Balloon Flights and Countermeasures

Since posting Ref. 1,  I have heard a lot of nonsense in the media and on the internet about “steering” balloon vehicles.  Since the first manned hot-air balloon flights in the 1700’s,  people have tried and failed to actively maneuver round balloons with various steering surfaces and propellers.  It requires an elongated shape,  in order to reduce the drag enough to make that work.  We call them “airships”,  “blimps”,  and “dirigibles”.  They work well,  as long as the wind is not too high or the weather too bad.

Round balloons have always gone,  and apparently will always go,  only where the wind blows them.  The only thing you can do with a round balloon is change its altitude,  looking for winds going in the direction you desire.  Even if you can do that,  it is still very imprecise steering,  at best. 

The recent revelation that the Chinese launched a spy balloon to fly over America,  got me to looking at how such a flight across the Pacific might be made,  using the prevailing winds (really,  the jet streams).  As it turns out,  this same means was tried by the Japanese military during World War 2,  to transport incendiary and explosive bombs by balloon to the continental US. 

Figure 1 shows information about the jet stream winds,  obtained (as the figure says) from Wikipedia.  In the northern hemisphere,  there is the polar jet,  and there is the subtropical jet.  These flow with varying speeds,  usually fairly strongly,  and at different altitudes for the two jet streams.  Paths are quite variable,  but some basic trends are shown.  There are some other winds up in the stratosphere,  but these are weaker,  and not very predictable,  there being no particular pattern to them.   The subtropical jet would be the way from east Asia to North America,  but with enormous uncertainty! 

Figure 1 – Basic Information About The Jet Streams,  Particularly Over the Pacific

During World War 2,  the Japanese launched a lot of “Fu Go” balloon bombs from Honshu island,  some of which actually made it to Canada,  the US,  and a very few to Mexico.  Information about these is given in Figure 2,  again obtained from Wikipedia,  as the figure says.  The bulk of these were the paper Type A flown by the Imperial Japanese Army.  Payloads varied somewhat,  but the main intent was to start forest fires.  Not many actually reached North America,  and little damage was actually done.  Fear was the greatest result.  They used hydrogen as the lifting gas.

Figure 2 – About the Japanese “Fu Go” Balloon Bombs of World War 2

Balloon-borne spy equipment would be a cheap alternative to launching spy satellites.  If photography were the goal,  smaller,  cheaper,  lighter camera optics are suitable,  since the range camera-to-target is only a few miles,  not several hundred miles.  A satellite-borne camera must have large,  heavy,  and expensive folded-path optics to serve that function.  That kind of satellite is what set the size of the Space Shuttle cargo bay.  Note that targeting precision is required to make spy photography worthwhile,  and balloons cannot deliver that,  except by the merest chance.

If intercepting communications were the goal,  then one would fly a set of antennas,  each capable of a particular radio frequency band,  along with some sort of receiving equipment,  some means of transmitting the information for its recovery,  and a power supply for this stuff.  The lower range-to-target makes this type of communications intercept a lot more feasible than a satellite-based form,  particularly if the lower-power networks are part of your goal.  The purpose might even be to support cyber warfare,  among many other ends.  For communications interception,  targeting precision is not required,  since the ”target” is quite diffusely spread about.  Balloons could well serve that function.

While balloon vehicles have long been considered obsolete as a military technology,  the more recent introduction of stealth to military systems may actually help provoke a reprisal of balloon technology for spy purposes (and we’ve learned a lot since the Civil War).  Stealth coupled with the shorter-range effects of atmospheric flight may offer opportunities to collect communications intercept information that is not so easily obtained otherwise.  This may in fact be why the Chinese have done this. 

If constructed in the right way,  a balloon can actually be quite stealthy.  The modern gas bag is clear non-metallized polymer film,  transparent to radar,  and able to show no infrared signature,  merely by soaking out cold.  There’s no optical signature on a cloudy day or at night;  it only stands out against a dark sky on a clear bright day.  You only see it if you happen to be looking in that direction.

It is the balloon payload that will have the radar and maybe the infrared signature,  and even then not very much,  simply because it has to be so small in comparison to the gas bag size,  in order to fly very high.  Optically,  there is little payload signature for the same reason:  small size.

