Stumbled across a picture of the old **Space:1999 **Eagle ships the other day and remembered that many times they landed them on a planet with atmosphere. Since the Eagles were hardly streamlined, I got to wondering if it would be possible to enter atmosphere without excessive heating by matching the rotation of the planet and just lowering the ship straight down into the atmosphere? I assume it would be fuel-expensive, probably prohibitively so, but still is it possible?
Sure, it’s possible. The only reason a vehicle heats up on reentry is because it’s moving fast relative to the air. If you mitigate that velocity difference by other means, then you don’t have a problem.
Unfortunately as you note, those other means are likely to be impractical.
If you have power/energy coming out the wazoo (of the spacecraft) then yes you can.
The trouble is, consider how much energy it takes to launch, and you need about the same order of magnitude for a controlled descent.
Specifically you are circumnavigating the globe in orbit - 25,000 miles or more in an hour and a half is about 17,000 mph.
You need to shed 17,000mph plus you need to control your descet, unless your craft is aerodynamic and flies after that.
Then it will be painfully long on the descent unless you plan to be hypersonic, which has its own heat and engineering challenges.
Or you could ride up real high in a balloon and parachute down.
Could you capture the energy from the heat and then turn around and use it for braking somehow?
Well, maybe some kind of scramjet design…
- emphasis mine
Physics 101 - The rotation of the planet really has nothing to do with it. For Earth, you’re traveling at +17K mph while in orbit because of Earth’s gravitational pull. The moment you slowed down below that speed you’d begin to descend toward the surface. If you had some kind of theoretical hyperdrive that slowed you down from 17K to zero in just a few seconds then yes, you’d just drop straight down. Your terminal velocity would depend on the size & shape & altitude of your craft (i.e. the aerodynamic qualities).
Note that without futuristic ‘inertial dampers’ or something you’d never survive slowing down that fast. Also, it would take a tremendous amount of energy to slow down any significant mass that quickly. Far beyond any conventional power source.
The thing you have to remember about the high speed of reentry: It isn’t because you’re *falling *from such a high altitude, it’s because you have to be traveling at that high speed to be in orbit in the first place…
In fact, it is because you are “falling” around the Earth at such a high speed that you completely miss the ground entirely, over and over again. In order to hit the ground you have to stop going so fast. This seems counterintuitive to most people because they would expect that you would be moving the fastest at the point of impact (or at least, when you are starting to hit the atmosphere) but in fact, you have to slow down to go down, and the faster you want to go down, the slower you have to go.
It is probably best not to think to much about it and just let it happen.
Anyway, if your only means of slowing down is direct propulsion, e.g. a rocket, the amount of energy required is nearly the same it took to go up (minus losses for gravity and drag on the way up). That means that if you needed, say, an Atlas V to get to orbit, you need the equivalent of an Atlas V to return from orbit. Since an Atlas V 501 masses about 330000 tonnes loaded and has a payload of around 8000 kg, it would require about 400 Atlas Vs to put into orbit enough fuel for you to return the same mass payload back to the rotation speed of the Earth. Although it isn’t physically impossible, I wouldn’t call this practical in any sense of the word.
Fortunately, there is a neat trick that every child riding in a car with the window down learns; if you stick your arm out, the drag from the atmosphere puts a considerable force on it, which also reduces fuel economy and causes your mother to scream that she will pull over this car if you don’t get your damned limbs back in the vehicle right now. This drag is very useful for vehicles reentering the Earth’s atmosphere because it can be used to transfer the high momentum (mass times velocity) of the vehicle directly to the atmosphere, slowing the vehicle for free. Technically, you are adding heat to the Earth, so it isn’t truly free, but it doesn’t cost the returning spacecraft anything other than a need to withstand the elevated temperatures.
While the drag is sometimes called “friction” it is not like Coulomb friction that you learn about in physics class. (There is some small amount of interaction between the air and the body which is termed “skin friction” which is due to viscous drag and will provide some modest amount of heating, but it only a major consideration with low Reynolds number flows.) The forces and resulting heating experienced during reentry are actually due to the compression of air in front of the body. This compression causes the air to be heated, and is concentrated at a shock boundary where it stagnates and then flows around the spacecraft, so there is actually very little contact between the fast moving fluid and the spacecraft skin. If the vehicle form is designed properly, the shockwave will stand off from the heat shield by several centimeters or more, and so the majority of heating is just radation through the shock boundary layer. This is why having a blunted body for reentry is preferable to one with sharp edges or changing profiles (like the Shuttle Orbiter did). By using this drag to convert kinetic energy to atmospheric heating, a spacecraft can go up on a large rocket, but only require very small orbital manuevering engines to adjust the orbit to intersect the atmosphere and then worm its way down to the surface.
