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Modelling Rocket Planes with X-Plane

Posted by: Horuseye - Fri Jun 04, 2010 3:51 am
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Modelling Rocket Planes with X-Plane 
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Post Modelling Rocket Planes with X-Plane   Posted on: Fri Jun 04, 2010 3:51 am
I got myself X-Plane, the "... most comprehensive and powerful flight simulator for the personal computer, bar none..." a while back. I just recently got around to play with it. It is quite realistic and is more like an engeneering software than your regular flight sim. One of the niftier features is that you can customise any plane or even build a completely new one from scratch. The physics engine will automaticaly calculate aerodynamic model for every aircarft based on their shape in real time. Quite an achievement for a simple desktop sim.

When i was just a kid, I used to play "get in space on rocket plane" game armed with nothing more than overactive imagination. Having never really grown up, I jumped at the chance to recreate my childish fantasy with the help of a laptop, cheap joystick and a little rocket engeneering. The results were quite enlightening and I think would be of interest to any who are into the idea of space planes. So I decided to share it with you people.

Where better to start the journey to the edge of space than with X-15; the quintecential rocket with wings and the current holder of speed record for atmospheric aircraft? Build with early 60'es technology it could reach Mach 6.7, fly above atmosphere and explode only once in two hundred flights. There hasn't been quite anything like it untill Spaceship One came along. Naturally I couldn't be bothered to read the manual (even if there was one) before I jumped into the cockpit and jammed the throttle to the max. 'Course if I did, I would've known that X-15 wasn't designed for runway takeoff, oh well. After half a dozen spectacular runway explosions I decided to modify my winged rocket.

To start with I didn't do anything major, just gave it proper landing gear and removed that stupid vertical fin. Oh, and I also changed from 240 Isp hydrazine or ammonia or whaterver rocket engines to 360 Isp (I was rounding off) Kerosine/Oxi fuel mix, I mean we do live in twenty-first century and there is only so far you can take the retro tech. After another half a dozen DOA launches I've found that you need to go surpersonic just for the takeoff and this is bad because:
A. You need Area 51 runway, because it is the only one long enough to get to these speeds even at 2g accelleration.
B. The wing flaps keeps falling off for some reason. Not that it matters much, you can take off without flaps just fine.
C. The tires get blown from over speeds. I bet all of you rocket scientists didn't think about a little thing like that raining on your parade.

One I got these nagging little issues sorted, I went in ta space! I tell you flying a rocket plane is quite a rush, I spent the whole weekend flying X-15 under all kinds of scenarios. Nothing like flying through the thunderstorm at Mach 3 or burning up on reentry at Mach 12. He heh...

Okay so the main things that I learnt are that while rocket planes can get to insane speeds real fast, you got to watch the fuel real close, it takes half the fuel tank just for the take off and to gain a decent altitude. Another thing is that if you want to go far and fast you gotta gain high altitude, the higher the better, because air resistance eats A LOT into your fuel consumption, 100 kilo ft would be ideal but I havent yet been able to design a wing configuration that would get enough lift at Mach 3-5. The best I've been able to do is cruise at Mach 3 at 75 kft altitude (25 km above sea level). When flying at hypersonic speeds the atmosphere seems two dimensional, you are flying at a very narrow flight envelope between dence lower atmosphere and airless space, you can't execute the majority of vertical manuvers that other aircraft take for granted. Turning around can also be a problem, even at high-G turn it takes minutes to do a 180 and you loose a lot of energy while doing so. And the last most important thing is the atmospheric heating, surface temperature routinely reaches 900C and even at cruising speeds it hovers around 600-700C. If you want to get around the world at Mach 4, you would need to withstand these kinds temperatures for hours. Thats an afully long time to spend in an oven.

Anyway I experimented with several versions of X-15 and all of them have little diffiulty in reaching Mach 6-7 from runway. If you put a rocket playload on it, only 6km/s Delta V is required to get into orbit. That's mass ratio of 5.5 with proven safe Kerosine/Oxi engine or one kilo of playload for 9 kilos of rocket. Thats one order of magnitude better than the convetional rockets and therefore an order of magnitude cheaper too. The best thing is that you don't need any new technology to achive this. You only need a 60'es tech rocket plane and off the shelf ballistic missile to lauch a few hundred kilos of playload into LEO at the tenth the cost of current prices. X-15 flew two hundred times and was lost only once, Spaceship One could be refurbished to launch every two weeks or so. There is no reason a commercial rocket plane couldn't match and exceed these numbers.

