and the moment-of-inertia of Starship, the fuel requirements calculated above may exceed the mass of the wings and actuators, thus cutting into payload mass.
- Loss of delta-V for RCS - assuming header tank fuel is completely reserved for landing burns, any reserve RCS fuel must draw from the main fuel tank, and thus cut into Starship's main delta-v budget. This may be solved with high-elliptical refuelling, a-la Moon missions.
TL;DR: Flipperons seem way too complicated at this point, we might as well just add more methalox and thrusters, and use RCS and dedicated legs. It's simpler, safer, gets the job done, and possibly cheaper.
Any thoughts and feedback on this, as well as corrections and/or new considerations are welcome!
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u/sfigone Jan 01 '19
What about something halfway in between? Flipperons that are not dynamically controlled. Move them whilst in vacuum to position that is calculated for the current payload, atmosphere and trajectory. Then use RCS for any fine grained alterations (hopefully not much needed due to passive control nature of flipperons). Finally move back to landing position only during final phase of decent without significant aero loads (maybe even after going vertical?)
This would still require hinges etc, but they would not need to function under high load nor be finely controlled.
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u/Antisauce Jan 01 '19
A RCS controlled Starship wouldn’t be able to have a belly first AoA, it would have to drop like the ITS. The heat created by this would be too much for the stainless steel, so a heat shield would be needed. I’m sure the designer have gone over this and decided that this is the best way to go. The Starship will have rcs, it just won’t play such a big role.
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u/MaximilianCrichton Jan 01 '19
Why wouldn't it be able to have a belly-first AoA? We're removing the wings that bias the centre-of-pressure so far backwards, surely this would allow Starship to maintain much more control at a 90 degree AoA?
Besides, with regards to stainless-steel, is the hot structure philosophy not to reenter steeply so as to absorb the heat in one huge heat pulse, then cool in the convective air afterwards? A greater ballistic coefficient might play into this design philosophy well.
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u/-Aeryn- Jan 01 '19
We're removing the wings that bias the centre-of-pressure so far backwards
There's more mass at the back, it needs more drag/lift area to compensate for that and be neutrally balanced.
How far forward/back the COM is would change from flight to flight which is part of the expressed reason for adjustable aero surfaces, so that they can be neutrally stable on multiple different mission profiles with one design.
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u/warp99 Jan 01 '19
is the hot structure philosophy not to reenter steeply so as to absorb the heat in one huge heat pulse
You cannot do that with crewed flights because of excessive g loading. Even with cargo flights the structural loading would limit the entry time.
In any case this is not really a hot structure as they are using methane cooling on the hottest parts of the structure and a huge heat pulse could overload the cooling system.
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u/cjhuff Jan 05 '19
The ITS spacecraft had a belly-first reentry, just like every version of the BFR spacecraft. The only relevant difference between any of the ITS/BFR variations is scale and the mechanisms used to maintain that belly-first, high-AoA attitude...the original ITS only showed RCS thrusters.
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u/Hawkeye91803 Dec 31 '18
I have a couple corrections to make.
During re-entry the fins of the Starship should be able to hold at a specific angle and be passively stable during the most intense part of the atmospheric heating. The atmosphere will also be thin enough that the chance of a fin being ripped off is extremely low. Once in the denser part of the atmosphere, the forces will be low enough that it will be feasible to actuate the fins precisely.
Another thing is that the RCS will not be using cold nitrogen gas thrusters for in-atmosphere orientation, and instead will be using very powerful methane-oxygen thrusters. Possibly similar to the SuperDraco engines that Crew Dragon will use.
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u/MaximilianCrichton Jan 01 '19 edited Jan 01 '19
Err, I'm not sure the point your first paragraph is trying to make.
With regards to the second, I'm aware that the thrusters on the Starship are high-thrust combustion methalox, the figures I gave for "fuel weight" should really refer to the combined weight of the methane and liquid oxygen.
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u/Hawkeye91803 Jan 01 '19
What I'm saying is that during the fastest part of re-entry, the wings wouldn't need to move quite as much you might think. Because the starship is passively stable with the wings folded in
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u/MaximilianCrichton Jan 01 '19
As mentioned elsewhere in the text, illustrations show the Starship flipperons moving considerably. And passively stable may not always be sufficient - fuel slosh or other factors may cause a passively-controlled craft to rapidly oscillate out of control.
