Making a boom, before Boom Aerospace - Messing around with breaking mach.

In light of the "beat Streaga" thread, and seeing far to much stuff on Concorde lately, I thought I might bring this subject back to the table.

Well, at least some variant of the subject: Breaking Mach, in a homebuilt airplane.

So how would it be done? How would you do it? The only limit I want to put on this, is we need to talk about ~available powerplants~. And we're not going to talk about buying used military aircraft.

To cut to the chase, you can break mach with a balloon and a pressure suit. EG: Felix Bomgartner jumping from 70,000'. But that's not exactly controlled flight.

The BD-10 was supposed to be the answer, but I can't find any record of one actually breaking mach, and they seemed to like to shed control surfaces.

Serious Options

So lets talk about how this could be done. I'm going to start with powerplants.

Powerplants:

Somehow, we need to get this plane up to altitude. And unless we're just doing a stunt, we want to sustain the speed for a while. If "just hitting mach" was the goal, maybe just climb and dive?

Props:

Since most of us have familiarity with props, lets start there. Doing some research, shows that the XF-85 Thunderscreach hit mach .83. There's reports of at least one spitfire hitting mach .95 in a dive. And the TU-95 Bear flies in that ballpark too.

I'm not sure anyone, other than boaters, have really considered airfoils for use above the speed of sound in their working fluid. Would wedge type blades work? Would it help when prop tip speeds are over mach? Would it reduce drag?

Or is going with a propeller just nonsense. Between trying to deal with high prop speeds, interference drag from the prop, and piss poor prop efficiency at low speeds? Maybe that's just crazy.

Now.. someone was trying to break mach with a prop, but the machbuster project was dead in the 90's.

Ducted Fans:

So, ducted fans have the advantage of not having all those tip losses. And a good duct design can keep the airflow through the fan under mach, even when the airframe is above mach. The fan could be used to drive engine cooling as well. However there's a big weight penalty from all that ductwork. And wetted surface drag. Sadly, I don't have the math handy to calculate a ducts efficency. Does anyone want to tackle that? I'm really curious.

Jet engines:

Well, why not? Small jet engines are available on the market. They produce quite a bit of thrust. As I understand it, the magic to going over mach is to get the intake just right. Allowing for good pressure recovery, and low losses, no matter the airflow speed.

Could.. a small plane get enough thrust from say, the engines that Sonex sells? Or would we need to look at "real" engines?

This is the conventional answer, and probably the easiest of the bunch. There's the least guessing using an actual jet. I think that's the direction I'd go in.

Airframe Materials:

I'd love to hear your thoughts on this. I.. have no good answers, but I have a heavy preference for aluminum.

Retracts:

I think that anything that might want to break mach, would need retracts. This starts to get complicated, because all of the "usual" methods for dealing with retracts start to have real issues when we start talking thin wings for low drag, and then having a fuselage that wants to have air ducts in it, instead of space for wheel bays. This has lead to some really oddball solutions on military aircraft. Such as outword retracting gear, specialized thin wheels and tires. Wheels that pivot at the end of the landing gear so they can sit upright in the fuselage. Tiny, but tough tires.

... I suppose that's best re-examined after the wings.

Wings:

So what to do about wings? They're definitely not going to be wood. The plane, should still manage a 60-70kt landing speed, so we'll need a lot of wing area, or lift adding devices. Frontal area, starts to matter more than airfoil as you go faster, so the plane should have some very slim wings.

Delta? Straight wing? Swept wing? Tailed Delta? I have a thing for tailed deltas (aka, Mig21)

Fuselage:

Now things get interesting. Essentially all popular home-built designs have room for two, ideally two, and luggage. Frontal area matters monumentally when it comes to flying faster than mach, so this means we're talking a very cozy side by side, or tandem seating.

What's it take to build a good, smooth duct? Should that be fiberglass? Is area rule going to be a concern?

Maybe just aim for a scaled down copy of a F104, or F106 and be done with it?

Mission:

Because it's important, why would someone want this. First, bragging rights. Second, time compression, sure you can't actually break mach over most of the US, but if you go offshore a bit, you could make some quick trips up and down coasts. It also looks like if you can get the thing above 45,000' you can go ahead and do some machbusting. At least according to this quora article: https://www.quora.com/Are-there-speed-limits-for-aircraft-civilian-or-military

That also means building for high altitude flight. 45kfeet means pressurization, and reasonable cabin air conditioning.

I believe I just talked myself out of this ~really cool~ idea. But... I'd like to hear your thoughts. and it would definitely give Strega a run for it's money.

 

You and the wife from LA to Hawaii in 3.5hrs makes weekend getaways pretty exciting......