Given the off the shelf technology requirements, the large potential demand for the service (low playload mass – more launches) and order of magnitude cost reductions of playload delivery, I think it is quite possible to create a profitable commercial orbital launch delivery service with a reasonably small start up capital on the scale of Space Ship One project. What are the people's thoughts on these matters?

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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Sat Jun 05, 2010 2:10 pm
First, welcome to the forum, and nice writeup! I laughed my head off at your description of take-off :).

I have to wonder a bit about the masses involved. You mention changing the rocket fuel and oxidiser to HC/LOX, but that would also imply the addition of cryogenic tanks and cooling systems. Did you take that weight increase into account? Same thing for the second stage, can you get your X-15 to 6 km/s with the second stage attached? I suppose it would also add drag.

As for rocket planes in general, the key issue seems to me whether the extra lift is worth it compared to the extra drag. I'd think that the extra lift mostly counts at low speeds and altitudes, whereas the drag would mainly be a problem at high speeds and altitudes. Will X-Plane let you launch a ballistic missile and fly along in a winged rocket and compare?

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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Mon Jun 07, 2010 12:37 am
Thanks for the Welcome! To tell you the truth I've been here before, the recent events with NASA promted me to come back after a long hiatus, it seems my profile got cleaned out.

To make things easier, I'll just list all the variables I've used and the assumptions behind them.

I gave the fuselage air drag coefficient of 0.05 which I think is a requirement for any hypersonic plane. The dry mass is 14,000 pd and fully loaded is 34,000 pounds. There are also two external fuel tanks 5,000 pds each (I do not truly know how much they hold in reality but they didn't look that big to me). At this stage I assume that dry mass includes the playload of I donno 5000 pds seems reasonable. There is one rocket engine that pulls 57,000 pd at sea levels and 70,000 at 60 kft/space (Okay, the original X-15 specs I got were more than a little doggy with the rocket engine being less efficient in space so I got my numbers off the Wiki)

The Specific Fuel Consumption is about 10 pounds per hour per pound of thrust which comes to about 350'sh seconds Specific Impuls. With the Liquid Oxi I assume passive boiloff cold storage. The engines will burn out after half-hour even at cruise thrust so there is no point in thick insulation or active refrigeration, plus the original X-15 DID use cryogenic fuels.

Overall I think that the mass ratio estimate is well within the proven tech, it is rather conservative, having been taken off an actual aircraft. Having said that I do not believe that the mass ratio can be improved much in a practical design.

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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Mon Jun 07, 2010 1:29 am
Answering your second question. There is no ballistic missile per see in X-Plane, BUT, I could easily take off the wings from X-15 and change dry mass/fuel ratio to a typical one stage rocket (two-stage if you take drop tanks) and launch it from airdrop. I've never would have come up with this kind of vandalism on my own, my thanks! I shall surely do so ASAP!

Now the question of Lift/Drag. At 75,000 ft altitude the air pressure is only about 3% atm, so ther is bugger all drag but there is still quite respectable lift. I was more interested in how feasible intercontinental ramjets were, and for this purpose 100,000 ft is the holy grail, I've been able to go at Mach 4 for 15min on half tank at this altitude, at 1/10th thrust. Which is just incredible for a rocket plane, I got all the way to Huston,Texas from Area 51! There is quite a lot of difference in fuel consumption between Mach 5 and Mach 4. Generally the extra speed ain't worth it if you going for distance.

Now the advantage rockets with wings have over simple rockets is not the lift, but the fact that they can return and land on airstrips. Personally I think that giving wings to spaceships with anything less than nuke engines is sheer idocity, hence I think that Space Shuttle retirement is the best thing that has happened to NASA in years. Your milege may vary, as always. I do not have the flight test result handy right now, but of the top off my head the theoretical max speed of X-15 with 350s Isp is abount Mach 9. I could get to Mach 5 after burnout off the runway and Mach 7 from airdrop without external tanks. As a rule 1 km/s is lost to air drag. Conventional rockets loose about the same to drag and gravity losses.

