A main battle tank's engine exists to move about forty-five tonnes of steel across broken ground. It is not a generator. Yet for a great many of the hours a tank spends deployed, generating electricity is the only thing that engine is being asked to do — and it does it badly.
On 21 September 2026, the Ministry of Defence signed a contract worth ₹586 crore with Accurate Industrial Controls Private Limited for Auxiliary Power Units for the Army's T-72 and T-90 tanks. The contract was signed in New Delhi in the presence of Defence Secretary Rajesh Kumar Singh. The Ministry describes the APU as an alternate source of electrical power for the fire control system, firing circuit, gun stabilisation system and other ancillaries, and notes that the units were developed under the Make-II route.
Both halves of that description — what the device does, and how it was procured — are worth understanding properly.
A tank is an electrical machine that happens to move
Consider what is drawing power inside a stationary T-90 in a forward position at night.
The thermal imaging sight is running, because it is the only way to see. The laser rangefinder is on standby. The ballistic computer is powered, holding the corrections it will apply when a target appears. The radios are on, and so is the battlefield management terminal. In a sealed tank the NBC overpressure system runs. The turret drive is live. And the gun stabilisation system is powered, because a stabilisation system that has been switched off cannot be relied upon at the instant it is needed.
None of this moves the tank. All of it consumes electricity continuously, and in quantities that a modern armoured vehicle's batteries cannot sustain indefinitely.
That leaves two choices. Either run the main engine to keep the batteries charged, or find another source.
Why idling the main engine is the wrong answer
Running a tank's main engine to make a few kilowatts of electricity is wasteful in four distinct ways, and each of them is examinable as a principle.
Fuel. A multi-fuel diesel in the thousand-horsepower class burns a substantial quantity of fuel per hour merely to keep itself turning over. A small dedicated generator producing the same electrical output consumes a small fraction of that. Over a long deployment the difference determines how far the formation's logistics chain has to stretch — and fuel resupply forward of the line is one of the most dangerous tasks in armoured warfare.
Engine wear. A large diesel run at very low load for extended periods suffers for it. Combustion temperatures stay below the design range, unburnt fuel and carbon deposit on cylinder walls and in the exhaust, and the glazing and fouling that result shorten the engine's life and degrade its performance when full power is actually required. Prolonged idling is not a neutral state for a diesel; it is a form of damage.
Signature. This is the operationally decisive one. A running engine produces two signatures that an enemy can detect. The acoustic signature carries, particularly at night and across open or hilly terrain. The thermal signature is worse: a hot exhaust plume is a bright, distinctive, unambiguous object to a thermal imager at considerable range. A tank that must run its engine to see is a tank that can be seen while seeing.
The crew. Noise, heat and vibration accumulate in a confined space over hours. Crew fatigue is not a comfort issue in armoured operations — it degrades gunnery, degrades judgement and degrades the reaction time on which the first shot depends.
An APU addresses all four. It is a small engine-generator set, sized for electrical output rather than propulsion, feeding the tank's electrical bus and keeping the batteries charged with the main engine shut down. The operating state it enables is called silent watch: the tank sits with its main engine off, its sights and computers alive, listening and watching, and producing very little noise and very little heat.
Foreign implementations give a sense of the scale involved. The American M1A2 in its current configuration carries an APU built around a small two-cylinder diesel driving a 28-volt generator, sized to sustain roughly twelve hours of continuous engine-off operation. The engineering is deliberately modest. The capability it buys is not.
What gun stabilisation actually is
The Ministry's list names the gun stabilisation system specifically, and it deserves explanation because it is the most demanding continuous electrical load on the vehicle and the least understood.
A tank moving across country pitches, rolls and yaws continuously. Without intervention, the gun barrel — rigidly attached to the turret, which is attached to the hull — moves with it. Aiming under those conditions would be impossible.
Stabilisation solves this by decoupling the gun's line of sight from the hull's motion. Gyroscopes sense the hull's angular movement in real time. A control system computes the correction. Servo drives move the gun in elevation and the turret in azimuth by exactly the opposite amount, so that the barrel stays pointed where the gunner put it while the vehicle underneath it moves. This is two-axis stabilisation, and it is what makes accurate fire on the move possible — the single largest tactical advantage a modern tank holds over an older one.
