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I rather doubt that aerocapture at Mars, mentioned as baseline in the plan quoted in the previous post, will ever prove very practical. That is because the upper atmosphere densities at Mars vary through a factor near 4 erratically. There are some figures for how variable that atmosphere really is, in the Justus and Braun EDL paper. They recommend using the MarsGram models of the Martian atmosphere, and not the averages in their paper, when planning missions.
With that much variation, you never know if you will really see enough drag to actually aerocapture. Which in turn means you must be prepared to essentially substitute an immediate engine burn for an aerocapture that failed to capture. If you have to be prepared to burn anyway, then what's the point of attempting aerocapture in the first place?
Direct entry is not so sensitive to density variations, if (and only if) you can develop some hypersonic lift to modify your trajectory real-time as you come down. A part of the smaller sensitivity is diving deeper to slow far below orbital-class speeds. Down there, the density is not anywhere near so erratically variable. It is seasonally quite variable, but that is pretty much predictable (which is what the MarsGram models are all about).
The other issue is the stated need for ISRU propellant from Phobos or Deimos to make trips to the surface. I really, really doubt there are any volatile ices to be had on either moon. They resemble nothing so much as the dry, loose rubble-pile C-type asteroids we have seen.
The way around that is not during exploration or initial base-building. It comes later as the settlement grows to significant size, requiring lots of two-way trips from Earth for various purposes. You do the ISRU down on Mars and base the shuttle ships down there, making round trips single-stage to orbit and back. The 1-way unfactored dV to theoretically reach the moons is near Mars surface escape speed. To reach only low orbit, it is nearer Mars surface circular orbit speed, some factor 1.414 slower.
That last is what you must trade off against the cost of building your orbital base, as a structure on the moon, or as a free-flying complete space station in low orbit. That may not be only a tradeoff in terms of money. There are psychological things, and other things, to consider.
GW
My Dad took his 170 to Oshkosh one year. He and a friend camped under its wing. That's a long way from Texas at only 110 mph and with only a VOR, but he said he had a great time flying it there and back.
Before the 170, Dad took me from D-FW area to San Angelo in the old J-5. It cruised at only about 75 mph, and we fought an enormous headwind getting there. That was back in the early 1960's, and the J-5 had no lights and no radio. It did not have a starter or a battery. It was restricted to day VFR, and it was a hand-prop start.
Took all day plus a refueling stop to get there. But the return flight sure seemed fast! Massive tailwind.
GW
Bob:
I pretty much agree.
I think it may be more due to young, inexperienced engineers not knowing about the mistakes made by others elsewhere in the industry. Some ideas-to-try can be dismissed early-on, before cutting steel, if you have somebody on the team old enough and experienced enough to know about the mistakes of others. SpaceX hires no one older than 45.
You can still use tapped combustion gases, but by means of a heat exchanger, not direct injection. Although making it big enough to actually work may be too heavy. Which may well be why helium is still so popular.
GW
Thanks, Void, that was a very good video. Unless I am mistaken, it was made right before flight test 4 of Starship/Superheavy.
GW
I don't see much point to going to Phobos other than for scientific curiosity. Tom does think it will prove attractive, mostly seemingly "because it's there".
I don't know myself, but if Tom is right, the "because it's there" psychology would apply equally well to anything else built anywhere in some Mars orbit. Like some sort of space station.
I would suggest a station in a far lower orbit than Phobos, in order to reduce the dV for routine travel between it and the surface. There will have to be a lot more ships flying that route, than ships reaching the station from Earth, in order to transfer goods and people to and from the surface.
Just trying to look ahead, and trying to keep an open mind.
GW
In todays news: NASA has pushed back Starliner's return to an undetermined date in July. This is to give them more time to figure out what has been going wrong with the helium leaks and the thrusters. This particular story gave enough detail to pin down the faulty thrusters to those on the service module, not the capsule itself, as I first thought. Supposedly, helium supplies are sufficient to re-enter at any time. The problem seems to be not fully understanding the sources of the problems.
GW
I did not see on the website exactly how the carrier vehicle is positioned on the slinger. I rather doubt it's tangentially-oriented. Would make more structural sense to be radially oriented, except that does not orient correctly at release.
I once did some propellant grain designs for a small motor to be flown in a gas gun shock tunnel at about Mach 25. This was for both impact stuff and for signature adjustments of decoys flying along with entering warheads. Launch was somewhere in the 5000 gee class, if memory serves and it may not).
