X-37C: plans for a crewed version of the X-37

[douglas100]

OK. Thanks for that.

[mr. mark]

Everyone will eventually have to face the fact that at least near term winged vehicles are only going to be for LEO spaceflight. Capsules, as command modules, will have to handle the load as far as BEO goes.

[simonbp]

I would guess a lifting body is also unneeded BEO.

More importantly, not possible from BEO. The reentry speeds and angles don't lend themselves to a gliding reentry. Unless you wanted to expend extra propellant needed to get yourself inserted back into a circularized LEO upon return from BEO, which is just plain silly. Better to design the spacecraft and TPS to take the heat of a ballistic reentry. And that's called a capsule.

Not quite.

A "lifting body" is a vehicle that produces lift without wings. Apollo was (and Orion is) a lifting body because it reentered at a non-zero angle of attack, thus producing a small amount of lift in order to guide the vehicle's trajectory (in concert with the RCS). This in contrast to, say, the Mercury reentry vehicle, which ballistically reentered at zero angle of attack (and needed a naval task force to find it afterwards).

Winged vehicles, like Shuttle and X-37, are not suitable for high speed entry (>10 km/s) because the mechanical loads on the wings are just too high without having to resort to exotic materials. Also, the wings are very heavy for the amount of internal volume they offer.

Lifting bodies that work while subsonic (HL-10, X-24A, etc), on the other hand, have much less weight and volume penalty than winged vehicles, and can have much lower mass recovery systems than antisymmetric capsules (landing gear+aero surfaces vs. parachutes+rockets/airbags). So, it would be possible to design an optimized BEO lifting body, but it would not look anything like X-37...

[vulture4]

Dreamchaser has a much lower lift to drag ratio than X-37C. The L/D of the X-37 is about 4.5, the DC is at about 3.5, too low for a safe landing without powered lift.

Do you have citations or other data showing these L/D ratios? It is my impression that the DC lands un-powered as a glider?

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070017478_2007014601.pdf

This is an impressive piece of work, careful and meticulous, and worthy of a careful read. It explains what is known, and the level of uncertainty. You have to download it and read in acrobat; the preview doesn't show the figures quite properly. When I looked at Figure 3, I at first noticed the slow, painful struggle to improve lift and reduce drag through all the lifting bodies, and suborbital craft like the X-15. The Shuttle was slightly better than any of the wingless lifting bodies.

Then I noticed an outlier in Figure 3 that did not even seem part of the same group, and it struck me. The Shuttle, with its huge engine bells and OMS pods, was burdened by a totally unaerodynamic tail section that caused most of its drag, while the wingless lifting bodies all had streamlined tails. When the field was leveld by adding the Shuttle tail cone (still not particularly streamlined) the Shuttle, with traditional wings and fuselage, absolutely blows the doors off every wingless lifting body, past, present or future. In a field where an increase of 5% is a struggle, the L/D of the Shuttle with tailcone was about 7.5, twice that of the HL-10.

It's not rocket science. But it is basic aerodynamics, and we seem to have forgotten it. Wings need a thin airfoil optimized to generate lift with a reasonable aspect ratio to minimize induced drag. A fuselage needs a circular cross-section to maximize volume, minimize parasitic drag and eliminate bending loads when the cabin is pressurized.

The X-37 was based on the lessons of Shuttle; the delta wing had reasonable lift and a soft transition from high-alpha hypersonic entry to gliding flight. But the Shuttle was sensitive to CG because of the short moment arm available to the elevons and flap, and the body flap caused so much nose-down trim it could not be used as flaps normally are, to increase lift. The vertical tail was of no use during hypersonic entry because it was in the wake of the body. So with the X-37 the wing was kept as a delta but moved amidships, and a V-tail was added with each surface in the airstream and useful during entry, separated from the wing to give a longer moment arm and much better pitch control. The wings even got conventional flaps, useful now that they were farther forward. As an orbiter, it has only a small OMS engine aft; the tail is otherwise streamlined, so the L/D will be close to that of the orbiter with tailcone. The X-37 has so much lift it would have needed a drag chute to drop it from the NASA B-52. I would be surprised if it was much over 100 knots at touchdown. The next time you fly, ask the pilot whether it is safer to land an aircraft with a stall speed of 100 knots, or 200.