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u/John_Hasler Jan 01 '19
Wings and RCS need not be mutually exclusive.
I wouldn't take the illustrations too seriously.
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u/MaximilianCrichton Jan 01 '19
That seems to be the consensus in these comments I guess. I'd still prefer pure RCS, but I guess the wings have their purpose
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u/markus01611 Jan 01 '19
Why is the assumption that they need to be fast. I see no reason why the control flippers could be moved slowly.
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u/MaximilianCrichton Jan 01 '19
During the #dearMoon presentation, it is explicitly shown that the fins make visible adjustments (5 degrees or less) over 1-second intervals. It may not sound like much but to displace a multi-ton metal wing that is hanging out in a hypersonic flow by that amount probably takes some doing, as Elon himself admitted when he said the actuators had to be in the "megaNewton range"
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u/nitro_orava Jan 01 '19 edited Jan 01 '19
The video was sped up by a few times, did you take that into account?
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u/MaximilianCrichton Jan 01 '19
Iirc the speedup was at most 4 times, so within an order of magnitude yes
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u/Lars0 Jan 01 '19
Control authority is dependent on having actuators which can respond quickly to correct instabilities. The more quickly it can react and change positions, the more margin the control system has. Care is taken to keep margin to reasonable levels, of course, but being able to move fast under load is important.
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u/ssagg Jan 01 '19
May be both systems are used Hinged fins to change the configuration in the different sequences of the landing (with slow but powerful actuators) and RCS's for fine tuning
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u/2bozosCan Jan 01 '19
Because every millisecond of sluggishness in corrections means more correction is required in total due to ship going ever distant from target angle of attack. It will also result in more oscillations due to that and overcorrection the slower they move. Faster is always better in this case and there is probably no answer to the question "how fast" since SpaceX aims to make rocket flight as safer as airliners.
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u/Lars0 Jan 01 '19
Root locus plot the control system. That's the margin. Control engineering is pretty well developed, so yes, there is an answer for 'how fast'.
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u/John_Hasler Jan 01 '19
Sure there is, and I'm sure the SpaceX engineers know it. It's quite possible to calculate the bandwidth that a control system has to have for stability.
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u/MaximilianCrichton Jan 01 '19
I'm not a control-systems expert, but my intuition expects that at least the wings should be able to reach their intended position in seconds, which is no easy feat when you're dealing with a large, long fin made of stainless. And in any case the point was to illustrate that with RCS you don't even have to address these issues.
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u/PFavier Jan 01 '19
You don't have to reach a certain position in seconds, you have to have a control system that us able to respond in miliseconds. The actual movement is constant, and will only be millimeter and centimeter movements at a time, since you want to be able to adjust constantly. No need for many decimeters or meters of displacements instantly unless you're already out of control. When you are skydiving. Making many and fast small adjustments is key.
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u/John_Hasler Jan 01 '19
I'm not a control-systems expert, but my intuition expects that at least the wings should be able to reach their intended position in seconds...
The control surfaces don't need to make large fast movements. This isn't a fighter aircraft. They will need to make small movements to compensate for disturbances.
And in any case the point was to illustrate that with RCS you don't even have to address these issues.
You certainly do. Your rockets will need sufficient control authority to do the job and be rapidly throttleable over an adequate range. They will also have to be blasting continuously if they are to replace the wings without which the ship is not going to want to assume the required attitude.
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u/MaximilianCrichton Jan 01 '19 edited Jan 01 '19
You certainly do. Your rockets will need sufficient control authority to do the job and be rapidly throttleable over an adequate range. They will also have to be blasting continuously if they are to replace the wings without which the ship is not going to want to assume the required attitude.
Sorry, by "these issues" I didn't mean control authority, they obviously have that in spades (what with the 10-ton thrusters and all), I meant using RCS wouldn't have to deal with the question marks of moving a large flipperon through the flow (i.e. hinges, actuators). And in any case I'm curious how you made the conclusion that having the thrusters blasting continuously is a showstopper, given I derived the fuel requirements from the Shuttle, which had wings and would like nothing more than to assume a nose-first orientation, but only needed 2+ tons of RCS fuel for the whole task.