External tanks give about 1km/s extra in final speed, but the air drag is a BIG issue. The most efficient tragectory seems to use drop tanks to gain altitude at Mach 1 and burn out at 35-45 degrees climb. The fastest I could get was Mach 10.5 with airdrop and drop tanks on ballistic trajectory. I swear I got to Mach 12 with the older design that had bigger drop tanks. But I can categoricaly state that 6km/s is a pure fantasy unless you have nuclear engines at the back. The X-15 is already close to optimal design and I doubt that you can significantly imrproove on these figures.

I'll experiment some more and get back to you with result later on.

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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Tue Jun 08, 2010 2:52 pm
Shape your drop tanks into airfoils and use them for the climb thru the atmosphere. Drop them at high altitude where lift is less effective/necessary.


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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Thu Jun 17, 2010 12:13 am
All right then, I did some quick and dirty mod on X-15 to see how it flies as a ballistic missile. I removed the wings but left the control fins, changed dry mass to 5,000 pds and max fuel to 25,000 pds. I then air drop tested it, Mach 0.94 initial V at 45,000 feet, no drop tanks (aerodynamic stability issues).

With mass ratio of 5 I got to about Mach 18-19 (Mach 21.5 with drop tanks)

With mass ratio of 4 I got to Mach 16.5

With mass ratio of 3 I got to Mach 12.8

As a contrast with default X-15 (Isp=350s) air drop at same speed and altitude:

With no external tanks I got to Mach 8 (Mach 5 on ¾ Empty)

With external tanks I got to Mach 10.5 (Mach 7.3 on ¾ Empty)

With external tanks X-15 has mass ratio of 2 roughly (14,600 pds dry plus 19,400 pds fuel and two 5,000 pd fuel tanks)

Now you have to remember that by the time they got air dropped, they were already going at subsonic speed, so the air friction losses seem to be about Mach 1-2, you can also put another Mach 2 on top of that for gravity losses if taking off the runway.

The incredible thing to me is that once you get to Mach 2 it doesn't seem to matter if you have wings or not, you can fly around just fine. Your flight ceiling is 10 k lower and you wont be able to take off OR land on runway but otherwise simple control fins are quite enough. The fuselage alone seems to give a lot of lift, I feel if you mod it for flying body configuration (ala X-51) you won't even need conventional wings.

I am gonna write up a summary of what I think it all means for practical rocket plane designs and any engineering issues likely to come up.

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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Thu Jun 17, 2010 7:06 pm
Great work, and interesting too. So what about a wingless fly-back booster that flies back at Mach 2+ and then lands vertically? Could be interesting :-). I'm looking forward to your write-up.

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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Thu Jul 01, 2010 12:28 am
There are two similar but distinct applications for rocket planes. One is using them as first stage for orbital launches and another for long distance high speed air travel. Of the two the former is far easier to achieve than later as high Delta V is about the only requirement with safety and operations of secondary concern. On other hand long distance travel rocket plane must have not only long range and speed but also compete with established airline industry with stringent safety and flight schedule requirements. So I am going to limit my analysis to orbital playload delivery with hypersonic air travel as special case for future deliberation.

Commercial Spaceflight start up companies face one issue that overshadows all other concerns and places stringent limits on any design concepts for practical rocket plane. That is the issue of cost. Ideally the R&D budget should be under $20 million and overall capitalization for such risky venture is unlikely to exceed $100 million even with a proven business plan. While these are peanuts for established aerospace corporations, technological and bureaucratic stagnation as well as conflict of interest makes it unlikely that they would undertake such a project. This rules out development of any new technology, there had been many paper studies on advanced propulsion concepts and after decades of work and billions of dollars spent they've all come to naught.

The limited budget necessitates use of off-the-shelf technology. Thankfully basic rocket plane is a mature technology, we've been making them since World War II. There are now many amateur rocket planes as bored retired engineers build working V1 replicas in their spare time. Even a third world country can make space planes these days, all they have to do is take a Scud and bold some wings onto it.

A smaller aircraft would be cheaper to build and develop, however it must be large enough to carry a useful playload. Thus jet fighter sized aircraft would be ideal.

As computer simulations show there is some benifit in utilizing air drop technique to maximize delta V. However the factor two advantage in orbital playload is countered by having to design and build mothership aircraft with all the cost and maintance associated with it. All the while simple drop tanks are almost just as good.