Yet none of these factors seem to have been considered in the commercial crew selection, which dropped OSC's wing-and-fuselage Prometheus in favor of the Dreamchaser. Looking at the selection criteria, it is hard to see anywhere actual aerodynamic preformance, and particularly safety margins in gliding flight, was even considered.

[sb]

I don't think anyone has ”forgotten” how aerodynamics works, it's just that high L/D isn't necessarily the be-all & end-all of spacecraft design. If you read the NASA history on lifting body research you'll see that actually while a certain minimum L/D was found necessary for a safe flare, the steep approach path of the lifting bodies at least wasn't disliked for precision approaches.

Which raises another point: you have to distinguish between L/Ds in different flight regimes. The sharp-edged lifting bodies were designed for improved hypersonic L/D & hence cross-range. Subsonic performance was only really of interest as to whether they could be landed safely.

[vulture4]

I agree that aerospace vehicle design is always a compromise, but for a design that works we should never give away an asset unless we get something more valuable. Capsules have low L/D (low crossrange in hypersonic entry, require parachute for landing) but gain volumetric and mass efficiency, structural simplicity, and, in theory, the ability to re-enter under manual control (the Soyuz ballistic mode). The parachute increases turnaround costs in a reusable system and is difficult to scale to large reusable vehicles landing on land like the shuttle, meaning reusability is less feasible.

Runway landing is the strategy used for almost all commercial aviation because it minimizes cost in reusable flight operations and permits almost unlimited scalability in size. If we see human spaceflight as ever being available on a large scale at an affordable price we need vast improvements in cost, and that was the rationale for runway landing for the Shuttle.

The Shuttle's landing performance was very close to the safe limits and had imposed inflexible weather limits. It required a 15,000 foot runway, a rare commodity, and very accurate weather prediction. There was no room for error. It is possible to operate under these conditions, but it is not something one does if there is an alternative. And, as I pointed out, given equal requirements for engines at the tail the Shuttle is vastly superior to any lifting body. The Shuttle also has greater hypersonic crossrange than a wingless design. It's quite true the extremely sharp-edged waverider concept has even higher L/D at hypersonic speed, but it sacrifices a lot of cargo volume for a given dry mass.

The question I keep asking is, what do we gain by eliminating the wings? A wingless lifting body that is even marginally capable of runway landing has a flattened fuselage with more hull per unit volume than a round fuselage, and more weight if it is pressurized due to the need for internal bracing and bending moments in the hull. The original rationale for the lifting body was to avoid sharp leading edges, and the development of high-temperature composites for this purpose was one of the reasons the Shuttle went back to wings. No commercial aircraft flies without wings because even though it is just barely possible, it adds cost and risk and provides no benefits. The few aircraft, like the B-2, that do not have both wings and fuselage give up the fuselage, not the wings.

That's why a direct comparison of the X-37 and Dreamchaser would be so revealing. Does anybody know how fast it was going at touchdown?

[Rocket Science]

190kts...

[vulture4]

Interesting. Faster than I would have guessed.

[BrightLight]

Interesting. Faster than I would have guessed.

The X-37C is stated to be 48 feet long as compared to the X-37B which is 29 feet long. The X-37B is about 11,000 lbs
Scaling Law:
"When a physical object maintains the same density and is scaled up, its mass is increased by the cube of the multiplier "
The ratio 48 to 29 feet is 1.65
following the mass scaling law, the multiplier is 4.53,
the scaled mass is 11,000 * 4.53 or almost 50,000lbs!
The DC is roughly 30,000 lbs and lands somewhere between 190 and 200 knots.  The caveat here is maintaining the same density, as I am not a aeronautical engineer (arm waving) I can't define this but I assume that the density will be lower on the X-37C when compared to the B model and the mass will be less than 50,000 lbs loaded.