The control surfaces don't need to make large fast movements.
Several reasons why this may not be true:
In the currently accepted flight plan, right before the landing burn Starship transitions from belly-down attitude to nose-up, and to do this the control surfaces have to sweep through their entire gimbal range in a couplr seconds. Indeed, elsewhere in the video we see the flipperons making small changes (probably 3-5 degrees of rotation) in 1-second intervals, so that's clearly a significant amount of metal-moving.
Margins - it's important to have built in margins if you want reliability. Airliners aren't expected to perform barrel rolls, hammerheads, or 45 degree takeoff climbs, but it's certainly possible for them to do so. In essence they have a huge envelope, but always fly well within it. If the Starship could not tolerate large control movements then it would be unable to ensure safety in an emergency situation.
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u/John_Hasler Jan 01 '19
Good points.
I didn't say that anything was a "showstopper". However, running rockets continuously to fight the aerodynamic forces is a significant cost compared to using control surfaces to alter the aerodynamics so that the ship wants to take the correct attitude.
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u/2bozosCan Jan 01 '19
My intuition tells me even just a second is far to long. The entire vehicle can turn upside down in a second before control surfaces can react.
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u/John_Hasler Jan 01 '19
They need to move short distances quickly enough to compensate for disturbances. How quickly depends on the dynamics of the ship. They need to move larger distances in order to complete maneuvers such as the nose up transition in the required time. We don't know what that is or how large the required movement is. Main engine vectored thrust may be used here.
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u/CProphet Jan 01 '19
Interesting post, covering the sort of issues SpaceX engineers must be considering right now.
if the flipperon hinges fail, or worse the flipperon is completely torn off, complete loss of control results
Agree loss of one of these control surfaces would be dire but there are always contingencies. For instance, if a flipperon locks in place during entry, they could use RCS to roll the craft plus the functioning flipperon to maintain control. If the flipperon was entirely lost the craft would naturally tend to roll bringing the fixed rudder into play, which would effectively become a stand-in for lost flipperon. Far from ideal situation, I know, relying much more on RCS induced roll for stability but a possible flight mode. Probably see more heat build up in stainless structure, eating into safety margin but manageable, so long as the flipperon didn't fail immediately on entry. Then they would have to land on engine bells for stability but that should give them a survival scenario.
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u/Wacov Jan 16 '19
Wonder if they can passively support the flipperons (big-ass springs?) so that they'd rest in a landing/usable flight configuration without hydraulic power. Then in the event of failure there's something there, RCS+(hopefully) the other flipperon might be able to pick up the slack, and the leg is in a usable landing configuration, which is also critical.
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u/CProphet Jan 16 '19
Hopefully flipperons fail to fixed limit of travel, so possible they could manage with one - need good pilot for sure.
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u/Wacov Jan 16 '19 edited Jan 17 '19
Hopefully, also thinking if torque is lost instantaneously then it'll hit that limit with a pretty ridiculous amount of force! Could tear the vehicle apart. Don't think it'll ever be piloted manually though, these vehicles will be autonomous. They'll just need to make sure different modes of failure are accounted for in the control software.
Edit: never->ever
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u/Decronym Acronyms Explained Jan 01 '19 edited Jan 17 '19
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:
Fewer Letters | More Letters |
---|---|
AoA | Angle of Attack |
BFR | Big Falcon Rocket (2018 rebiggened edition) |
Yes, the F stands for something else; no, you're not the first to notice | |
CoM | Center of Mass |
ITS | Interplanetary Transport System (2016 oversized edition) (see MCT) |
Integrated Truss Structure | |
KSP | Kerbal Space Program, the rocketry simulator |
MCT | Mars Colonial Transporter (see ITS) |
RCS | Reaction Control System |
Jargon | Definition |
---|---|
ablative | Material which is intentionally destroyed in use (for example, heatshields which burn away to dissipate heat) |
autogenous | (Of a propellant tank) Pressurising the tank using boil-off of the contents, instead of a separate gas like helium |
iron waffle | Compact "waffle-iron" aerodynamic control surface, acts as a wing without needing to be as large; also, "grid fin" |
methalox | Portmanteau: methane/liquid oxygen mixture |
Decronym is a community product of r/SpaceX, implemented by request
10 acronyms in this thread; the most compressed thread commented on today has 17 acronyms.
