Gemini: We can rebuild it, we have the technology

[mrmorris]

Well -- the report at Spaceref that I referenced has good news and bad news about the landing of an Apollo-style CM. <br /><br /><b>The Good News:</b><br />They note that the landing accuracy was great: <i>"The landing accuracy was very high, however and resulted in a landing dispersion of approximately one mile from the target point. "</i>. As I've mentioned before (though in reference to Gemini), the computing power and positioning systems of the day are relics compared to what is available OTS today. It's inconceivable that modern electronics would not reduce the landing error by 50%. I would expect an error reduction on the order of 80-90% -- generating a landing dispersion of around 500-1000 feet.<br /><br /><b>The Bad News:</b><br /><i>"Given that the propulsion module will follow the CM CRV in an entry trajectory of its own, not far behind the CMCRV, the water landing sites would have to be selected on the West coast of land masses, or near islands. "</i> Oops. I wasn't actually thinking about any of the DO module making it all the way to the ground. My mental picture was of it burning up completely on re-entry, seeing as it has no heat shielding. Mind you I was planning on a west coast landing -- namely Mojave Airport. However -- they don't say just <b>how</b> far behind the CM those debris might be. To say that California is densely populated is a bit of an understatement...<br /><br />It should be possible to do a couple of things reduce the possibility of this being an issue. The flight path in can be made to use the L/D in 'maximum lift' to extend the flight path of the CM as much as possible. Unfortunately, this simultaneously reduces its capability to 'make up ground' if there's any shortfall on the re-entry burn.<br /><br />It's also possible (in fact desireable) to make the de-orbit framework/etc. in such a fashion as to maximize the degreee to which it will burn up on re-

 

[najab]

Also, make sure that it's tumbling as it comes in, so that it doesn't maintain a stable attitude.

 

[mrmorris]

<font color="yellow">"...make sure that it's tumbling ..."</font><br /><br />With a light and non-aerodynamic shape -- I don't think we'd have to worry <b>too</b> much about tumbling occurring. But yes -- this helps too.<br /><br />One thing I did forget to mention about the use of composites, though, is that having the structure being extremely lightweight means that it will decelerate due to atmospheric friction <b>much</b> faster than the CM itself -- putting it that much further behind the flightpath if debris *do* make it to the ground.

 

[mrmorris]

The following statement from the ISS-Apollo-CRV study closes a space-time loop:<br /><br /><i>"A 15 psi internal environment based on stored high pressure 02 or missed gas. Redundant flash evaporators would provide internal thermal control, but at the penalty of substantial water storage."</i><br /><br />Namely it makes me re-think about the possibility of using a fuel cell (i.e. what I proposed in the first post of this thread). The main reason I got rid of the fuel cells is because I didn't want to have to store hydrogen, and there was no need for water (for drinking) since the trips were intended to be so short. However, if I need water for the flash-evaporator <b>anyway</b> -- then the fuel cell starts to look attractive again. Especially because the Ballard Nexa specifications are so sweet. Top-end output of 1.2kW (about six times whatI calculate for the top-end power requirement) means that there's power to spare and the FC can generally run at very low consumption levels. The water generation can be used for both flash evaporation and drinking. It expands the capabilities for staying on-orbit which expands the ability to be precise in selecting the landing site. Comparing mass and volume against the use of batteries is impossible without further data, but shouldn't be *too* far off. <br /><br />*Edit* We can use the sublimator assembly Hamilton Sundstrand developed for the X-38 (along with the water created by the fuel cell) to eliminate excess heat from G-X3.<br /><br />Envelope: 5.5 in x 8.0 in x 14.5 in <br />Weight: 20.5 lb <br />Heat Rejection Capacity: 35,000 Btu/hr max <br />Coolant Fluid: Water <br />Coolant Pressure: 0-200 psig Flow Rate: 0-600 lb/hr <br />Coolant Water Pressure Drop = 2 psid max at 500 lb/hr

 

[rancamp]

Out of curiosity, what are the chances, (how much of a penalty) would there be to incorperating the de-orbit thrust into the capsule itself?<br />That would eliminate the de-orbit stage and the problem with it following the capsule down.<br /><br />If we leave the orbital stage at the 'station' and just use internal thrusters, it might actually work out better all the way around.<br /><br />There is also the possibility of using something similar to the Mercury 'strap-on' solid pack only with a set of hybrid rockets. <br /><br />Thinking outloud again <img src="/images/icons/blush.gif" />)<br /><br />Randy