Altitude control for lifting-gas balloons is by venting lifting gas or by adding new lifting gas from on-board reserves.  Or it can be by dropping selected ballast.  Or by using both methods.  Until recently,  helium has been the usual lifting gas,  but flammable hydrogen is the stronger lifting gas.  Any degradation of the gas bag by the effects of hydrogen exposure is of little concern for a one-way spy flight,  as long as it takes several days to have effect.   The greater lifting power of hydrogen confers the capability of reaching higher altitudes,  with a given set of design proportions.   See Figure 3.

Figure 3 – The Why and How of a Spy Balloon

You must traverse the Pacific at an altitude low enough to catch the subtropical jet,  to have a practical travel time to North America.  Once you reach North America,  you need to rise to an altitude that is relatively invulnerable to any countermeasures against it.  Spy balloons are subject to the same international customs as spy planes.  Such intruders may be,  and often are,  shot down,  if territorial airspace is violated.  Such are not to be shot down until such territorial airspace actually is violated.

Over the last 30-40 years,  we have fielded fighter aircraft with service ceilings in the 58,000-65,000 foot range.  If an infrared-guided missile is to be used to shoot down the balloon,  the fighter has to get within about a mile or two of the balloon’s position,  or the seeker cannot lock on,  if it can at all.

If a radar-guided missile is to be used,  the seeker lock-on range is significantly longer at a single handful of miles,  but there is less likelihood of a radar signature large enough for the seeker to acquire at all,  especially as that range increases. 

For a gun attack,  the fighter needs to be co-altitude,  and at a real “up-close-and-personal” range under 1000 yards.  If some sort of dogfight-style air-to-air laser is to be used,  the co-altitude and short range requirements relax a little,  but only a little!  There are some lasers that can aim from much longer range and lower altitudes (even from the surface),  but the beam guidance equipment required is still rather large and heavy.   That’s why very little of those laser technologies are yet fielded.

The Chinese spy balloon was flying somewhere near 60,000-66,000 feet,  if the news reports are accurate.  The F-22 that shot it down was reportedly flying at 58,000 feet,  but has a service ceiling of 65,000 feet,  per Wikipedia.  The Sidewinder infrared-guided air-to-air missile that it reportedly used has a seeker lock-on range of about 2 miles,   or maybe 3 miles at most.  Had that Chinese balloon been flying above about 70,000-75,000 feet,  the F-22 could not have shot it down with a Sidewinder. 

We still have a few U-2 aircraft flying,  mostly the TR-2 variant,  but these are unarmed.  They can reach altitudes around 80,000 feet,  although adding armament would likely lower that.  There was an interceptor version of the SR-71 spy plane called the YF-12A,  which was armed.  Those could fly around Mach 3 at around 85,000 feet,  like the SR-71.  But all of those high-speed craft are long-retired now.   

Basically,  what that says is that if the Chinese spy balloon had been flying up nearer 70-75,000 feet,  we simply could not have shot it down.  If we were to arm a U-2,  then all the Chinese need do to counter that capability,  is to fly up nearer 100,000 feet.  Even if we pulled a YF-12A out of retirement,  it could not reach a balloon flying at or above 100,000 feet. 

Can a balloon fly that high with a significant payload?  Yes!  The third Project Excelsior flight in 1960 carried a gondola with a space-suited man (then-Captain Joe Kittinger,  USAF) to about 103,000 feet on its third mission,  from which he tested bail-out procedures and equipment.  The lifting gas was helium.  Kittinger set a long-standing record for high-altitude free-fall parachute jumps on that third flight.

In more recent years,  there have been two civilian free-fall parachute-jump flights that far exceeded Kittinger’s record.  Both were from manned gondola-bearing balloons,  at around 130,000 feet.  Again,  the lifting gases were helium.  You can fly even higher with hydrogen.  See Figure 4. 

A few of the old experimental rocket X-planes could reach such altitudes,  near or above 100,000 feet,  but they required hours-to-days to prepare for launch,  and were dropped from old bombers that served as carrier planes.  Fast response is a requirement here.