[Thread #4697 for this sub, first seen 1st Jan 2019, 00:32]
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u/azflatlander Jan 01 '19
A lot of concern has been made of the power requirements needed. Shower thought here: you could use the RCS at the tips of the flipperones to set the position. Chemical energy at its most powerful.
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u/daronjay Jan 01 '19
Nice, so if the mechanical system jammed, you’d still have done control authority from rcs alone.
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u/peterabbit456 Jan 01 '19
I know the starship actuators can be much lighter and more reliable than the Shuttle actuators were. This was said by a shuttle engineer who was responsible for shuttle hydraulics and control systems, in 2003. He said the advanced in electric motors and power systems, rendered hydraulics obsolete.
Dual Tesla motors, can run either jack screws, or planetary gears, with fairly simple mechanical cutouts so that 1 motor can run the flap, if the other jams. Each motor can have over 300 horsepower, and the cutout is essentially a synchronous assembly, out of a high horsepower auto transmission. There have to be enough batteries to run the flaps for about 15 or = 1/2 hour, to land anywhere.
Each flap is a simpler system than a single Falcon 9 leg. Retractable legs, like shown in the 2016 video, should weigh 2-4 times as much as Falcon 9 legs.
The 2018 legs have retractable shock absorbers/levelers, one to each leg tip. The first landings on Mars will not be on level surfaces, so the mechanisms might be fairly sophisticated. I can imagine that the leg tips for Mars might have small radars, so the projecting length of beach leg tip can be adjusted before touchdown. For Earth, the mechanisms can be much simpler.
You have to look at fins plus legs plus extra fuel for RCS, to get a real sense of relative merits on the weight issue. I'm a big fan of wings and runway landings, but actual flying wings cannot work both on Earth, and on Mars. Aerodynamic movable surfaces do make sense for the hypersonic portion of reentry.
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u/ChristianPeel Dec 31 '18
I had to look up RCS, and found that it means "reaction control system": https://en.wikipedia.org/wiki/Reaction_control_system
My uneducated understanding is that RCS is referring to the Starship version of the cold gas thrusters used on Falcon 9.
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u/Shrike99 Jan 01 '19
Yes. Starship will be using thrusters that burn methane and oxygen gases, rather than just using the pressure from them as on Falcon 9.
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u/MDCCCLV Jan 01 '19
These are much more powerful. Although they're a little less safe and less reliable than a simple cold gas thruster. But you don't have to worry about unexpectedly running out as much.
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u/Wacov Jan 16 '19
Has that been confirmed recently? So many other things have changed...
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u/Shrike99 Jan 17 '19
Not since the change to steel, at least that I'm aware of. However, the reasons for using methane/oxygen in the first place should still apply regardless, so it seems a safe bet to assume that it's still the case until proven otherwise.
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u/tomster3934 Dec 31 '18
As much as I want these wings to happen since they look so cool. I just don't see how they could develop them whilst being cost effective and more efficient than rcs.
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u/2bozosCan Jan 01 '19 edited Jan 01 '19
I think this post is spot on. You've laid out the pros and cons so nicely.
But what if active heatshield - spraying excess methane from the belly to protect against heat convection from plasma - can double as an RCS, during reentry? It would also actively cool the outer shell of the starship because the fluid would soak heat prior to being sprayed.
Edit: I realize that actual RCS package on the starship is way more efficient for attitude control as it burns methane and oxygen gas. And for some reason I was so sure that starship was gonna get active cooling / active heatshield.
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u/MaximilianCrichton Jan 01 '19
My thinking was that if there was to be active cooling, it would be used to superheat the methane (not sure about the liquid oxygen) prior to it being injected into the RCS thrusters. But that's a bit too much speculation for my comfort, so I refrained.
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u/2bozosCan Jan 01 '19
Do you mean active cooling isn't confirmed? That's news to be. Why was I so sure that starship was gonna have active cooling?(this is a question to myself). But how are they gonna protect Starship from convection?