[Lars_J]

I'm certainly looking forward to the DC drop tests - we should get a wealth of new information to debate.

[vulture4]

It is odd no attempt was made to streamline the tail of the X-37, since it has only one small, centrally mounted OMS nozzle. That square tail adds a lot of drag, maybe 30% or more of the total.

It would be interesting to know the weight for the DC drop tests; will it be the planned operational gross weight of somewhere between 9 and 11 metric tons? Most of the testing of older lifting body designs was done at much lower vehicle mass. I.e. X-24A about 3 metric tons at landing (3.8 for the X-34B).

[Rocket Science]

Interesting. Faster than I would have guessed.

You might want to re-visit as to what you define as “faster” as I answered you in the Q&A thread… So I’ll link it to you again…

http://forum.nasaspaceflight.com/index.php?topic=29126.15

[vulture4]

An interesting document. I am curious why DOD dropped the project as a NASA partner, why NASA then dropped it, and why DOD then picked up the entire project it had previously dropped. Despite the two missions it isn't clear whether it is "operational" in its current form or a development prototype for something else, as it was under NASA. In recent years DOD has expressed interest in fully reusable launch systems, and particularly flyback booster stages, as Jay Penn described:

http://www.nasa.gov/centers/kennedy/news/beyondnextgen.html

Perhaps the X-37 is part of a program to develop a reusable upper stage for such an RLV.

[Prober]

Interesting. Faster than I would have guessed.

The X-37C is stated to be 48 feet long as compared to the X-37B which is 29 feet long. The X-37B is about 11,000 lbs
Scaling Law:
"When a physical object maintains the same density and is scaled up, its mass is increased by the cube of the multiplier "
The ratio 48 to 29 feet is 1.65
following the mass scaling law, the multiplier is 4.53,
the scaled mass is 11,000 * 4.53 or almost 50,000lbs!
The DC is roughly 30,000 lbs and lands somewhere between 190 and 200 knots.  The caveat here is maintaining the same density, as I am not a aeronautical engineer (arm waving) I can't define this but I assume that the density will be lower on the X-37C when compared to the B model and the mass will be less than 50,000 lbs loaded.

Need to keep in mind the X-37B is a "test" model.
Your numbers are off.

[BrightLight]

Interesting. Faster than I would have guessed.

The X-37C is stated to be 48 feet long as compared to the X-37B which is 29 feet long. The X-37B is about 11,000 lbs
Scaling Law:
"When a physical object maintains the same density and is scaled up, its mass is increased by the cube of the multiplier "
The ratio 48 to 29 feet is 1.65
following the mass scaling law, the multiplier is 4.53,
the scaled mass is 11,000 * 4.53 or almost 50,000lbs!
The DC is roughly 30,000 lbs and lands somewhere between 190 and 200 knots.  The caveat here is maintaining the same density, as I am not a aeronautical engineer (arm waving) I can't define this but I assume that the density will be lower on the X-37C when compared to the B model and the mass will be less than 50,000 lbs loaded.

Need to keep in mind the X-37B is a "test" model.
Your numbers are off.

I realize that the X stands for experimental. The exercise was to demonstrate that it's not as simple to scale a spacecraft, what numbers are wrong? What is the mass of the x-37C? How does one scale a body?

[vulture4]

Interesting. Faster than I would have guessed.