Those little motors were squat cans of an L/D a bit less than 1. The nozzle was mounted re-entrant into the center of the aft end. The propellant was a thick pancake shaped like a washer, about that nozzle, supported by the aft surface of the motor case, and by the case on its outer diameter, and by the nozzle on its inner diameter. Propellant does not have the strength to stand unsupported at such gee levels.
GW
Void:
I think you can capture samples of gases from a very low satellite, but these will essentially be closed containers of extremely low-density gas when you pull them aboard. Very tiny masses inside the containers.
I do not think you can capture such gases in quantity rates sufficient to use those gases for any kind of propulsion.
The density is just too low, and at orbital speeds, the compression capability of any imaginable inlet will be vanishingly low. The shocks are simply too strong to manage in any effective way. And there would be no materials that could withstand contact with such plasma and sill maintain a useful shape, and hot plasma it would be: crudely an effective 8000 K at 8 km/s velocities.
GW
There are always "studies" justifying why we cannot go. I've helped to debunk the cosmic ray excuse on these forums.
What the ISS has proven is that there's a whale of a lot more to microgravity diseases (yes, plural!) than the muscle and bone weakening we originally thought. It starts pretty quickly, and even with all the exercise gear, gets fairly threatening to health after something on the order of a year.
All that really says is that you do artificial spin gravity. Despite the design inconveniences.
It all gets down to a question of will. Do you, or do you not, want to go? If you do, we already know how:
Do artificial gravity and radiation protection from the giant solar flare events. Provide enough space for people to congregate, and to be alone. If you cannot do a closed ecology (and we still cannot, although we can do some pieces), just build it bigger and pack the stores. If you want to go, you will pay the price to do this.
Simple as that.
GW
About 3+ decades ago, there were war games being flown at Nellis to evaluate the F-15 being introduced back then. Because of the longer range radar, it was outflying the aggressor squadron using F-5's to simulate Migs. No radar warning receivers in the F-5.
American ingenuity always surfaces, even in pilots trained to think and behave like Russians. They got tired of losing in the war games. They went to Radio Shack and bought police radar warning receivers, plus the gear to jerry-rig them to aircraft power. This stuff was not large, and fit in the flight suit pockets along with rolls of duct tape.
Preflight inspections revealed no contraband, so once they were ready to taxi, the warning receivers were duct-taped to the glare panel and plugged into aircraft power. The games suddenly went lopsidedly in favor of the aggressor squadron, which really pissed of the generals, who for the longest time, could not figure out why. (Eventually the truth was revealed.)
The point is, you put a really good pilot into a just-barely-good airplane, and you still have a winner!
I agree with you, simpler is better and more reliable. Which is also more combat-capable.
GW
You lean the trajectory just a tad to offset the effect of the spinning Earth. That is what makes it go up, seemingly straight up, and straight back down. Lean it any further than that, and you incur two very serious problems: (1) lower apogee (mechanical energy must be conserved), and (2) your carrier vehicle comes down supersonic at some remote site.
The lean angle is not something designed into the prototype as adjustable. You would have to rotate the whole shell, there is only one small exit portal, covered with a thin diaphragm that the carrier vehicle bursts through. I'm not sure what the design apogee altitude is, but the prototype has yet to achieve "exit" from the atmosphere.
Even the notion of "exit" depends upon speed. At low to only supersonic speeds, the air is thin enough to ignore drag forces at only about 150,000 feet (roughly 45-50 km). At orbital-class speeds, you need to be above 140 km in order to ignore drag forces. That is why the "entry interface altitude" for Earth is set to 140 km (135 km at Mars).
Basically, the slinger "poots-out" the carrier vehicle at something near 5000 mph, which lets it coast up to an apogee somewhere above 90-100 km, hopefully. Hopefully more like above 140 km. The carrier vehicle opens up, freeing the payload and its 2-stage solid booster rocket. That rocket then takes the payload to speed in the desired direction. The carrier vehicle fall back for parachute recovery at the launch site.
I do foresee an extreme difficulty getting a two-stage solid rocket design with enough dV to reach orbital speed, fired from stationary at apogee. Because of the 10,000 gee exposure during spin, feasible solid propellant grain designs that could survive are extremely restricted, basically to flat pancakes at the bottom of a squat can. It will be quite difficult to pack enough propellant into each stage, under such restrictive circumstances.