 

[ve7rkt]

Gemini's deorbit module is pretty much Mercury's 'strap-on' solid pack, with some airframe wrapped around it to serve as an adapter between other modules (right down to the straps; Gemini's deorbit module was held on with three metal bands cut by explosives). The G-X3 deorbit module would be pretty similar, I think.<br /><br />I would choose solids over hybrids for the deorbit module. I'm not that familiar with 'real' hybrids, but the ones used in high power model rocketry have lousy Isp compared to solids, they're much harder to get lit (that's bad enough on the ground), and they also work in a narrow temperature band (on a hot summer day the oxidizer is all gas and no liquid; on a winter day... something... I forget... I fly solids!) I guess 'real' hybrid rocket motor manufacturers have some of this sorted out, after all, SpaceShip One was hybrid powered, but solids work.

 

[mrmorris]

<font color="yellow">"...incorperating the de-orbit thrust into the capsule itself..."</font><br /><br />There's a few problems.<br /><br />1. Internal space is already at a premium. While I can see increasing the CM size by 3-7% for other reasons (namely to provide more internal volume for pasengers/cargo) -- I'd prefer not to do it for propellant/thrusters.<br /><br />2. The additional size/weight of internalizing the DO thrusters makes the landing impact that much harder to attenuate.<br /><br />3. SRMs as a DO booster are *very* reliable, as opposed to either internal liquids or external hybrids. I think I've mentioned hybrid earlier in the thread (at the very least I *thought* of them. I like them for the additional control available -- but reliability and simplicity trump that for G-X3.<br /><br />4. Where do the nozzles for an internalized DO booster go? I know this discussion was thrashed out early in the thread. You have a choice of either poking holes in your heat shield or having nozzles that hang over the edge of the capsule base. Neither presents a pretty picture.<br /><br />Overall -- I think the DO thrust needs to come either from a strap-on solid or from a 'Service Module' equivalent.

 

[ve7rkt]

<font color="yellow">4. Where do the nozzles for an internalized DO booster go? I know this discussion was thrashed out early in the thread. You have a choice of either poking holes in your heat shield or having nozzles that hang over the edge of the capsule base. Neither presents a pretty picture.</font><br /><br />I like the idea of the disposable DO pack, but just for the sake of considering all options, if you wanted to internalize the DO motors, they could be pointed out the pointy end (the nose on launch, the tail on reentry). They could be pointed sideways if you really wanted, so long as they were directed at the craft's center of gravity, but that's just getting silly. If the RMS doesn't have the oomph required to flip the craft around between DO burn time and atmospheric entry, we had issues anyhow.<br /><br />Doesn't solve issues 1 & 2, though.

 

[najab]

>...<i>if you wanted to internalize the DO motors, they could be pointed out the pointy end...</i><p>Which makes a fully functional RCS a <b>requirement</b> for reentry.</p>

 

[tap_sa]

Whether DO system is solid, liquid or tennis ball cannon, a working RCS is a requirement.

 

[ve7rkt]

It's far better to have working RCS than not, but it wouldn't be absolutely essential. Think of a lawn dart. Its center of pressure is back in its fins, its center of gravity up in the tip. If there is air moving around it, it WILL travel through that air heavy-end-first.<br /><br />As an example, Soyuz 5 failed to separate its service module after the DO burn. The CP was back in the service module, the CG was up front, so when it started biting air, it re-entered HATCH FIRST. Thankfully the module straps were weaker than the hatch seals, the straps burned through, the service module finally let go, and the re-entry module flipped around, heat shield first, because that was the new stable position. Now, it wasn't a happy day, because he was way off track, made a 9g reentry, the landing rockets failed, he broke his teeth, went to a state celebration, and was shot at in an assassination attempt. And yet the pilot survived all that, and flew again. <br /><br />There's no way to adjust your reentry path without RCS though, so you could end up anywhere in a very large footprint.

 

[mrmorris]

<font color="yellow">"...the landing rockets failed, he broke his teeth, went to a state celebration, and was shot at in an assassination attempt..."</font><br /><br />Note to self: Add mouth-protectors and bulletproof vests to G-X3 list of standard equipment.

