As can be seen in the comments section of the last post on this subject, David Perron remains skeptical of XCOR's ability to do a suborbital vehicle, let alone an orbital one. But Jeff Greason, head of the company, has provided a (characteristically) informed and polite response. It's nice to let someone from the company speak for them directly.

We certainly are well aware of the X-15 and similar vehicles. Some of our consultants are old Dryden retirees who were around in those days. Doing our mission requires a propellant fraction of about 70-75% by mass. The Xerus design is wet wing, so the volume available for propellant storage internally is a bit higher than might at first be apparent.

We don't release most of the details of what we're doing not because we don't know them, but because we're well aware that the details will change during development. We don't want to fall in to the all-too-common trap of feeling that we can't change things because of PR pressure.

Certainly when we are approached by a serious investment prospect, we go through a due diligence process where our estimates on vehicle performance, subsystem masses, etc. are challenged. We believe we have good answers for those questions.

To pick one example, if you look at the X-15 structural weight, the high heat load caused by lengthy maximum heating flights at relatively low altitude, combined with a heat-sink TPS approach, required a high structural weight just to keep from melting. Initial results for a more modern insulating TPS approach for suborbital vehicles look more promising.

As for going to orbit -- that's a lot further down the road. I think it's far more important for space startups to demonstrate *profit* than any particular level of performance. Private space development is seriously starved for capital -- and the way you improve that is by establishing a track record of investors making money by investing in space companies.

Lastly, we are *well* aware of the regulatory issues raised by one of the posters above. One of the reasons for doing the EZ-Rocket demonstrator was to put some urgency behind some of the regulatory development we and others are working with the FAA. Lots to do yet, but so far, so good.

[Tuesday morning update]

For those interested, Bruce Hoult, who manned the XCOR booth at Oshkosh, provides some additional detail, in a post to sci.space.policy last night, responding to another newsgroup member's (Mike Walsh) analysis of the Xerus.

The concept shown is a rocket airplane. A big advantage of a rocket airplane is that you don't need the complicated inlet system of an aircraft and don't have the corresponding weight of the turbo-machinery. The trade-off is that you really are not going to go very far before you run out of propellant.

The Xerus will have 3 - 4 minutes of fuel, not a terribly large increase in duration over the EZ-Rocket, but it will have rather more thrust (and fuel flow) to allow it to accelerate near-vertically to Mach 3.5 or so. It's not optimized for distance, but could probably manage on the order of 500 miles or so on a different (lower) trajectory designed to set up a supersonic glide from high altitude.

XCOR should be well qualified and prepared to build the rocket propulsion system. I believe they said it is a scale-up of the engine used on the EZ-Rocket.

Current thinking seems to be 4 x 2000 lb engines. That's four times larger than the EZ-Rocket engines. Note that XCOR has already built engines spanning more than a 20:1 range, and the same engineers built a 5000 lb engine at Rotary Rocket (and they own the rights to that work).

The drawing of the aircraft does not show much space for propellant storage. I assume it is pretty much all available.

It should be about 75% fuel at takeoff.

[He means propellant here--fuel plus oxidizer--ed]

I note that at least the first version has a pilot and one passenger. It is apparently not an X-Prize candidate.

The X-Prize is 1) a one-off, not a sustainable revenue stream like tourist rides, and 2) will probably have already been claimed by the time Xerus flies. In the event that it hasn't been claimed I'm assured that there will be a way to squeeze a 3rd person in. :-)

I would hazard a guess that the success of the concept depends on just how hard it is to design a supersonic aircraft that can make repeated trips out of and back into the atmosphere. The short travel time should limit thermal problems. I will guess that the heat sink of the vehicle will do the job.

The reentry speed is only about 1 km/second, so the heat load is about 1/50th of that for an orbital vehicle.

The drawing does not show any reaction control motors and I would guess that they would be needed to orient the craft for reentry.

XCOR already has 50 lb motors suitable for this application.

Just how much does it cost to develop this type of rocket airplane? My assumption would be more than required for a ballistic rocket. If I remember comments from XCOR people and some charts I saw earlier they were estimating $10 to $20 million.

The numbers I've been told are around $3m - $4m to get to flyable hardware, $10m to complete the test program necessary to get permission to fly paying passengers.

Mr. Greason later endorsed these comments.

Detailed remarks on the previous item in this thread. Thanks for the response, Jeff.

Thank you too, Rand.

Dave

Xcor's work is some of the most exciting stuff going on now, and their approach seems the most likely to work. Another proponent of the small and gradual approach is the work by John Carmac (sp?), who has been flying small vtvtl. rockets for a while now.

John Carmac's Armadillo Aerospace is an interesting venture as indicated.

http://www.armadilloaerospace.com/n.x/Armadillo/Home

VTVL craft reminiscent of a stripped down LEM.

John is the well off author of the computer game Doom.

Interesting. 8000 lbf of thrust is just what I used as a strawman exercise after I learned of the propellant loading fraction. Given the numbers that I admittedly made up out of nothing, Xerus would have an initial thrust-to-weight ratio of approximately 1 (not good for a rocket; excellent for an airplane) and a terminal axial acceleration of about 4g. I assumed a ton of inert mass, which is awfully small considering that about half of it is flesh (pilot plus 4).

After reviewing Bruce Hoult's comments, I have to say those assumptions seem to be consistent with a 3-4 minute burn at 8000 lb, depending on specific impulse assumptions.

It'd be interesting to know whether the engines are throttleable. From what I've read, it seems the X-15 folks probably gave away a lot in terms of propulsion flexibility when they required a throttleable main engine.

For reference, an F-15 has achieved an altitude of just over 30km, and it's an airbreather. And it did so in about 3.5 minutes. Given that the Xerus design seems to have an initial thrust-to-weight ratio similar to F-15 and the final ratio is of course much higher. A modified Foxbat purportedly reached 36 km with a thrust-to-weight of about 2.

At risk of repeating myself, this is great stuff and is exactly what I was looking for.

Dave Perron