I rather doubt the spin launch folks have yet looked at this solid design problem. They might be better off marketing cheap suborbital instrument shots, at least at first.
GW
'Starship" is not configured to provide artificial spin gravity or significant radiation protection, and probably never will be.
It will be one of the internal spaces just barely large enough for all aboard to crowd into, that will be the radiation shelter. With some lightweight polymer-based fibrous insulation installed thick on the walls of only that space, plus the added effect of the outer wall of the ship, that should be enough shielding for all but the most violent of solar events to provide survival, but not safety.
The outer hull wall in all crew spaces will have to be insulated thermally anyway, so that's two layers of wall and modest-to-thick thermal insulation that constitute the shielding. It won't be the recommended 20-25 g/cm^2, but every little bit helps.
Bigger solar events probably will lead to serious radiation sickness cases. That would seem to be inevitable, but with two insulated walls, you get the effects of two spacecraft hulls, not just one, to protect you.
"One hull" is not enough: the really-bad 1972 event between Apollo 16 and Apollo 17 would have killed an Apollo crew in only several hours after the end of the exposure, with only one thin hull's protection, and that rather thinly insulated.
GW
On the desirability of this vs. that mission plan: it isn't always about the numbers. It's fundamentally about what you really want to do.
On supersonic/hypersonic inlets: this topic occupies two full chapters of my ramjet book -- one on pitot/normal shock inlets, the other on supersonic inlets. It includes real-world band-aids for various real-world problems seen in these devices. The topics are important because inlets are what actually make ramjets work at all. I also have a good "exrocketman" article about supersonic inlets: "Fundamentals of Inlets", posted 9 November 2020 to http://exrocketman.blogspot.com.
The focus of all the ramjet and gas turbine applications is delivery of subsonic-decelerated air to the compressor face or ramjet combustor entry. That requires a diverging subsonic diffuser passage after the air is captured at the cowl, whether there is an external compression spike or ramp, or there is not.
For supersonic inlets operating supercritically, the shock system is swallowed, and the final normal shock-down from supersonic to subsonic internal flow occurs in this diverging passage. Supercritical operation is the lower drag scenario, and it is what must happen with ramjets, which need and will use all the airflow the inlet can scoop up.
For subcritical operation, the shock system is not swallowed, instead normal-shocking to subsonic flow just ahead of the cowl lip. That allows for variable air spillage around the cowl lip, required for matching turbine engine airflow demand with inlet scoop capability. It is a higher-drag scenario.
Supersonic combustion situations in scramjet are quite different. Those can use exactly the same cowl capture and external compression spike or ramp features, but what is done with the air after capture into internal flow is quite different. Once the air speed has been reduced to the desired supersonic internal Mach number, there is no divergent diffuser. There is a fairly significant-length very nearly constant area "isolator" duct that delivers the still-supersonic flow to the scramjet combustor. This geometry is entirely unlike, and extremely incompatible with, the geometry required for either gas turbine or ramjet air delivery.
The real-world problems of the supersonic-delivery "hypersonic" inlet are also quite different. That isolator duct is required for stable scramjet operation. Without it, scramjet combustors tend to explode, violently I might add. It is not exactly constant area, but very slightly divergent, just enough to offset the constricting effect of boundary layer growth and thickening.
The biggest bugaboo of all is injecting the fuel into a scramjet without provoking a shockwave upon each of the fuel jets, which invariably forces an unintended shock-down of the entire combustor flow to subsonic (defeating the entire purpose of the design). And yet, unless you risk those fuel jet shocks, or include a draggy separated-flow step (or multiple such steps), you cannot get the extremely-fast mixing rates that you simply have to have. This is still an extremely-difficult technology to implement, even today!
GW
As I indicated in post 20 above, there is a size scaling effect on the wing loading achievable. This is a square-cube law thing. It is inherent.
Mass (weight) scales as size cubed. Area scales as size squared. Double the size, and weight increases by a factor of 8 (all else equal, although it is usually not). Wing area only increases by a factor of 4.
So wing loading increases by roughly a factor of 8/4 = 2, for a factor-2 increase in size. Wing loading is crudely proportional to absolute size, at the same velocity and altitude.
Does that shed any light on what might be possible?
There is also the design speed range, which is an even stronger effect. W/S = CL q from the lift equation. Dynamic pressure q varies as velocity squared, and as proportional to atmospheric density. CL is pretty much fixed by the range of allowable angles of attack, typically only around 10 degrees wide. The faster you want to fly at a given altitude, the larger is q, and the higher W/S (wing loading) is going to have to be. Period, end of issue.
