Robotic Space Plane Launches In Mystery Mission This Week

mpicpp writes: The United States Air Force's robotic X-37B space plane will carry a NASA experiment into orbit when it launches on its next mystery mission Wednesday. The liftoff will begin the reusable space plane's fourth mission, which is known as OTV-4 (short for Orbital Test Vehicle-4). Since it's classified it's not entirely clear what the space plane will be doing once it leaves Earth Wednesday. This has led to some speculation that the vehicle might be a weapon, but officials have repeatedly refuted that notion, saying X-37B flights simply test a variety of new technologies. The X-37B looks like a miniature version of NASA's now-retired space shuttle. The robotic, solar-powered space plane is about 29 feet long by 9.5 feet tall (8.8 by 2.9 meters), with a wingspan of 15 feet (4.6 meters) and a payload bay the size of a pickup-truck bed. Like the space shuttle, the X-37B launches vertically and lands horizontally, on a runway.

Comments

What's up with that motor?

[gman003]

Okay, I know it's probably the least important thing about the craft, but still...

Why are they using such an ancient, decrepit-ass rocket motor? The AR2-3 is incredibly old - it dates back to a Gemini-era trainer, basically a modded F-104 that NASA used for early tests and training for spaceflight. It was made back when rocket chemistry was still in the "even the experts don't know much" stage, so it burned jet fuel and high-test peroxide (90%+ H2O2 in H2O).

Jet fuel is not good for rockets - basically, the restrictions on what compounds can be present is fine for jet engines, but leads to horrible problems with rockets. There's a specific petroleum-product blend designed for rockets, called RP-1, which clamps down on things like sulfur compounds or alkenes that love to gum up the works. This rocket was originally used on a jet fighter and shared fuel with it, so that was understandable... but the USAF recertified the engine for modern JP-8 instead of the old JP-4. So they already went through the effort of making it work with a new but similar fuel. Unless the X-37 hides a jet engine on itself somewhere, I don't see why they couldn't have used RP-1 instead.

Further, rocket science moved away from peroxide for a reason - it's dangerous. It will explode for basically any reason - peroxide decomposes exothermically, so once it starts reverting to H2O + O2, it's nearly impossible to stop. And it reacts with tankage surprisingly often. Oh, and it does horrible things to your specific impulse, which really hurts you on a last-stage engine like this one.

Honestly, using the engine at all is a weird choice. Sure, maybe they had some laying around... from the sixties... but that's like putting an F-104 engine in a prototype aircraft, it just doesn't seem right when other off-the-shelf systems work better. An AJ-10 would have worked beautifully. An RL10 might not have fit the aero package (hydrogen is a bulky fuel), but would have given them some impressive dV if they wanted it. Aestus would be a perfect match as well, if they didn't mind outsourcing to the Euros. Even Kestrel would work (although it first flew around the same time as this, so understandable not to use it). Point is, they had options, and being the Air Force, they could easily have just had an engine custom-made for it if they so wanted.

So what are the implications? All I can think of is a) they don't care how badly the rocket performs, b) they probably aren't going to keep that engine in whatever "production" version they build, or c) they have some other reason to use peroxide or JP8. Maybe peroxide is also their monoprop for RCS? That isn't really worth it though, particularly when UDMH works better as RCS and in the main motor.

Re:What's up with that motor?

[lourd_baltimore]

There is some speculation that the AR2-3 may not be the engine used in current flights (see other replies to you post).

However, the AR2-3 is human rated. The X-37 is nominally unmanned, but hey the missions are classified, and because Halo Orbital Drop Shock Troopers. Of course some of the alternative engines you mentioned have been used on stages of previous manned flights and are thus presumably man-rated as well.

I did some Binging on the AR2-3 and found a NASA/Rocketdyne/OSC presentation that looks to be drafted around 2000.

Here that seem to make the case high test peroxide (HTP) technologies are the way of the future for upper stage propulsion:

Hydrogen peroxide was selected over liquid oxygen because it is dense, storable, capable of tolerating months in orbit, and meets safety restrictions for being part of the payload in the Space Shuttle.

Of course the Shuttle aspect is no longer a factor, but the other factors still seem to be in play.

Further into the paper, the USFE 10k peroxide motor is mentioned as a project to develop new HTP technologies. These technologies would be used for future HTP-based upper stages. They even have a goal of over 100 uses of an engine before it has to be removed for overhaul. Is that a lot in the world of rockets? As this paper was drafted around 2000 I would guess that the X-37 is using something a bit different that the bog-standard AR2-3 or has moved away from HTP technologies altogether.

Now to say that rocket science has moved away from HTP is not quite true. I don't think there are any big HTP engines used in lower stages. However, the Bloodhound SSC is using a HTP hybrid motor they are designing.

There is also research in to using HTP as a monopropellant for thrusters using a catalytic bed. I suppose the advantage here is that you have HTP as your oxidizer for an upper stage and then it can be used for maneuvering once on orbit. Similar, as you mentioned, to UDMH.

Take a look at the Introduction from the ESA paper referenced above. They cite several reasons why HTP is desirable and advantageous. Cost and safety being paramount. They also mention that Soyuz has been using HTP in its maneuvering systems for over 40 years. I think that HTP safety concerns have been effectively mitigated from the "explode because you looked at it funny" era.

As for performance it seems that HTP is as good as some other technologies, but it's no dog either and it seems to be a good fit for the X-37 or other small stages. Quote from the ESA paper:

The propulsive performance of hydrogen peroxide monopropellant rockets is about 20% lower than hydrazine, but the volume specific impulse achievable with 90% H2O2 is higher than most other propellants due to its high density. This is particularly useful for systems with significant aerodynamic drag losses and/or stringent volume constraints. With respect to bi-propellant and hybrid rocket engines, hydrogen peroxide yields a specific impulse comparable to other liquid oxidizers like dinitrogen tetroxide, nitric acid and even liquid oxygen..

It seems that HTP has many uses and rocket science has not moved away from HTP, indeed, it is being actively researched. It may or may not be used on the X-37 right now. It may or may be used on the X-37 in the future. With further attention to cost, safety, and, increasingly, environmental impact, HTP seems to be coming for you...

Re:Republicans and their unhealthy space obscessio

[fnj]

NASA had nothing to do with the microwave oven. Diathermy (therapeatic heating of human tissue by radio waves) was being used in 1930. Westinghouse demonstrated cooking food using short waves in the 1933 Worlds Fair. The cavity magnetron was perfected early in WW2. Percy Spencer noticed a candy bar in his pocket melting when he was working close to an operating radar in 1945. He experimented with heating food in a metal box fed from a magnetron the same year; Raytheon filing a patent for it. Raytheon built he first "Radarange" in 1947. A public vending machine was producing hot dogs in Grand Central Terminal in 1947.