Stranger
Yeah, I know that technically that’s what it’s called (and is), but to most people the colloquial definition of ‘falling’ just seems very different from the orbital dynamics one. As you said, best not to think about it…
Riding up real high in a balloon != earth orbit. Orbit means an even higher altitude, plus a huge horizontal velocity that you don’t have while riding in the balloon. In other words, you’ve got a lot more kinetic energy to deal with.
Just for kicks, I plugged 500,000 feet into my “Felix Baumgartner Jump Simulator” spreadsheet (though I’m pretty sure a balloon couldn’t get that high). I figure he’d hit about 3000 MPH on the way down, and see peak temperatures of about 580 F. So that’s still pretty damn hot. A parachute designed for a reasonable landing speed probably wouldn’t slow him down much at the high altitudes where that peak speed would happen (around 160,000 feet).
Not necessarily. One could hover in a fixed position, relative to the “fixed stars,” above the earth at a fixed distance from the earth (and still orbiting the sun together with earth). The earth would rotate beneath such a craft. It would take enormous amount of propulsion to do this.
It’s all about the propulsion.
The Delta Clipperwas intended to experiment with the concept of vertical descent. It appears that carrying the extra fuel needed to slow descent for a vertical landing requires the removal of substantial heat shielding and wings that would allow aerodynamic flight in order to reduce weight, so if anything goes wrong, the craft would have no means of slowing down, and burn up. Though perhaps a parachute escape capsule could be incorporated for emergencies like that in a manned craft.
Well if you match the rotation of the planet, are you really moving at 17,000mph? The Earth doesn’t rotate that fast. I understand the problem with slowing down and aero-breaking, but I was speaking more about just a straight descent. I guess you don’t even need to match the rotation, if you’re just coming in on a straight line from, say, the moon, and do not go into orbit at all, you wouldn’t need tons of fuel for slowing down, just to have a controlled landing, correct?
Of course, antimatter drives wouild help enormously, but I wasn’t really speaking in terms of practicality, just feasibility.
IIRC the Delta Clipper was still intended to use aerobraking - hence the “capsule” shape. Then it was unaerodynamic enough that it would fall at a relatively slow rate, only needing to use rockets to slow its descent at nearly the last minute.
Note it would be incredibly light for its size, hence the relatively(!!) slow rate of fall - more like a hollow metal shape (which it would be) than a solid chunk of metal. After all, it was going to be 90%-plus fuel tanks, and most of that fuel inside was burned off in ascent.
The attempt to turn this into a functional SSTO (Single Stange to Orbit) vehicle apparently failed because of the demand for incredibly structurally sound but incredibly light fuel tank combined with rocket structure. IIRC they were trying to use fancy carbon fibre technology for the new design and it would not work. the problem still is - the amount of fuel needed to take off and achieve orbit in a single stage is simply far too high a percentage of total vehicle weight - hence the “disposable stages” most rockets use today.
You are confusing the Delta Clipper with the VentureStar vehicle. Both would have been SSTO launchers, the Delta Clipper being VTOL, the VentureStar being vertical launch and horizontal landing. Nasa decided to pursue the VentureStar vehicle instead of the Delta Clipper, developing the X-33 prototype based on a scaled-down version of the VentureStar design. The X-33 turned out to be technologically unfeasible for multiple reasons, including the failure of the complex-shaped carbon fiber fuel tanks during pressurization tests. The Delta Clipper might also have turned out to be technologically impossible had it been pursued, but it never reached that point.
“Coming in from the moon” means “falling toward the earth from a height of 240,000 miles.” Earth’s gravity is accelerating you during your entire trip (weakly at first since you’re so far away, but more strongly at the end). If you don’t employ some sort of retrorocket during the trip, by the time you reach the upper edge of the earth’s atmosphere you’ll be traveling somewhere close to 25,000 MPH straight toward the ground.