There was one trainer plane (intended for training space plane pilots) that routinely flew to almost 130,000 feet,  and with a fast response (and no carrier plane).  That was the mixed-propulsion NF-104,  which was an F-104A “Starfighter” modified with attitude thrusters,  plus a rocket engine in the base of the vertical tail.  It flew as a normal jet aircraft until pulling up for the steep “zoom” ascent.  The rocket engine powered that “zoom” ascent.  The jet engine had to be windmill-restarted on the way back down.  Ref. 2 has more information about the NF-104.

Figure 4 – What Altitudes Would Be Relevant

From the viewpoint of the spy balloon builder,  you need a balloon that flies across the Pacific at around 30,000 feet in the subtropical jet.  At just the right time of year,  that will take you near the Aleutians,  into Alaska,  western Canada,  and the northwestern US.  From there it crosses the lower 48 towards the Atlantic.  Although,  “exactly where” is uncertain by at least several hundred miles!

You want to raise the altitude above about 75,000 feet over Alaska and Canada,  and on up to around 130,000 feet or more,  over the continental US.  You can track the balloon by satellites,  and change its altitude on command,  again by satellite,  a capability the Japanese did not have during World War 2.   Your range to the communications targets is about 24 miles vertically,  and closer to 36 miles at 45 degrees to either side of your ground track. You retrieve your data and send it to the satellites overhead,  and they send it “home”.

All of this is indicated on the left side of Figure 5. 

From the viewpoint of the people trying to intercept and down these things,   you need a fighter craft capable of zooming up well beyond 130,000 feet,  at least briefly,  and it needs an appropriate set of weapons to use against the balloon.  Anticipating a sort of arms race in balloon altitudes,  I’d recommend at least 150,000 foot initial capability.

You have two sub-missions:  (1) identify and evaluate the threat of the balloon,  and (2) if it really is a threat,  shoot it down.  The weapons could be missiles,  guns,  or lasers.  Those two sub-missions could be two flights by two separate aircraft.  If a long-range air-to-air laser is used,  the shoot-down flight does not really need the high-altitude “zoom” capability.  But it does need to be a fast aircraft,  in order to obtain fast response.   Otherwise,  your best bet is guns,  and maybe infrared-guided missiles,  on the “zoom” aircraft.

Seeker lock-on being an “iffy” problem due to the inherent stealth,  I’d recommend using guns.  But I would modify the ammunition:  from projectiles to scatter-shot shells.  That offers more holes in the gas bag per hit,  with a much lower risk of collateral damage on the ground from your ammunition falling back to Earth.   Scattershot falls a lot more slowly than projectiles,  and each particle is much smaller.

All of this is given in the right half of Figure 5.  

Figure 5 – Mission Characteristics for the Balloon and Countermeasures Against It

In Ref. 1,  I ran some rough-sizing numbers for modifying an F-16C to the mixed-propulsion form needed for “zoom” missions like these,  and for sizing the rocket engines that need to be added.  There are plenty of these planes available in the inventory.  As jet aircraft,  they have far superior flying and handling characteristics,  compared to those of the old F-104A.  Being a far more modern design,  the flight control hydraulics should not fail with engine stoppage,  the way they did in the old F-104A.   That would eliminate a very serious failure mode experienced long ago with the NF-104.

In the F-16’s,  there is a 20 mm gun in the left wing root,  and Sidewinders get carried on the wingtips.  I doubt there is enough signature from the balloon payload for a radar-guided missile to lock onto,  so removing the on-board radar and deleting those missiles is the way to get the weight allowance for the attitude thruster modifications and the rocket propellant tanks.  That propellant is carried in tanks on the inboard underwing pylons,  near the center of gravity.  Doing it that way makes the weight-and-balance problem with the modified F-16C far less severe. 

References

#1. G. W. Johnson,  “Thoughts On the Chinese Spy Balloon”,  posted 5 February 2023,  on http://exrocketman.blogspot.com

#2. G. W. Johnson,  “Early High-Speed Experimental Planes”,  posted 3 July 2022,  on http://exrocketman.blogspot.com

To find articles on this site,  you can scroll down,  but the navigation tool left side of page is faster,  especially if it is an older post that you want.  You need the date and title.  Click on the year,  then on the month,  then on the title if there was more than one article posted that month.