 

[tap_sa]

Have Volvo design airbags into the cabin to protect the passengers from all impact angles <img src="/images/icons/smile.gif" /><br /><br />ve7rkt: After de-orbit burn you can trust aerodynamics to do attitude control but the burn itself requires RCS to orient yourself correctly. OK you <i>might</i> succeed without it if your de-orbit motors have thrust vector control or you wait till you freely rotate about the right way and try your luck. AFAIK Mercury and Gemini deorbit solids didn't have TVC.

 

[najab]

As ve7rkt said, it's real nice to have RCS (meaning reaction control system - don't you just love the way that the Shuttle RCS is the Gemini OMS and vice-versa <img src="/images/icons/smile.gif" />), but in a pinch, after <b>everything</b> has gone wrong, with the DO system (liquid or solid) beneath the capsule you just have to get the heatshield pointed into the velocity vector and get it to ignite.<p>With the DO nozzles in the top of the capsule, you have to be able to do the deorbit burn and then pitch the craft through 180 degrees before you start getting reentry heating.</p>

 

[ve7rkt]

<font color="yellow">but the burn itself requires RCS to orient yourself correctly</font><br /><br />Um... well, yes, there is that... <img src="/images/icons/laugh.gif" /> <br /><br />The capsule will reorient itself heatshield-first when it kisses air, long before it gets warm... but yeah, before that happens you do need to be able to point the DO motors the right way to make that burn, and that requires RCS. You're absolutely right.

 

[najab]

Worse come to worst you can open the capsule door and fart in the appropriate direction.<br /><br />(Best read with a fake French accent.)

 

[mrmorris]

<font color="yellow">"...open the capsule door and fart..."</font><br /><br />I think we can do a bit better than that if we think outside the box (hopefully while keeping the rest of us safely <b>inside</b> the box). <br /><br />How about a propellantless RCS system? No -- I'm not going all Unitel on you -- just suggesting that G-X3 is small enough that we might consider the use of... wait for it... reaction wheels. There's not a nice way of having three reaction wheels on G-X3 -- but I can see the potential for adding two. They should provide (no guarantees <b>at all</b> that I'm getting p/y/r desingations right) roll and yaw control. To get pitch control -- the other two can be used in combination (i.e. roll 90 degrees then yaw). <br /><br />The CM might well have to be scaled up just a bit for this -- they're certainly not *small* by any means. However, there's already been consideration of scaling the CM up by 3-7% anyway for additional cabin space -- this would also make plenty of room for the wheels. In addition, if they are constructed with the mass primarily at the outer edge -- they should provide an extremely capable maneuvering capability for a very reasonable weight penalty. Looking at it in terms of failure-proofing -- the wheels will be more reliable than any thruster-based RCS from the get-go. To add an extreme backup -- the same 'pedal-power' system I suggested as an electrical backup can also be used to drive the reaction wheels if *everything* goes haywire (i.e. batteries/fuel cells die or electric motors for the wheels fail). At that point -- the only failure point to initiate a de-orbit would be the SRMs themselves.

 

[najab]

Those reaction wheels are *huge*!!! I guess they wouldn't have to spin at a great rate of turns to get a decent amount of torque.

 

[mrmorris]

<font color="yellow">"Those reaction wheels are *huge*!!! "</font><br /><br />I wasn't setting a particular size so much as I was showing the largest diameter that's possible given the space available. However, the most efficient r/w possible is going to be one with the maximum diameter for which there is space, and that has the mass concentrated along the outer rim.<br /><br />No comments on the concept itself though? I've never seen any reference to using RWs for a craft like this. Either I'm a moron because there's a very good reason why this isn't even considered -- or it's a breakthrough concept and IR brilliant. I give it even odds either way. <img src="/images/icons/smile.gif" />

 

[ve7rkt]

Possible drawbacks to the reaction wheel plan... wouldn't they have to remain spun up the whole time while the capsule is parked in order to be usefulf or emergency evacuation? That would require tending them, there's some kind of procedure that has to be done to a reaction wheel to get rid of extra energy, I don't understand it very well. Cold gas thrusters are ready from T+1, just open a valve. <br /><br />Interesting note: LockMart's CEV plan is out, lifting body with the dock in the aft, looking very much like the Russian Kliper drawn by American artists. The diagram shows ethanol/LOX tanks near the aft RCS sets, and NO2 in the nose. I don't see any evidence of an OMS motor, so my guess is they plan similarly to us, use another module for the deorbit burn and ditch it before hitting air.<br /><br />One thing I like about G-X3 is that two G-X3s could dock. With the RCS clusters and body flap where they are, I don't think two of these CEVs could connect.