Understood - of course you would slow down instead of just plunging to Earth, that’s a given.
md2000, I think you may be conflating the McDonnell Douglas DC-X demonstrator with the Lockheed Martin X-33. The DC-X was an unmanned vertical ascent craft with a fairly conventional S-glass prepreg layup aereshell manufactured by Scaled Composites (the same company that build SpaceShipOne and SpaceShipTwo suborbital vehicles)… It was not intended to be an SSTO or to demonstrate adequate mass fraction, but rather a suborbital testbed for technologies that would be used on a follow-on SSTO system. Although the terminal landing mode did desend base first under thrust, the orbital prototype (DC-Y) and follow-on production articles (DC-n) were actually intented to use a nose-forward reentry mode using the structure as a lifting body rather than blunt base reentry, which was required in order to achieve cross range requirements for a single orbit return trajectory. The intended mission for the Delta Clipper was as a satellite/weapon system delivery system for the Strategic Defense Initative.
NASA took over the program when SDIO shifted away from space-based systems and applied a series of questionable upgrades including a problematic aluminum-lithium alloy oxidizer tank from Russia. The NASA effort never really had a clear direction for the program–it wasn’t intended to develop into a launcher or a testbed for a STS replacement–and the program was funded at a low and irregular level, leading to errors in testing and a lack of budget for maintenance and repair. While the system did perform well, achieving rapid turnarounds and setting time and altitude records for hovering flight, cumulation of damage and a hard landing which resulted in failure of the oxy tank and a fire damaged the vehicle beyond repair, after which NASA cancelled the program. The core concept has been carried on, however, by Blue Origin with the Goddard testbed and New Shepherd suborbital systems.
The LM X-33 (also sometimes known as the commercial VentureStar) was a testbed for a vertical takeoff, horizontal landing spaceplane. It was cancelled in 2001 as a result of cost overruns due to problems in testing and manufacturing the composite fuel tanks, fuel cells, and active thermal protection systems without ever flying or even completing a full airframe (although Lockheed has apparently completed a testbed and flown the system under IR&D funding after NASA cancelled the contract). It did develop and qualify a linear aerospike engine intended to be used on the VentureStar and necessary for optimal performance from ground level ambient to vacuum conditions.
While it is often asserted that SSTOs are not practicable due to the mass fraction issue (i.e. the required maximum fraction of fuel to dead weight such as tankage, engines, payload, and supporting structure) a scaling study reveals that the mass fraction for SSTO is achieviable even without advanced composites, simply by using lightweight metallic structures and altitude-compensating propulsion (the afformentioned aerospike or plug nozzle). The Titan II first stage was actually capable of SSTO performance merely by throttling back and extending the burn, albeit with minimal payload and without altitude compensation, using engines that have mediocre performance compared to modern liquid engines. The same would be true for the Saturn S-IVB stage 3 with an altitude compensating nozzle, and this was proposed as the basis for the Phoenix B and Osiris.
Chrysler Aerospace also advanced a proposal for the STS (“Shuttle”) called the SERV which was a blunt base SSTO that formed an aerospike nozzle of the entire base. Chrysler did an extensive feasbility study and concluded that the concept was not only feasible but that it could be done with only modest modifications to existing infrastructure. An independent review by TRW agreed with the feasibility and only took issue with meeting the cross-range requirement imposed by the Air Force for “Blue Shuttle” (single orbit return) applications. NASA, however, was wedded to a two stage shuttle design and did not give the proposal serious consideration, actually refining the RFP to exclude SSTOs.
A reusable SSTO design is technically achievable with only moderately more cost and effort than developing a conventional multi-stage vehicle. However, it is unclear that it would be cost competitive with expendable launch vehicles owing to maintenanace and refurbishment costs, and would not carry as much payload in proportion to structural mass as expendable vehicles. The applications are really limited to ascent and return of small to moderate payloads. Heavy lift capability is unquestionably more economically provided by two stage expendable launch vehicles.
Stranger
Well then the technical challenges are fundamentally the same as one would encounter when trying to descend from earth orbit: in both cases, if you don’t want encounter deleterious levels of aerodynamic heating, you’ve got massive amounts of energy that you need to get rid of somehow before you reach the atmosphere.