There is also the structural strength to resist the loads of pulling high gees. That is another square-cube law effect. The gees are a velocity-squared thing at any given altitude. The material stress capability is not! Which is exactly why you do NOT try to fly high angles of attack, when you are flying at high velocities, no matter what altitude you are flying! And yet surviving entry depends inherently upon flying at high angle of attack!
All these effects acting together are what determines your entry design. Focusing upon only one is a serious mistake!
GW
Tom:
I have actually designed solid rocket propellant grain designs able to survive 20,000 gees. These were for ICBM fly-along decoy applications. Such were tested in a shock tube facility at orbital speeds.
GW
Most of the people who propose such things have absolutely no idea how supersonic inlets really work. I do, which is why I am more-than-skeptical of such notions.
GW
Brian:
My old 170 had old-time round analog gauges for everything. It had a decent radio and a decent transponder, adequate before the switch to the ADS-B thing. It was a perfect example of a well-equipped stick-and-rudder airplane from the paper chart and E6B era. It had a VOR that was all my dad and I ever needed. No glass cockpit there! I inherited it from my dad when he passed away. He rebuilt it from a wreck. I bucked rivets for him as a young teenager, when he was doing that.
I had a very mild stroke that did no damage anyone could see, just before I could solo. I had no idea who I was or what was going on, during the hours after that stroke. I did not get grounded by the FAA, I grounded myself. There was no way in hell I would want such a thing to ever happen again, with me alone at the controls. After a while, I sold the airplane to a fellow EAA member who would use it for basic flight instruction. It was a more-than-perfect airplane for that purpose. Even better than a cub!
And my dad's original airplane was a J-5 Piper Cub Cruiser, rebuilt from a wreck. That's a 3-seat cub with a real door. I sanded the airframe tubes with crocus cloth when I was 8-10 years old. My dad was both an aeronautical engineer and a certified A&P. He retired from LTV aerospace back in the 1980's as their chief of airframe design. The F-8 Crusader and A-7 Corsair-2 are his proudest designs, among many things.
My proudest achievement was becoming an advisor to him at home, that he trusted.
GW
My guess is that the total round trip dV from Mars to Phobos and back is less than the 1-way dV up from Earth's surface to Phobos. That means you mine the ice and ship it up to Phobos, using reusable vehicles. The carbon probably needs to come from Earth, as what's in "carbonaceous" asteroid materials is strongly chemically bound in those materials, requiring lots of energy input to recover. Charcoal or graphite from Earth can be utilized with little or no energy input to free the carbon. It'll cost you more dV to send those vehicles back to Earth, if they are to be reusable.
You have a big enough energy problem turning carbon and water into methane and oxygen. You don't need to try to "free" your carbon from local rocky minerals on Phobos. Solar power per square meter of collector is about half the power density we see here at Earth. And you will need to include batteries to get through the intervals when view of the sun is blocked by Mars. Plus these panels have to track the sun; it is not in a constant position overhead, even when the view is not blocked by Mars. Nuclear might be better. Something to think about.
GW
All of the discussions above aside, I'm curious about why we would want to go there? The min one-way dV to reach the surface of Mars with direct aerobraking entry off the interplanetary trajectory is the min possible value. It is larger than the departure dV plus the course correction budgets by only the retro-propulsion needed to touch down (crudely only 10% of what is needed to get there at all).
That's only one-way, and it's how most of the lander probes were sent. There's a bloody good reason for that, since the lander payloads were at the throw weight limits for the boosters that sent them there. Going home is actually worse than getting there, since it takes propulsion to escape from Mars, plus at least some course corrections to achieve a really high-velocity free aerobraking return at Earth. Plus whatever it takes to touchdown somehow.
The largest one-way dV requirement occurs when you decelerate into low Mars orbit. That's in the neighborhood of 2 to 2.5 km/s more than the sum of departure dV and the course correction budgets. If you want to visit more than one site on Mars in the one trip there, you stage out of orbit, and that's the least dV requirement for your landers. To go home, you will need that whole orbit trip budget again! The higher dV is just the price you pay for multi-site capability staged out of low orbit. There is no way around that. But the lander descent dV is a trivial de-orbit (50 m/s) plus a touchdown dV (around .7 to 1.4 km/s). Lander ascent dV is only bit more than low orbit speed (3.6 km/s).