 

[mrmorris]

OK -- carrying this idea a bit further down the path...<br /><br />1. Since the RWCS can provide all of the orientation changes for GX-3 (in particular -- the oversize RW at the rear should be great for rolling on re-entry) -- let's eliminate the thrusters from the CM entirely. The strap-on de-orbit module will be enhanced with tanks and thrusters to provide forward/reverse dv for the CM. The degree of dv needed is dependant on exactly when the second stage of Falcon-V exits the scene -- as described in an earlier post. Depending on when it separates -- G-X3 might need no thrusters at all, or a minimal cold-gas system, or a very capable OMS. One of the great things about the OMS being on the strap-on DO frame is that <b>all</b> of these are possible. G-X3 need not be saddled with a single system that has to be everything and a bag of chips. The OMS system can be mission-dependent. In particular -- this means that the early version can use a hydrazine system -- then upgrade to a better technology at a later date.<br /><br />2. The RWCS is going to use additional electrical power, so I initially thought this is further evidence that I need to use fuel cell power in lieu of batteries. However -- I was thinking (my wife knows this is a bad sign)... how could the mass of the reaction wheels actually <b>be</b> a component of G-X3 itself (i.e. instead of simply being dead weight). Most of the thoughts I had weren't workable. You wouldn't want to spin your electrical equipment around. Spinning propellant or LOX/LH tanks is contraindicated. However, I thought: "What about the batteries?" Initially I thought about using the main batteries for the craft, but I decided against that -- mainly to be conservative. However -- what if each of the wheels had the batteries that would act as <b>their</b> primary power source distributed about the circumference of the wheel. "Batteries" are actually a collection of "cells" -- banks of which are connected in series to get

 

[mrmorris]

<font color="yellow">"Possible drawbacks to the reaction wheel plan... wouldn't they have to remain spun up..."</font><br /><br />You're confusing reaction wheels with flywheels. <br /><br />- Flywheels are a source of potential energy -- spin them up and then pull power out of them over time.<br /><br />- Reaction wheels are a source of action-reaction energy. If the RW is motionless and you spin it clockwise -- the craft itself will experience a counter-clockwise force... and vice-versa. It's a common means for making propellantless orientation changes in satellites.

 

[ve7rkt]

The RWCS is not going to be an off-the-shelf component then... so make the hub the motor. Take an electric motor, tape the batteries to its casing, and mount it to the wall by the shaft instead of by the case. You'll still need brushes or something similar to pass the speed up / slow down commands, and to recharge the batteries. The extra mass of the motor isn't going to add much to the effect of your reaction wheel because it's right at the center, but it should make the whole system a touch more compact.

 

[ve7rkt]

<font color="yellow">You're confusing reaction wheels with flywheels. </font><br />Entirely possible! <img src="/images/icons/smile.gif" /> I might also be confusing it with the gyros that keep failing on the ISS.<br /><br />In that case, correcting my post above, make the hub of the wheel a stepper motor.<br /><br />Will a system like this be capable of rolling the craft enough when it's actually in the atmosphere? If we're still going Gemini-style for reentry, the roll angle is how you steer to a particular landing point, right?

 

[mrmorris]

<font color="yellow">"...confusing it with the gyros..."</font><br /><br />Yes -- Gyros are a third device that makes use of rotational velocity. Gyros are in some senses the antithesis of reaction wheels, though. They make use of the fact that once an object is spinning -- it resists changes in orientation at right-angles to the line of the axis of rotation. So reaction wheels are used to create motion and gyros used are to prevent it. As with flywheels -- gyros must be spinning to function.<br /><br /><font color="yellow">"... be capable of rolling..."</font><br /><br />It's a matter of practical rather than capable. Yes -- a reaction wheel could generate sufficient energy to do this. The amount of force of a reaction wheel is dependent on its mass, diameter and rotational speed. It is certainly possible to make a flywheel with a combination of mass, size and speed to perform these maneuvers. What is *not* certain (and I don't really have a means of performing the calculations) is what those numbers will add up to. It may very well be that an <b>impractical</b> combination of weight, size, and motor strength is required. I don't believe that to be the case, though.<br /><br /><font color="yellow">"...the roll angle is how you steer..."</font><br /><br />Correct.