The Space Reviewin association with SpaceNews

Rocketplane XP in flight
The Rocketplane XP is the latest project for David Urie, who has worked on cutting-edge aerospace efforts from the SR-71 to the X-33/VentureStar. (credit: Rocketplane Ltd.)

Rocket plane venture star (part 1)

<< page 1: operating the Rocketplane

Human factors engineering

TSR: So, I am interested in the engineering. I asked you a little bit about the pressure. What other human factors challenges are you going to tackle?

Urie: We have to remove heat. People produce heat and water and carbon dioxide. We have to make sure all of that is taken care of. But the flight is brief enough, you know, so we are not concerned. We have got plenty of bottled air. We are going to carry good air, compressed air. We will just replace the air in the cabin at a rate so that none of that is built up.

TSR: You are going to have a spare canister.

Urie: We are going to have a big bottle of air ‘cause we use it for other things. So, it is just like a diver with a couple of tanks on his back. You know, we are just going to have plenty of air and just replace the air so there is no CO2 or moisture or heat buildup.

TSR: Heat, water, and CO2. It’s kind of like we are batteries in The Matrix or something like that! Kind of an engineering diagram. What are your plans for people floating around?

Urie: Right now you are going to stay in your seat.

TSR: So kind of like Branson? You get to float up a little bit off of it or something like that?

Urie: Our fighter pilot board members and others say “you’ll get plenty of sensation of zero-g without floating out of your seat.” And the concern is it is not like the Vomit Comet where you go from two g to zero to two. In this case, you are going from three to zero to three and a half.

TSR: Now, is that eyeballs out like SpaceShipOne?

Our fighter pilot board members and others say “you’ll get plenty of sensation of zero-g without floating out of your seat.”

Urie: Some of it is. It is mostly going to be seat of the pants. Just down across the runway and down the road is the Civil Aerospace Medical Institute (CAMI). It is an FAA operation. They are in the business of qualifying civilian astronauts. That is their charter. And they are MD’s and physiologists and so on. And they have done a lot of research on seats, human constraints, that sort of thing. They say if we have our seats tilted back just 30 degrees from the vertical, even in a fixed position like that, with our trajectory and our vehicle attitude in our trajectory, the g-vector will be going through the best place.

TSR: Takeoff is going to be?

Urie: Takeoff is just an airplane takeoff. Then you go to [7,000 m]. I think that is our current rocket start altitude. And then in the horizontal direction, you are in horizontal flight. You start the rocket, and then do the pull up. See that way, when you are up at a high angle, you have plenty of thrust. You do not want to do that on the jet engines because it just, you just do not have enough thrust. You are going to stall.

So you start the rocket. And so you get an axial—an eyeballs-in—g load of about a little less than two g’s. You are still full of LOx. Then you get three plus at the end of the burn when you are in vertical flight. That’s all eyeballs-in. In the mean time, you’ve done a three-g pullup. That’s all seat of the pants.

So you’ve got eyeballs-in plus seat of the pants. And the total is about three at the beginning. So the total g’s stay at about three all the way up. But by the time you reach the end of the burn it’s all eyeballs-in.

The vortex engine

TSR: Tell me a little about the vortex engine. Is that a solid?

Urie: No. It exists in two forms. There is a liquid version that we were baselining. I always say “were” because in my experience, engines are always late. Sometimes they do not show up at all.

TSR: [This from the man who championed the linear aerospike.] So you have got some backup plans.

Urie: Vortex is available in either a liquid or a hybrid, and the liquid is the baseline. And in that case, we use a fuel-cooled nozzle and throat (with kerosene). And the vortex itself is best described on Orbitec’s website. They introduce the LOx at the exit end of the combustion chamber in the wall, the perimeter of the chamber wall. At about eight different locations little jets that bring the LOx in with sufficient momentum that it maintains angular momentum all the way to the top of the chamber. So they have a rotating sheet of liquid oxygen along the wall up to the top. Well, then it has nowhere to go except inward. Increases its angular velocity because it is losing radius.

TSR: Top in this case is towards the plane.

Urie: The top of the chamber is towards the pointy end of the plane.

TSR: Right and the bottom is towards the—

Urie: Toward tail end of the plane.

TSR: Where the exhaust comes out.

Urie: Right. It comes from the tail end and works it way up uncombusting liquid. Then converges and increases velocity. You know, conservation of momentum forces the velocity to increase. Then you have your injector for the fuel. It is kind of like a showerhead for the kerosene.

TSR: So how does the igniter work?

In my experience, engines are always late. Sometimes they do not show up at all.

Urie: The igniter is a separate chamber. There is an ignition sequence. You start a jet with gaseous oxygen and follow it with liquid. And you have a combustion igniter with a hot jet chamber. You have got a spark plug and a burn chamber. A lot like a diesel engine. And then the hot jet goes into the combustion chamber. Up at the top you are spraying in the kerosene, which is meeting this converging spiral of LOx, which of course is now gasified. And you have a combustion vortex, a second vortex which, maintaining that same direction of momentum, now descends towards the nozzle. And so you have this hot column of combusting materials headed down toward the nozzle and out the nozzle, while this vortex sheet of LOx keeps the walls cool. It has got a single wall. It comes up the outside, heads to the center, mixes with the fuel, burns, then descends again. There is an ever so slight amount of rotation in the gas coming out of the nozzle.

TSR: You have them counter-rotating? Do have one or two?

Urie: Just one. If we had two, it would be easily counter-rotated. It is not so much torque that a control system will not take it out. For most of our rocket burn we have enough aerodynamic authority to do it with a control system.

TSR: I know with the SpaceShipOne they had that spin, so you guys are going to be countering even a little bit once you lose your aerodynamic surfaces you have to get rid of it.

Urie: They had an interval between loss of aerodynamic authority and when they turn on their reaction control system.

TSR: Oops.

Urie: And so they probably got some kind of a small residual moment out of the aerodynamic phase when the air was gone. It was an inertial coupling. It just started to precess. That is where they probably had a little bit of yaw into a roll when there was no damping. He did not have [reaction control system (RCS)] yet. So then he configured to the mode, his reentry mode essentially, and accessed his reaction controls and recovered from the roll.

TSR: Sounds like the plot of [the movie] X-15.

Urie: Well, those things happen.

TSR: So I asked Mitchell Burnside Clapp this (See “X-15 and today’s space planes”, The Space Review, August 9, 2004) He said a no-light is no big deal. Because you just keep going with the jet engine.

Urie: You just fly home.

TSR: Would you fly home or retry a few times?

Urie: No, you would fly home. Anything does not work, you come home. There is no fail operation case on this. If something does not work, you go home.

TSR: The fail operation is immediately go home.

Urie: Yes.

TSR: So, what kind fail-safes do you have built in?

Urie: Well it has got redundancy on all vital systems. The structures will be designed to meet FAA standards, which have enough margin, safety factors and so on to give you that kind of reliability. So it is a matter of redundancy, factors of safety and safety margins. Pretty much an aircraft approach.