Going to Phobos one-way has a dV requirement between those of the other two scenarios. The real question is why? If your goal is to reach the surface of Mars from Phobos, the landing requires a modest (but NOT trivial) departure dV, aerobraking, and a touchdown dV. But the return costs a lot: your one-way lander ascent dV is closer to Mars escape at 5 km/s one-way, not orbit speed of 3.6 km/s. You just made the lander ascent problem much worse, by about 40%.
Post 112 above gives some overall results and points to an "exrocketman" article, where I figured all these numbers about 4 years ago.
There are differences of opinion about this next item, here on the forums, but I rather think there are no volatiles in Phobos that could be made into propellants for refueling anything! More than anything else, it resembles the dry rubble-pile asteroids we see elsewhere in the inner solar system. Just loose chunks of rocky minerals. To find significant ices to mine, you have to go out past the giant planets. Closer in, the solar heating evaporated all the ices long ago, quicker in the smaller ones. Or so most astronomers say.
So, if the lander problem is much worse, and there are no suitable resources from which to make propellants, then why would we ever want to go to Phobos in the first place, other than just once or twice for scientific curiosity?
GW
Nice tale about the 172 with the broken rudder cable. In tail draggers like my 170, the rudder control problem on the ground is even worse. Those want to swap ends while taxiing. Quite insistently, too!
I quite agree with the criticisms of Boeing and especially NASA top management. The only inquest anybody actually learned anything from was Apollo 1. NASA management never learned the lessons from Challenger or Columbia.
One of the problems is time: the people who actually learned something from Apollo 1 were all dead or retired by the time shuttle was flying. There is a built-in human resistance to learning from the mistakes of others, especially when those others are not there in person talking to you. Procedures are supposed to help overcome that, but they're not perfect, even when followed.
GW
I quite agree with Kbd512: just avoid most of the medical troubles and do artificial gravity. It's fairly incompatible with minimalist vehicle design, but that is the price you must pay to make the long trips. Partial gee at some level is likely "good enough", and partial daily exposure to zero-gee is likely not much of a problem. Too bad the experiments have never been done that could tell us how much gee is enough, and how much of a day we can be at zero-gee and still be healthy. You have to have some sort of spinning space station to do them!
Radiation in the form of cosmic rays is a canard often used as an excuse not to go, but it's just that: a canard, a lie. In the inner solar system near Earth, it varies between 24 and 60 REM/year, with the solar cycle. The astronaut exposure limits were set for many years at 50 REM/year, 25 REM in one month, and lifetime career limit that was age and gender-dependent, but peaked at 400 REM over a career. These exposure limits were supposed to allow a 3% increase in late-in-life cancers.
The real radiation danger is flares and mass ejections from the sun. Those can be as bad as standing outside in the fallout right after a nearby surface burst atomic explosion. 5000 REM/hour class stuff. They are only hours long, and are very directional, occurring very erratically. There are typically more of them during solar peak activity, as has been happening recently. This has even been seen by the instruments on Mars.
To "calibrate" these numbers, consider that 100 REM over a short interval like an hour will give you serious radiation sickness, 300 REM in a short interval will kill about 50% of those so exposed, and 500 REM over a short interval is lethal to 100% of those so exposed.
GW
The Starliner will stay docked at ISS not to June 22, but to at least June 26, per today's CBS news website.
Reading between the lines, it seems NASA and Boeing still cannot figure out whether Starliner can bring Butch and Suni home alive. They need more time to convince themselves it can. Which tells me it cannot.
GW
I saw it on their site some time ago.
As for hardening against launch gees, you are looking at something in the 5,000-10,000 gee range. It can be done; naval 5-inch-54 shells launch at about 20,000 gees.
GW
I hope Dreamchaser succeeds. It is amazing that Sierra Nevada has kept this alive and going, despite a lack of government funding. If it does succeed, it could replace Boeing's troubled Starliner as the alternate to SpaceX's Dragon.
It was NASA that insisted crew Dragon return via parachutes to ocean landings. SpaceX's original design called for thrusted landings on land, with the parachutes as only a backup to the retro-propulsion. Which NASA decision was before SpaceX proved retro-propulsion to be feasible and reliable with Falcon cores.
GW
What they ought to do, failing a servicing mission, is reboost it to prevent entry too soon, and then bring it home in a "Starship" to be a public monument. That does require a Starship/Superheavy system actually ready to take on real missions. SpaceX still has a ways to go yet.
GW