 

Well, that adds a range thing to the equation. What's the minimum there? Say, 3000 mile range, to cover for weather and delays at either end? That said, at 1000 miles an hour, that's just 3 hours of flight time. Hah!

I'd start with the minimum mach airplane, and stretch the airframe to meet the fuel requirements.

.... Drop tanks aren't kosher on civillian planes, are they?

 

https://en.wikipedia.org/wiki/Bede_BD-10

Sounds dangerous......

 

SubSonex VVLJ jet aircraft from Sonex Aircraft could be a start. See the video here:
https://www.youtube.com/watch?v=t7uTcfNqaaE

 

Yeah, to say it had problems is an understatement. A couple of them flew, a couple also crashed. It's not a good ratio.

That design screams "less than mach" for a whole bunch of reasons. There are things that could be stolen... the landing gear setup is clever. But most of that airframe starts screaming in pain as you approach mach. Interference drag and area rule are all screwed up for any attempt at going ~fast~.

 

Interesting topic, sounds like a kickass funn project! I watched the Machbuster come to naught, and had a close look at two BD10, and here is this pundit's response:

...Basically a CEO-seeking missile. Though way too chunky for a single non-AB CJ610/J85 to push through M1, the approach and basic design was reasonable for a personal jet runabout. Pity Bede couldn't focus enough to get the details right, because details count.

CJ610/J85, I think Bede was right about that. Except with some sort of afterburner. You buy an old Lear 24, and you have one to burn and one to turn.

Total non-starter. You'd have to innovate to do that, and innovation along those lines takes testing and development in the $ seven-digit territory. Easier to just buy lots of kerosene.

How about a ducted non-fan? This might actually be a reasonable application for a simple ramjet, once you get going.

I'd probably start with aluminum wings and molded carbon fuselage. The wings might go either way, but female molded carbon would probably be the easiest way to reconcile the cockpit volume and shape requirements with necessary ducting and area ruling.

Probably indispensable. Though for a one-and-done project you could probably launch from a jettisoned dolly and land on a skid ala Me163. But operationally that would be a real pain.

To get through M1, you need a minimum of frontal and wetted area. With a J85, you'd probably end up with barely room for a single 5'8" pilot in a steeply reclined couch. No baggage.

--Bob K.

 

So lets throw some "useful information" into the thread.

https://en.wikipedia.org/wiki/General_Electric_J85

Something between 2500 and 3500lb of thrust, and 400gph at sea level, with 100gph at cruise power. Whatever that might be...

An hour at full power, would be 2800lbs of fuel.

https://en.wikipedia.org/wiki/General_Electric_CJ610

Here's the "more civil" version of the engine. ~3000lbs thrust.

"Specific fuel consumption: 0.96 - 0.97 lb/(lbf·hr) ( 97.89 - 98.91 kg/kN/hr )"

What other engines might be sane to look at?

 

Given that this is planned as a homebuilt I would suggest that following points

Limited engineering time means the calculations will need to be conservative, hence one needs to accept that there will be quite a bit of fat in the design, so start with a slightly oversized payload.

Small engines mean it is a flying fuel leak, thus a large fuel fraction would be required. External fuel tanks are of low added value for this type of config.

High landing speed & secondhand engines would tend to drive one to twin engine config

Small thin wings i.e BD10 are going to consume lots of engineering time and have little fuel volume.

Pure Deltas tend to have very high landing speeds and long landing lengths.

So to me that would suggest a tailed delta to get sufficient fuel volume and reasonable airframe stiffness, that would however require one to still address the wing drop issue of a delta (unless one goes for a YF23 type config). A civilian airframe capable of supercruise could be very cool.

As for engines and airframe parts this place is always interesting http://www.everettaero.com/

 

T-38. Excellent safety record, great performance. The F-5 version is highly under rated as a combat plane, except for the Top Gun instructors and the Air Forces that use them.

A local Mig-21 pilot told me he's basically in a fuel emergency on take off. It only takes minutes to fly from Buffalo to Rochester NY, but he doesn't take off until he has a dead solid landing time assigned to him.

Drop tanks are fine, unless you drop them. I'll point out that many airplanes, like the Douglas A-4, fly with drop tanks all the time, and only drop them when they are engaged in combat and need the additional maneuverability to evade SAMs or Migs. The Skyhawk also drops it's tanks when empty if they are on a nearly one way run, like a LABS strike.

If you want real inspiration for design excellence, the A-4 Skyhawk is a good choice. The proposed nuclear capable Carrier bomber was expected to be huge. By utterly ruthless attention to weight & detail, the Skyhawk came in TONS lighter than expected, and is still an active combat plane.