The system is a closed feedback loop that must run continuously. It cannot be started at the moment of need, because a stabilisation system takes time to settle and a gunner who switches it on when a target appears has already lost the engagement. Continuous operation means continuous power, and that is precisely the load an APU is there to carry.
Make-II: who paid for the development
The procurement route named in the contract is not a footnote. It changes who took the risk.
Under the Defence Acquisition Procedure, development projects fall into two broad sub-categories.
Make-I is government funded. The Ministry supports prototype development with up to 70 per cent of the cost — a figure reduced from the earlier 90 per cent — subject to a ceiling of ₹250 crore per Development Agency. It is used for large, expensive, high-risk development where no company would reasonably fund the work alone.
Make-II is industry funded. There is no government funding for prototype development at all. A company identifies a requirement — typically import substitution or an innovative solution — builds a prototype at its own cost and risk, and submits it for trials. A proposal may be brought forward by the Services or offered by a firm on its own initiative, in which case it can be progressed even where only one vendor emerges.
The consequence is worth stating plainly. Accurate Industrial Controls funded the development of these APUs itself. If the units had failed their trials, that money would simply have been lost. The ₹586 crore contract is not a grant towards development; it is the order that follows successful development, and it is what makes the risk rational to have taken in the first place.
This is why the Ministry described the contract as demonstrating that the mechanism works — that industry-led research and development can in fact be translated into production and induction. A procedure that never produces an order teaches industry not to use it. Our explainers on the iDEX route for defence innovation and the review of defence public sector undertakings cover the neighbouring parts of the same system; the significant feature here is that the developer is a private company rather than a defence public sector undertaking.
Old platforms, new sub-systems
The T-72 entered Indian service in the 1980s. The T-90S followed from the early 2000s, and is licence-produced at the Heavy Vehicles Factory at Avadi. Together they constitute the great bulk of the Army's armoured strength, and they will remain in service for many years yet.
Replacing a tank fleet is enormously expensive and enormously slow. Upgrading sub-systems within an existing fleet is neither. A platform designed around the electrical demands of the 1970s can be made to meet the demands of the 2020s by adding capacity rather than by replacing the vehicle — and that, rather than the headline value, is what the APU contract represents.
It is the same logic as the overhaul programme described in our piece on the Jabalpur tank overhaul line: an armoured fleet stays capable through sustained investment in what is inside it. The Ministry's framing of operational readiness, reliability and sustainability for the Army's mechanised formations is an accurate description of what a sub-system contract buys. It buys neither more tanks nor faster ones. It buys tanks that can sit still, in the dark, watching, for longer — and that is very often the thing that decides an armoured engagement.
🔑 Revision block
- Contract: ₹586 crore, signed 21 September 2026, New Delhi, in the presence of Defence Secretary Rajesh Kumar Singh
- Vendor: Accurate Industrial Controls Private Limited — a private company, not a DPSU
- Item: Auxiliary Power Units for T-72 and T-90 tanks
- Function: alternate source of electrical power for the fire control system, firing circuit, gun stabilisation system and other ancillaries
- Silent watch: main engine off, sensors and systems running on APU power
- Four gains from an APU: fuel economy, reduced main-engine wear from idling, lower acoustic and thermal signature, reduced crew fatigue
- Gun stabilisation: gyroscopes sense hull motion; servos counter-move the gun in elevation and the turret in azimuth; enables accurate fire on the move; requires continuous power
- Procurement route: Make-II — industry funded, no government funding for prototype development, aimed at import substitution and innovative solutions
- Make-I contrast: government funded, up to 70 per cent of prototype development cost (reduced from 90 per cent), ceiling ₹250 crore per Development Agency
- Fleet: T-72 in service since the 1980s; T-90S licence-produced at Heavy Vehicles Factory, Avadi
- Framing: indigenisation of critical sub-systems of armoured platforms; Aatmanirbharta in defence
🎯 Practice MCQs
Q1. The Auxiliary Power Unit contract signed on 21 September 2026 is valued at: (a) ₹250 crore (b) ₹586 crore (c) ₹1,200 crore (d) ₹2,500 crore
→ (b)
Q2. An Auxiliary Power Unit on a tank principally allows the crew to: (a) Increase the vehicle's top speed (b) Improve armour protection (c) Fire the main gun more rapidly (d) Operate electrical systems with the main engine shut down
→ (d) — the operating state this enables is called silent watch.