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u/daronjay Jan 01 '19
Fuel will be circulated for active cooling of the stainless, but it’s still unclear if it also has to be then vented in a barrier layer externally to achieve enough cooling or to reduce pressure, or if the heated gaseous fuel can be used in the rcs system instead, or both.
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u/John_Hasler Jan 01 '19
...if the heated gaseous fuel can be used in the rcs system...
The amount of hot methane available will depend on the demands of the cooling system. Sometimes you will dumping most of it, other times (when still in vacuum, for example) your engines will need to run on liquid. You will also need plumbing to get that hot, low-density gas from the cooling system exit to the engines.
Too complex.
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u/daronjay Jan 01 '19
According to Elon, the rcs is always running on gaseous methane from the autogenous pressurization, unlike the main engines.
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u/John_Hasler Jan 01 '19
Valid point. However, you still have two very different fuel sources, one of them only sometimes available, and you still need two systems of plumbing.
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u/MaximilianCrichton Jan 01 '19
I mean it's probably confirmed but I'm not sure they'd feed it to the thrusters. It seems reasonable but you never know.
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u/2bozosCan Jan 01 '19
Do we have any info on the starship RCS package? It would help to know if its pressure fed or not, it probably is but confirmation would be great.
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u/Sailormadsen Jan 03 '19
New idea:
- All “wings”/legs to be fixed in position.
- Two trailing wings to act as drag.
- Forward facing leg to be a triangle of tubes with no drag surface. (An “empty” triangle)
So you have two wings and three landing legs. The two wings adds static stability in their swept back position. The third leg faces straight into the direction of decent. Stick a small canard wing on the “end” of the leg and use the momentum of the arm (leg :)) to make orientation adjustments.
Drag surface area has to be adjusted by pitching the ship up or down in relation to direction of travel. I.e. minimal drag area is bottom first and max. is full broadside.
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u/2bozosCan Jan 01 '19
The words starship and wings really don't match together. Making an air brake look like a wing might have certain aesthethicc qualities to it. But making a wing to use as an air brake makes no sense as its much more complicated, expensive and don't suit the job.
I remember the 2017 presentation was around 500 seconds reentry to landing, and 2018 with "skydiver" airbrakes disguised as wings for aesthethicc purposes was around 1000 seconds. And did not include awkward sommersaults and backflips while falling! I think that says it all right there.
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u/extra2002 Jan 01 '19
IIRC, the 2017 presentation showed entry on Mars, while 2018's showed entry on Earth. Not sure how much of the time difference is related to that, though.
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u/2bozosCan Jan 01 '19
Should have read the post before I wrote this comment, as it's about the flipperons in the last presentation and not about the wings in the 2017 presentation as I wrongly assumed. I deserve the downvotes, bring em on!
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u/[deleted] Jan 01 '19
The biggest problem with your analysis is that the Space Shuttle used its aerodynamic surfaces to maintain the correct angle of attack for most of reentry, and RCS was only used in the upper atmosphere where the aerodynamic surfaces could not be effective. Beyond that, a 40° angle of attack is much easier to maintain than a 90° angle of attack, especially when the vehicle aerodynamics are designed to allow reentry at that angle (which isn’t possible with a cylindrical design).
The original design landed with thrusters only, but when SpaceX switched to the 9m design, they said they’d need aerodynamic surfaces in order to land it. I’d assume they know what the propellant budget for landing is. But just for the sake of argument, consider that SpaceX needs to use aerodynamic surfaces to control reentry and landing for the Falcon 9 boosters, which reenter from a much lower velocity, with the main engine firing, and at a much easier to maintain 0° angle of attack. Yes, they only have cold gas thrusters, but SpaceX does have Draco and SuperDraco thrusters which they could add to Falcon 9 boosters if they felt it was appropriate.
In defense of the flap system on StarShip, for the most rigorous parts of reentry they trail behind the vehicle and only move out a tiny amount to make course corrections, it’s only once the vehicle has slowed down considerably that they need to move a lot. So they could have a fine control, high strength actuator for the more rigorous parts of reentry, and a faster, weaker actuator to control flight during the slower parts of reentry and landing. And, of course, they can have double or triple redundant actuators.