The X-37C is stated to be 48 feet long as compared to the X-37B which is 29 feet long. The X-37B is about 11,000 lbs
Scaling Law:
"When a physical object maintains the same density and is scaled up, its mass is increased by the cube of the multiplier "
The ratio 48 to 29 feet is 1.65
following the mass scaling law, the multiplier is 4.53,
the scaled mass is 11,000 * 4.53 or almost 50,000lbs!
The DC is roughly 30,000 lbs and lands somewhere between 190 and 200 knots.  The caveat here is maintaining the same density, as I am not a aeronautical engineer (arm waving) I can't define this but I assume that the density will be lower on the X-37C when compared to the B model and the mass will be less than 50,000 lbs loaded.

Need to keep in mind the X-37B is a "test" model.
Your numbers are off.

I realize that the X stands for experimental. The exercise was to demonstrate that it's not as simple to scale a spacecraft, what numbers are wrong? What is the mass of the x-37C? How does one scale a body?

It's just a rough approximation, an accurate estimate would take more detail. If scaled up to the size of the Shuttle (x4 in length) multiplying by 4^3 would give you about 300 metric tons, three times the actual Orbiter mass.

[Rocket Science]

The flaw in scaling is assuming a constant mass/volume ratio. In the case of Shuttle you would have a high M/V in the fore (cabin, RCS) and in the aft (engines, tanks). The center 60’ cargo bay would be relatively a low M/V and via a high percentage of overall dimension thus a source of error… Just for you to consider… Carry on

[vulture4]

The flaw in scaling is assuming a constant mass/volume ratio. In the case of Shuttle you would have a high M/V in the fore (cabin, RCS) and in the aft (engines, tanks). The center 60’ cargo bay would be relatively a low M/V and via a high percentage of overall dimension thus a source of error… Just for you to consider… Carry on

I'm  unclear as to whether the X-37 mass was with fuel. It seems to have a higher propellant mass fraction than the Shuttle Orbiter, though it's hard to tell what changes DOD made when they switched to hypergols. There is a Shuttle-like payload bay but it is relatively small.

Here's a review of a presentation by Arthur Grantz of Boeing that has some detailed drawings of the proposed "C" model. Maybe they left the stern flat because they planned to put the APAS there.

http://www.space.com/13230-secretive-37b-space-plane-future-astronauts.html

[Rocket Science]

The flaw in scaling is assuming a constant mass/volume ratio. In the case of Shuttle you would have a high M/V in the fore (cabin, RCS) and in the aft (engines, tanks). The center 60’ cargo bay would be relatively a low M/V and via a high percentage of overall dimension thus a source of error… Just for you to consider… Carry on

I'm  unclear as to whether the X-37 mass was with fuel. It seems to have a higher propellant mass fraction than the Shuttle Orbiter, though it's hard to tell what changes DOD made when they switched to hypergols. There is a Shuttle-like payload bay but it is relatively small.

Here's a review of a presentation by Arthur Grantz of Boeing that has some detailed drawings of the proposed "C" model. Maybe they left the stern flat because they planned to put the APAS there.

http://www.space.com/13230-secretive-37b-space-plane-future-astronauts.html

Yes, by the nature of the vehicle’s mission much is not known and all speculation. What we can discern is that in terms of overall M/V for the x-37, is that it is more evenly distributed throughout the craft relative to the Shuttle by nature of its short “close coupled” length, CoL (wings) and Cg. Note gear placement for example and compare it to the obiter.

In an exercise for my physics students we compared a modern day reptiles/raptors mass using a scaling-up to the size of a dinosaur like T-Rex for  example,  that the final weight of the animal would shatter its limbs under its own forces if it tried to run, if we limit the legs to what we see in the fossil record. I did this for them to exercise caution when trying to scale-up and its potential source of errors…

[Prober]

The flaw in scaling is assuming a constant mass/volume ratio. In the case of Shuttle you would have a high M/V in the fore (cabin, RCS) and in the aft (engines, tanks). The center 60’ cargo bay would be relatively a low M/V and via a high percentage of overall dimension thus a source of error… Just for you to consider… Carry on

or the easy way is to see how they scaled the x-40 to the x-37B.  A good hint for you.