A few details, make a difference. Wing small enough it didn't need folding mechanisms to fit on the elevator or hanger. Landing gear attached to the main spar.... everything attached to the main spar.... tailed delta to reduce trim drag....wet wing.... and a cockpit you have to turn your shoulders when lowering the canopy, so it doesn't hit you.

The Skyhawk is not supersonic except in a dive. Just like a F-86.

http://www.airbum.com/pireps/PirepA4.html

 

One minor issue is that non-governmental aircraft can't go supersonic over US territory, at least not over anyplace with people. NASA is working on sonic-boom-free supersonic aircraft (they tend to be very slender, e.g. 100 m long and 3 m diameter). Of course, you could make a homebuilt Busemann biplane.

On airfoils, to NeroBro (https://www.homebuiltairplanes.com/forums/showthread.php?t=28728&p=408289&viewfull=1#post408289). Boaters don't worry about supersonic flow because the speed of sound in water, at about 1500 m/s (http://hypertextbook.com/facts/2000/NickyDu.shtml) is much higher than it is in air, at about 350 m/s. What marine engineers worry about is cavitation; the wedge-shaped profiles (I can't use airfoils; they're not designed for air. Hydrofoils is used for something completely different) are designed to control cavitation.

 

The drop tanks on the A4 were also great in the event of failing to extend the gear. My understanding was the A4 would only go supersonic in a dive if it was not carrying any external stores.

 

NACA looked into supersonic prop planes during WWII, before they were told about jet engines. A supersonic homebuilt is going to be a challenge. I suspect that purloining some early designs would be useful.
Propulsion. What about a solid fuel ramrocket? A rocket provides takeoff and acceleration up to ramjetting speed. Solid fuels can be very cheap, so an atrocious consumption wouldn't hurt so much. polythene is one fuel. You just need to offer free shopping bag recycling at supermarkets!

So, my suggestion is something like a Lippisch P.13

 

Just a guess, but in simple(?) home building design and construction techniques I think the answer would look a lot like a scaled down Northrop F-20 Tigershark. Basically a pilot, a huge engine, just enough wing to hold it in the air plus enough tail to keep the pointy end forward, then fill whatever space is left with fuel.

 

If you had a mothership to launch then not much wings would be needed.
Might need a dry lake bed to land on skids.

 

I'd really like it to be a "practical" plane. I define that as something that can be parked on an apron, be fueled by the fuel service, self started, taxied and flown away. Ideally for a very fast $100 hamburger. And back. For the plane to "get used" and have a resale market, it needs a second seat. If we're just talking "drop off a plane, go mach, recover" things get a whole lot more simple.

Ok, it feels nice when "everyone else" has the same sort of ideas. Tailed delta looks to be the way to go. Especially when it comes to stability going through mach.

I feel like long ducts are a bad idea. So the intake in the nose is a no go. It also ends up making life harder for variable geometry intakes.

 

You're going to need to add at least another zero to that number...

To a point. But too short an intake causes problems too--see the F-111A through E models. With a nose intake you at least have clean air ahead of you and the variable geometry actually isn't that bad--use a translating cone like the MiG-21 and SR-71.

Then again, a properly-designed fixed intake will do you well enough below M1.6-1.8 or so.

A big part of your trick is going to be finding an appropriate engine. For the mission you're looking for--two people, plus an overnight bag, and a useful range of a few hundred miles, you're well beyond the TJ-100 engine range. You may also be subject to the engine noise limits, I don't know.


I, too, dream of such an aircraft. But I think there's a substantially larger commercial market (note, not large in an absolute sense) for a sufficiently-pointy ducted fan or pusher airplane that is reasonably fast (RV speeds or greater) and looks really fast.

 

This pointy enough?

The propeller duct was removed to increase the speed. There also was a two seat version. Turned out that not many kits were sold.


BJC

 

Since high AOA isn't really required for this profile, one could use a F107 type intake which would also greatly improve the FOD resistance to the point of being able to operate off grass. Would also make the gear layout much easier, noting that SAAB didn't use an F16 type intake as they couldn't get the nose leg to fit in such a small airframe with out it being too far forward.

 

Easiest way to make a supersonic homebuilt is to buy a surplus supersonic aircraft.

The problem is that supersonic flight over the US is not permitted for non-governmental aircraft except in special circumstances, mostly involving special permits.

NASA is working on boom-free supersonic aircraft (more accurately, the aircraft would be configured so the booms would dissipate above the ground). These aircraft are very slender -- fineness ratios of about 30 -- and very carefully contoured. I think a lot of people designing supersonic business jets hope that bribery lobbying will get rid of the restriction on over-land supersonic flight.