The same logic applies to runway assisted take-off. While maglev runways and beamed propulsion are extrimely cool and show promise, they are also prohibitively expensive.

The number one issue I've found is that ideal flight profile at hypersonic speed is very different from subsonic velocities. At high speed minimizing air drag is the priority and wings are hardly needed. While at slow speeds high lift is needed. While jet fighters can somewhat compromise on the wing design, such is not an option for a practical space-plane. Some kind of variable wing is needed and this is likely to be the biggest R&D ticket as well as source of safety and maintenance problems.

Number two issue is the overheating due to air friction. During a routine flight the average outside temperature has been in mid 600 C range with peak of 1000 C. While it is not a major issue for infrequent orbital playload delivery, it becomes a serious maintenance and safety issue as number and frequency of missions increases. The catastrophic break up of X-15 on flight XXX demonstrates the folly of ignoring this issue. The problem gets worse as long distance travel is considered. Somehow I don't think it would be easy to deal with oven-like temperatures for hours on end. On short missions the problem can be solved via cryogenic fuel cooling, however this would necessitate installing coolant tubes throughout the airframe with corresponding cost in weight and complexity. I also wonder about the corrosive nature of liquid oxi if employed as coolant and the potential for catastrophic failure of such a system.

The third important issue I've identified is actually a complex of persistent problems with safe take-off and landing. The notorious blown tire is only one of these. As I've mentioned before landing gear tends to blow at overspeeds, that is to say – every time it is used. If there was a design that would work at say Mach 2 or even better at Mach 3 it would allow even a cruise missile concept to land safely, which would conveniently solve the numero uno issue above.

A good steam catapult or something similar can potentially save up to a quarter of fuel tank and shorten the runway length and increase top speed, which is good news all round. Arresting gear would also be nice on shorter runways.

If these problems are solved I see no reason why a practical space plane couldn't be made if there was a demand for it. I am gonna think about the human side of the problem for the next time.

Any comments, suggestions?

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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Thu Jul 01, 2010 9:12 am
What about detachable rather than variable geometry wings. Make landing more difficult of course....but makes launch (with a full fuel load) much easier. The wings could parachute back, (or perhaps, if designed right, could fly back)

It's a bit Thunderbirds....but any mileage?


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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Fri Jul 02, 2010 2:35 am
Well,.. that's different,.. if nothing else. If you had a sledge with wings... maybe maglev support... Yes I see how it can work, that's pretty ingenious really - you can bolt the external fuel tanks to the sledge and be able to recover those as the sledge lands. You only need to get to Mach 2 for cruise missile wings to work and drop tanks are more of a liability at higher speeds anyway. Yes I am starting to like that idea a lot. Not sure how I am going to model drop wings, but I am certain its possible, maybe I just make wings fragile and snap them off with a high g climb or something...

About the two stage flying carpet. Even crappy wings have lift to drag radio in the twenties, that means that you can fly twenty times futher with wings than without for the same fuel expenditure. That's the reason jet packs aren't very practical. Having said that, I've found that you expend half a fuel tank for just take - off and altitude gain, and when you drop back down air resistance slows you down to terminal velocity REAL FAST. So if you had a decent parachute I see no reason why it can't work. There is one caveat though, the rockets must be reasonably small. Parachutes don't work very well for something the size of a tank (they did however manage to parachute an APC so its not completely out of the question), also it is far easier to salvage something that you can put on a truck.

It might work on something like a Scud, there is plenty of those around. Might be able to launch a micro-sattellite with those on the cheap. I am gonna play around with X-plane to see how far you can get with vertical climb on engines alone.

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Post Re: Modelling Rocket Planes with X-Plane   Posted on: Fri Jul 02, 2010 9:04 am
Just realised that what I wrote didn't quite match what I had in mind - I meant that some of the wing would drop off, not all of it. So, for launch you have large wings with high lift, then as you get up to speed you jetison a large chunk, leaving small stub wings which are more than enough at high speed. Since you have no fuel left when landing you need less lift, so could get away with less wing on landing.

You could also use the wing as fuel storage, sort of like a winged first stage but actually part of the craft.

Or perhaps make the drops tanks lifting surfaces, which is sort of the same thing.


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