Q3. Under the Make-II sub-category of the Defence Acquisition Procedure: (a) The government funds up to 90 per cent of prototype development (b) The government funds up to 70 per cent of prototype development (c) Prototype development is funded entirely by industry (d) Only defence public sector undertakings may participate
→ (c) — the 70 per cent figure belongs to Make-I.
Q4. Under Make-I, government funding for prototype development is capped at: (a) ₹100 crore per Development Agency (b) ₹250 crore per Development Agency (c) ₹500 crore per Development Agency (d) There is no ceiling
→ (b)
Q5. Prolonged idling of a large diesel engine at very low load typically causes: (a) Carbon deposits and cylinder glazing that degrade the engine (b) Improved fuel efficiency (c) Increased power output (d) Reduced exhaust temperature with no other effect
→ (a)
Q6. The most operationally significant disadvantage of running a tank's main engine while stationary is: (a) Noise inside the crew compartment (b) Increased ammunition consumption (c) Battery overcharging (d) The acoustic and thermal signature it presents to enemy sensors
→ (d) — a hot exhaust plume is a distinctive object to a thermal imager.
Q7. Two-axis gun stabilisation on a main battle tank controls the gun in: (a) Elevation and azimuth (b) Elevation and range (c) Azimuth and traverse speed (d) Range and deflection
→ (a) — gyroscopes sense hull motion and servos apply the opposite correction.
Q8. The principal tactical benefit of gun stabilisation is: (a) Longer barrel life (b) Faster reloading (c) Accurate fire while the vehicle is moving (d) Reduced recoil
→ (c)
Q9. The T-90S is licence-produced in India at: (a) Ordnance Factory Medak (b) Gun Carriage Factory, Jabalpur (c) Bharat Earth Movers, Bengaluru (d) Heavy Vehicles Factory, Avadi
→ (d)
Q10. Consider the following statements: 1. Under Make-II, a company that fails trials bears the development cost it has already incurred. 2. Upgrading sub-systems is generally faster and cheaper than replacing an armoured fleet. Which is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2
→ (c) — which is why a production order matters as a signal, not only as revenue.
📋 How this gets asked (PYQ pattern)
Defence procurement has become a steady source of questions in the NDA general ability paper, and it is one of the few areas where candidates can gain an advantage by understanding a system rather than memorising a list of contracts.
The contract question asks for the value, the platform or the vendor. These are simple to hold if you record them as you encounter them, and simple to lose if you do not.
The category question is the one worth preparing deliberately. Buy (Indian-IDDM), Buy (Indian), Buy and Make (Indian), Make-I, Make-II, and the iDEX route are distinct categories with distinct funding and eligibility rules. The Make-I versus Make-II distinction — government funded against industry funded — is the single most frequently asked pair, and it is asked precisely because it is easy to invert under pressure.
The technology question asks what a named system does. APU, fire control system, gun stabilisation, explosive reactive armour, thermal imager. Short definitions are enough, but they need to be correct definitions rather than approximations.
The platform question covers the T-72 and T-90 themselves: origin, service entry, where they are produced in India. Avadi recurs.
For the SSB interview, this contract is a useful example to have ready. Asked what indigenisation means in practice, a candidate who says that a private Indian firm funded the development of a tank sub-system at its own risk and won a production order for it is describing a working mechanism. That is a substantially better answer than a recital of the slogan, and it is the kind of specificity that distinguishes a candidate who follows defence affairs from one who has revised them.
Preparing for NDA? Defence procurement rewards learning the categories once, properly — after that, every contract you read slots into a framework you already hold. Build the base with our NDA general ability notes, follow the daily NDA current affairs, and prepare with our faculty in the upcoming Cavalier courses in Delhi.
✍️ Written by Maj Sunil Chopra — Co-founder and defence studies faculty at The Cavalier. Reviewed by the Cavalier Faculty Desk.